Networking Self Teaching Guide OSI TCPIP LANs MANs WANs Implementation Management And Maintenance

User Manual: Pdf

Open the PDF directly: View PDF PDF.
Page Count: 867

DownloadNetworking Self-teaching Guide OSI TCPIP LANs MANs WANs Implementation Management And Maintenance
Open PDF In BrowserView PDF
x

Edwards

ffirs.tex

V3 - 03/27/2009

10:42am

Page ii

Edwards

ffirs.tex

V3 - 03/27/2009

Networking
Self-Teaching Guide

10:42am

Page i

Edwards

ffirs.tex

V3 - 03/27/2009

10:42am

Page ii

Edwards

ffirs.tex

V3 - 03/27/2009

Networking
Self-Teaching Guide
OSI, TCP/IP, LANs, MANs, WANs,
Implementation, Management,
and Maintenance
James Edwards
Richard Bramante

Wiley Publishing, Inc.

10:42am

Page iii

Edwards ffirs.tex

V3 - 03/27/2009

Networking Self-Teaching Guide
Published by
Wiley Publishing, Inc.
10475 Crosspoint Boulevard
Indianapolis, IN 46256

www.wiley.com
Copyright © 2009 by Wiley Publishing, Inc., Indianapolis, Indiana
Published simultaneously in Canada
ISBN: 978-0-470-40238-2
Manufactured in the United States of America
10 9 8 7 6 5 4 3 2 1
No part of this publication may be reproduced, stored in a retrieval system or transmitted in any form or
by any means, electronic, mechanical, photocopying, recording, scanning or otherwise, except as permitted
under Sections 107 or 108 of the 1976 United States Copyright Act, without either the prior written
permission of the Publisher, or authorization through payment of the appropriate per-copy fee to the
Copyright Clearance Center, 222 Rosewood Drive, Danvers, MA 01923, (978) 750-8400, fax (978) 6468600. Requests to the Publisher for permission should be addressed to the Permissions Department, John
Wiley & Sons, Inc., 111 River Street, Hoboken, NJ 07030, (201) 748-6011, fax (201) 748-6008, or online at
http://www.wiley.com/go/permissions.
Limit of Liability/Disclaimer of Warranty: The publisher and the author make no representations or
warranties with respect to the accuracy or completeness of the contents of this work and specifically disclaim
all warranties, including without limitation warranties of fitness for a particular purpose. No warranty may
be created or extended by sales or promotional materials. The advice and strategies contained herein may not
be suitable for every situation. This work is sold with the understanding that the publisher is not engaged in
rendering legal, accounting, or other professional services. If professional assistance is required, the services
of a competent professional person should be sought. Neither the publisher nor the author shall be liable for
damages arising herefrom. The fact that an organization or Web site is referred to in this work as a citation
and/or a potential source of further information does not mean that the author or the publisher endorses the
information the organization or Web site may provide or recommendations it may make. Further, readers
should be aware that Internet Web sites listed in this work may have changed or disappeared between when
this work was written and when it is read.
Library of Congress Cataloging-in-Publication Data:
Edwards, James, 1962Networking self-teaching guide : OSI, TCP/IP, LANs, MANs, WANs, implementation, management, and
maintenance / James Edwards, Richard Bramante.
p. cm.
Includes index.
ISBN 978-0-470-40238-2 (pbk.)
1. Computer networks. 2. Computer network protocols. 3. Computer network architectures. I. Bramante,
Richard, 1944- II. Title.
TK5105.5.E28 2009
004.6’5 — dc22
2009004168
For general information on our other products and services please contact our Customer Care Department
within the United States at (877) 762-2974, outside the United States at (317) 572-3993 or fax (317) 572-4002.
Trademarks: Wiley and the Wiley logo are trademarks or registered trademarks of John Wiley & Sons, Inc.
and/or its affiliates, in the United States and other countries, and may not be used without written permission.
All other trademarks are the property of their respective owners. Wiley Publishing, Inc. is not associated with
any product or vendor mentioned in this book.
Wiley also publishes its books in a variety of electronic formats. Some content that appears in print may not
be available in electronic books.

10:42am

Page iv

Edwards ffirs.tex

V3 - 03/27/2009

10:42am

This book is dedicated to my brother, Joel, for all that he has done for so
many over the years. I sincerely hope that he will forever be able to enjoy
all of the good things that life has to offer. Whether he knows it or not, he
has always been a source of inspiration for me and his encouragement
has kept me going whenever a challenge was thrown my way. The best
brother in the world! That’s my brother, Joel.
— Jim Edwards
This book is dedicated to those who have supported me, not just during
the writing of this book, but throughout my life. There have been many
and too numerous to mention, but to all who have been there for me, I
am deeply grateful. Deserving special mention are: My son, Rich; his
wife, Michelle; my three grandchildren, Vanessa, Ethan, and Olivia; my
parents; my siblings, Margaret, Mary, Josephine, Frank, and Salvatore;
and the person who believed in me, unfailingly, even through all my
blunders, my deceased wife, Barbara.
— Rich Bramante

Page v

Edwards ffirs.tex

V3 - 03/27/2009

10:42am

Page vi

Edwards

f01.tex

V2 - 03/27/2009

10:44am

About the Authors

Jim Edwards has more than 10 years of experience supporting data networks
as a Premium Support Engineer. He has authored four books pertaining to
data networking, as well as served as a technical editor.
Rich Bramante earned both a bachelor’s and master’s degree in electrical
engineering from the University of Massachusetts – Lowell. He has worked in
the technology industry for more than 40 years. For the past 11 years, he has
worked for a major telecommunications equipment manufacturer, primarily
within the VPN technology area.

vii

Page vii

Edwards

f02.tex

V3 - 03/27/2009

Credits

Executive Editor
Carol Long
Development Editor
John Sleeva
Technical Editor
Don Thoreson
Production Editor
Angela Smith
Copy Editor
Lunaea Weatherstone
Editorial Manager
Mary Beth Wakefield
Production Manager
Tim Tate
Vice President and Executive
Group Publisher
Richard Swadley

Vice President and Executive
Publisher
Barry Pruett
Associate Publisher
Jim Minatel
Project Coordinator, Cover
Lynsey Stanford
Proofreader
Publication Services, Inc.
Indexer
Jack Lewis
Cover Image
© Chad Baker/Photodisc/Getty
Images
Cover Designer
Michael Trent

3:50pm

Page viii

Edwards

f03.tex

V1 - 03/27/2009

11:21am

Acknowledgments

First and foremost, Jim wants to thank Rich for being such a great co-author to
work with. Rich and Jim had the opportunity to work together on a previous
book and we make a great team. Jim is a bit of a pain in the neck,1 so Rich may
have other opinions on this whole team thing.
We would also like to send out a huge word of thanks for all of the
individuals involved in the development of this book. To Carol Long, thank
you for bringing the idea to us and trusting us to see it through. We really
enjoyed it as much as we all thought we would. We also want to send a word
of thanks to the development editor, John Sleeva, for keeping us in line. It
was a pleasure working with you, sir. To Angela Smith, thank you for all the
assistance you gave us during the production phase. It is always nice to work
with people who are as friendly and helpful as everyone we have had the
pleasure of working with at Wiley. Additionally, thank you to Don Thorenson
for being our technical guinea pig and to Lunaea Weatherstone for catching all
of our mistakes. Finally, to all the people who work behind the scenes, thank
you for your support of this project.

1 There

are times when a bit of a pain in the neck is a good thing. Rich would like to thank
Jim for his enduring good nature and understanding of the predicaments Rich finds himself
involved with from time to time. We do make a good team because we have come to understand
that although we work together each has his own methods when it comes to his work. Overall,
mutual respect and understanding have helped us endure some trials and tribulations, and at
the end of the day we can open a beer and still find a good laugh to share.

ix

Page ix

Edwards

f03.tex

V1 - 03/27/2009

11:21am

Page x

Contents

Introduction

xxiii

Part I

Networking Nuts and Bolts

1

Chapter 1

Introduction to Networking
Networking: A Brief Introduction
Internetworking
An internet
The Internet
Intranets (Give Me an ‘‘A’’, Remove My ‘‘E’’, Now Flip the
‘‘R’’ and the ‘‘A’’)
Extranets
Virtual Private Networks
Catenet
Area Networks
Network Relationships and Topologies
Network Relationship Types
Network Topology Types
Protocols
Transmission Control Protocol
User Datagram Protocol
Internet Protocol
History of Networking
Standards and Standards Organizations
American National Standards Institute
International Organization for Standardization
International Electrotechnical Commission
Telecommunications Industry Association

3
4
5
6
6
7
7
8
9
9
13
13
17
24
26
27
27

28
32
34
35
36
36
xi

xii

Contents
Electronic Industries Alliance
International Telecommunication Union
IEEE
IEEE 802 Working Groups
IEEE 802.1
IEEE 802.3
IEEE 802.5
IEEE 802.11
Internet Society (ISOC)
Internet Engineering Task Force

Chapter 2

37
37
38
38
39
41
41
42
43
43

An Introduction to the OSI Reference Model
All People Seem to Need Data Processing — A Mnemonic
Device
A Layered Approach
Layer 7 — The Application Layer
Layer 6 — The Presentation Layer
Layer 5 — The Session Layer
Layer 4 — The Transport Layer
Layer 3 — The Network Layer
Layer 2 — The Data Link Layer
Layer 1 — The Physical Layer
TCP/IP, Please (and Don’t Be Stingy with the IP)
TCP/IP Applications
TCP/IP Utilities
The TCP/IP Reference Model
Chapter Exercises
Pop Quiz Answers

45

53
55
56
57
58
60

LANs, MANs, and WANs
Local Area Networks
LAN Standards
802.2 Logical Link Control
802.3 CSMA/CD Access Method and Physical Layer
802.5 Token Ring Access Method and Physical Layer
The Collision Domain Battle
The Most Common Wireless Standards
LAN Topologies
Token Ring Network Topologies
Bus Networks Topologies
Metropolitan Area Networks
Fiber Distributed Data Interface
A MAN Example
Wide Area Networks
Whose POTS?

63
64
64
64
66
70
73
76
77
79
83
93
93
96
98
99

46
47
48
49
50
50
51
52
53

Contents
Integrated Services Digital Network
Point-to-Point WANs
Frame Relay
Using the Internet for Your WAN

Chapter 3

100
101
103
105

Chapter Exercises
Pop Quiz Answers

107
108

Network Hardware and Transmission Media
Stuff You Just Need to Know
Bits, Bytes, and Binary
Non-human Resources
Volatile Memory
Nonvolatile Memory
Encapsulation
Data Communication Equipment and Data Terminal
Equipment
All Your Base Are Belong to Us
Computer Buses
IP Addressing
Transmission Media
Network Cabling
Twisted Pair Cable
Coaxial Cable
Fiber Optic Cable
Wireless Communication
Network Hardware
End-User Interface Hardware Types
Connecting End Users
Network Interfaces and Adapters
Network Interface Controllers
To Boldly Go Where Data Needs to Flow (or, How Does that
E-mail Get to Brother Joel?)
Concentrators
Hubs
Media Access Units
Repeaters
Bridges and Switches
Routers
Layer 3 Switches
Upper-Layer Switch Types
Remote Access
Servers
Chapter Exercises
Pop Quiz Answers

109
110
110
112
114
115
117
120
120
121
121

123
124
125
129
131
133
133
134
134
136
138
139
140
141
142
143
143
146
148
148
150
154

154
155

xiii

xiv

Contents
Chapter 4

Operating Systems and Networking Software
Computer Operating System Basics
CPU Basics
Computer Basics
Read-Only Memory
Random-Access Memory
Mass Storage System
Input/Output System
Operating System Basics
Network Operating System Basics
Peer-to-Peer Networking
File Sharing on a Peer-to-Peer Network
Printer Sharing on a Peer-to-Peer Network
Other Operating Systems
Unix
Linux
Sun Solaris
Chapter Exercises
Pop Quiz Answers

157
158
158
161
162
162
164
166
167
169
171
181
183
185
185
188
191
193
194

Chapter 5

The TCP/IP Protocol Suite

197

The TCP/IP Layers
Popular TCP/IP Protocols
The Application Layer
Domain Name System
Simple Network Management Protocol
File Transfer Protocol
Trivial File Transfer Protocol
Simple Mail Transfer Protocol
Network File System
Telecommunications Network
Secure Shell Protocol
The Transport Layer
Transmission Control Protocol
User Datagram Protocol
The Internet Layer
Internet Protocol
Internet Group Multicast Protocol
Internet Control Message Protocol
Routing Information Protocol
Open Shortest Path First
Border Gateway Protocol
Internet Protocol Security
End of Chapter Hodgepodge

198
201
202
202
206
212
217
220
222
224
227
228
228
231
232
233
234
234
235
237
238
238
239

Contents
There Is Hope for Diskless Nodes
A Little More Information on Routing
Sockets and Ports Are Not the Same Thing

240
240
241

Chapter Exercises
Pop Quiz Answers

244
245

Chapter 6

Ethernet Concepts
The Beginning of Ethernet Technology
Ethernet Components
DCE and DTE Cabling Considerations
Interconnecting Like Ethernet Devices
Ethernet and IEEE 802.3’s Relationship to the OSI Model
Logical Link Control
Media Access Control
Ethernet Frame Format
Transmitting a Frame
Half-Duplex Transmission
Full-Duplex Transmission
Autonegotiation
Receiving a Frame
Traffic Optimization
Traffic Shaping
VLAN Tagging
Chapter Exercises
Pop Quiz Answers

247
248
250
253
255
263
265
265
267
270
270
274
277
279
280
281
283
285
285

Chapter 7

Not to Be Forgotten
Can’t Get Enough of Those LAN Technologies
Attached Resource Computer Network
StarLAN
Token Ring
Token Ring’s Modus Operandi
Token Ring Media
The Format of the Token Ring Frame
Fiber Distributed Data Interface
FDDI Does What FDDI Does
FDDI Node Types
The FDDI Frame Format
As If You Haven’t Had Enough of These Sweet Protocols
Digital Equipment Company Network
Xerox Network Systems
Internetwork Packet Exchange
Point-to-Point Protocol
PPP Encapsulation Method

289
290
290
291
292
295
295
295
298
298
301
301
303
303
305
306
313
313

xv

xvi

Contents
PPP Link Control Protocol
PPP Network Control Protocol
Please, Tell Us More
PPP Frame Format
X.25
X.25 Operations
Link Access Procedure, Balanced
Packet Layer Protocol
Asynchronous Transfer Mode
ATM Generic Cell Format
An Overview of ATM Operations
ATM Reference Model
Traffic Management
ATM Adaptation Layer Types
Frame Relay
Frame Relay Node Types
Virtual Circuits . . . Again?
Data Link Connection Identifier
Feckens and Beckens
Local Management Interface
Frame Relay Frame Format
Integrated Services Digital Network
Basic Rate Interface and Primary Rate Interface
ISDN Nodes
The ISDN Reference Model
AppleTalk
AppleTalk Physical and Data Link Layers
AppleTalk Network Layer
AppleTalk Upper Layers

314
314
314
314
315
318
319
320
321
321
322
325
326
327
328
329
330
330
330
332
332
333
333
333
334
336
336
337
338

Chapter Exercises
Pop Quiz Answers

339
339

Part II

The OSI Layers

343

Chapter 8

The Upper Layers
Background
The TCP/IP Model
TCP/IP Application Layer
TCP/IP Transport Layer
TCP/IP Internet Layer
TCP/IP Link Layer
TCP/IP Link Layer Protocols
OSI Application Layer
OSI Presentation Layer
OSI Session Layer

345
346
349
362
362
366
367
370
372
374
374

Contents
Chapter Exercises
Pop Quiz Answers

376
377

The Transport Layer
The Terms and Conditions of Chapter 9
End-to-End Delivery
Standards
ISO/IEC 8072
ISO/IEC 8073
This, That, and the Other
Types of Transport Service
Data Units
Classes of Transport Service
Types of Network Service
Multiplexing
Transport Layer Operations
Connection-Oriented Operations
Setting Up the Connection
Maintaining the Connection
Terminating the Connection
Connectionless Operations
Transport Layer Protocols
A Few More Words about TCP
The TCP Header Format
A Little More on UDP
The UDP Header Format
The Meaning of Control
Chapter Exercises
Pop Quiz Answers

379
380
380
381
381
382
382
382
383
383
383
384
387
387
388
389
389
390
393
393
395
397
398
399
399
400

Chapter 10 The Network Layer
Network Connection Types
Connectionless Network Services
Connection-Oriented Network Services
Domain Name Services
TCP/IP Network Layer Protocols
Internet Protocol
Internet Protocol Version 4
Internet Protocol Version 6
Internet Control Message Protocol
Ping
Traceroute
Internet Group Management Protocol
Internet Protocol Security

403
404
405
410
412
417
417
418
423
425
425
427
429
431

Chapter 9

xvii

xviii Contents
Chapter Exercises
Pop Quiz Answers

433
433

Chapter 11 The Data Link Layer
Concerns of the LAN
It Just Is
Highs and Lows
Accessing the Medium
Rules of Accessing the Medium
From Tokens to Contention
Using the Token Method
Using the Contention Method
Meet the Sublayers
Logical Link Control
LLC Framing
Subnetwork Access Protocol
The MAC Sublayer
The MAC Address
Access Control for the Channel
The ‘‘ings’’ — Casting, Detecting, and Addressing
Data Link Addressing
The MAC Address Format
Unicast Addressing
Multicast Addressing
Error Detection
Control of the Flow
‘‘Knode’’ the LAN
Diary of a Network Bridge
Unicast Operation
Multicast Operation
When the Bridge Just Does Not Know
The Address Table
Chapter Exercises
Pop Quiz Answers

435
436
436
437
439
439
440
441
442
443
444
444
447
449
450
450
451
451
452
453
454
457
464
465
466
467
469
469
470
472
472

Part III

475

Network Design and Implementation

Chapter 12 Design Methodologies
Your Task Is to Design a Network
Types of Organizational LANs
Other Things to Consider
Building the Foundation
Let’s Start Planning
Development of Scope

477
478
479
480
480
481
481

Contents
You Are Not Alone

A Hierarchical Design Model
Access Layer
Distribution Layer
Core Layer
Why Hierarchical?
5-4-3-2-1, Speed Is Not the Big Concern
Making Determinations
Determining Which Topology to Use
Bus Network Topology
Star Network Topology
Ring Network Topology
Determining Which Nodes to Use
Traditional Nodes
Node Evolution
LAN Switching Technology
Switch Types
By All Means, Be Redundant
I’m Loopy!
Link Aggregation
Virtual LANs
Determining What Other Determinations Need to Be
Determined
Talking to a WAN
Management and Security
Choosing Protocols
Proactive Thinking
Network Implementation
Chapter Exercises
Pop Quiz Answers
Chapter 13 Implementation
Planning
Totally New Network Planning Phase
Initial Planning
Finalizing the Plan
Network Revision Planning
Reworking Network Access
Upgrading a Network’s Core Routers
Upgrading the Network’s Distribution Components
Network Supporting Infrastructure
Budgeting
Staging
Rollout
Verification

483

483
483
485
486
489
491
492
493
493
494
495
496
497
501
505
506
506
507
513
514
518
518
519
521
522

522
523
524
527
528
529
530
542
544
544
546
547
547
548
549
550
551

xix

xx

Contents
Documentation
The Final Stretch
Chapter Exercise
Pop Quiz Answer

553
554
556
557

Managing and Maintaining the Network

559

Chapter 14 Network Security
Elements of Network Security
Network Security Policies
Network Access Control
Network Premises Access Security
Network Access Security and Control
Restricting Network Access
Network Data Integrity
Network Security Monitoring
Network Security Assurance
Network Security Methodologies
Authentication
Lightweight Directory Access Protocol
RADIUS
Certificates
Data Integrity
Point-to-Point Tunneling Protocol
Layer 2 Tunneling Protocol
Internet Protocol Security
Chapter Exercises
Pop Quiz Answers

561
562
562
566
566
568
571
573
575
576
577
578
578
584
585
588
591
592
592
595
595

Chapter 15 Network Management
Operation
Help Desk Software
Network Operations Staff
Network Monitoring
Administration
Network Management Staff Members
Executive Level
Department Heads/Managers
Maintenance
Provisioning
Tools
Simple Network Management Protocol
Packet-Capture Capability
Chapter Exercises
Pop Quiz Answers

597
598
600
601
602
604
604
605
605
610
612
613
615
618
620
620

Part IV

Contents
Chapter 16 Troubleshooting
The Little LAN that Cried Wolf
Feedback
End-User Feedback
Management Station Feedback
Hmm . . .
What Could Possibly Go Wrong?
Food for Thought
The Proactive Approach Beats the Reactive Approach Hands
Down
Baseline
Proactive Documentation
There Is No Such Thing as Too Much
Troubleshooting Tools
Helpful TCP/IP Utilities
Ping
Traceroute
Netstat
Route
Arp
Ipconfig
More Helpful Tools
Even More Helpful Tools
A Logical Order
Define the Problem
Consider the Possibilities
Determine the Issue
Find a Possible Solution
Test the Possible Solution
Develop an Action Plan
Implement the Action Plan
Monitor the Results
Another Fantastic Bonus from the Authors
Layered Strategy
Common Lower-Layer Issues
Layer 1
Layer 2
Layer 3
Thoughts Pertaining to the Upper Layers
Troubleshooting Examples
Example 1: PC Can’t Connect
Example 2: Reading a Sniffer Trace
Example 3: Identifying a Broadcast Storm

621
622
623
623
624
624
624
625
627
627
628
630
631
631
632
634
637
639
642
643
646
647
648
649
649
650
650
651
651
652
652
653
654
656
656
657
658
659
660
661
663
665

xxi

xxii

Contents
Example 4: VPN Client Can’t Connect to VPN Server
Example 5: Two Common LAN Issues
Duplex Mismatch
Spanning Tree

Chapter Exercises
Pop Quiz Answers

666
667
668
669

671
672

Appendix A Additional Exercises

675

Appendix B Exercise Answers

701

Appendix C Glossary

765

Appendix D Acronyms

793

Index

805

Edwards f04.tex

V3 - 03/27/2009

11:10am

Introduction

The tremendous growth of local area networks (LANs) into the organizational,
corporate, and home networks in the last 20 years has shown that there is a
need for individuals with networking experience, and that need will remain
for a long time coming. The U.S. Department of Labor forecasts an increase of
58 percent in the network and system support job market by 2016. With that
growth comes opportunities for individuals with networking knowledge to
secure their future.
There are very few instances where a business is run without a network
of some sort. Retail environments maintain inventory, report income, transfer personnel information, and many other functions are handled within a
LAN. LAN-to-LAN communication, secure tunneling, encryption and authentication, and many other functions are now handled by specific nodes and
application programs that are part of the network.
In the beginning, most LANs were created around a shared data communication channel. Although not very reliable, these networks laid the foundation
for the LANs of today. In the late 1980s, LANs migrated from a shared medium
to more standardized and reliable media. These were twisted pair cabling and
the use of a node called a hub. End-user needs were also a driving force in
some of the advancements made in all facets of networking technology. Today,
the advancements made in areas related to networking are far superior than
what one would have dreamed possible back in the days of punch card coding
and computers that filled huge rooms.
We have written this book to serve as a self-study guide for individuals
looking to move into a networking career. Written as a basic networking guide,
the book covers networking technologies, including the hardware, software,
transmission media, and data transfer processes, along with operating systems
and systems software; LANs, WANs, and MANs; and the interactions of
network components.
xxiii

Page xxiii

Edwards f04.tex

xxiv

V3 - 03/27/2009

Introduction

How this Book Is Organized
The book is divided into four sections.

Part I: Networking Nuts and Bolts
The first part of the book teaches the essentials of networking. It is made up
of seven chapters. The information covered in this part is a basic overview of
many technologies used in networking today.
Chapter 1, ‘‘Introduction to Networking,’’ provides a review of basic
networking concepts, including network types, relationships, topologies,
protocols, history of networking, networking topologies, and standards
and standards organizations. This chapter is intended as a primer for
the target reader of the book. It can also be a great refresher chapter for
those of us who like to get back to the basics from time to time. This
chapter sets the framework for the rest of the book. Some important
insights are provided into the relationship between network architecture
and implementation, along with a lot of the history behind the development of modern LAN technology and the relevant standards.
Chapter 2, ‘‘LANs, MANs, and WANs,’’ explains the details of area networks, including the practices, standards, and standards organizations
that operate at each level.
Chapter 3, ‘‘Network Hardware and Transmission Media,’’ takes
a glance at the hardware and cabling that make up a network.
Additionally, there is an introduction to binary numbering, IP
addressing, and Ethernet concepts that provides an introduction
to the in-depth coverage of these topics throughout this book.
Chapter 4, ‘‘Operating Systems and Networking Software,’’ covers the
programs that are involved in a given network. The chapter shows
how the operating systems interact with the components within
a node and some of the basic services that are provided because
of these interactions. Details are provided on how peer-to-peer
networking operates, and the services and standards that allow
this to happen. Finally, an overview of the more popular operating
standards that are found in networks around the world is provided.
Chapter 5, ‘‘The TCP/IP Protocol Suite,’’ explains how the suite allows
data communication to take place. No matter where a device is located,
if it has a connection to the Internet and the device supports TCP/IP, you
have a connection to the world. The chapter also covers the more popular TCP/IP protocols and what these technologies and standards do.

11:10am

Page xxiv

Edwards

f04.tex

V3 - 03/27/2009

11:10am

Introduction

Chapter 6, ‘‘Ethernet Concepts,’’ explains the term Ethernet and
how it is used to describe the most common network architecture
used in a majority of today’s networks. Beginning from the development of Ethernet all the way to current Ethernet technology, you
will gain insight in the predominant LAN technology of today.
Chapter 7, ‘‘Not to Be Forgotten,’’ provides a basic overview of the
most commonly deployed standards and technologies in networking
today. From standards that are the tried and true technologies
to the up-and-coming standards, this chapter will provide you
with the understanding of the protocol and how it is used.

Part II: The OSI Layers
The second part of the book builds on the fundamentals discussed earlier to
explore advanced features and capabilities offered in many of the standards
that we discussed in the first part of the book. We provide an overview of the
individual layers of the OSI model, and explain how the layers work with one
another to communicate.
Chapter 8, ‘‘The Upper Layers,’’ covers the upper layers of the OSI
reference model: the Application layer, Presentation layer, and
Session layer. The chapter also provides information relating to the
‘‘translators’’ used so that information can flow smoothly and without
error between these layers and eventually be sent over the network
medium to another network node and the device servicing that node.
Chapter 9, ‘‘The Transport Layer,’’ explains how the Transport layer interacts with the Network layer and the Session
layer. This layer is responsible for the end-to-end connection
and datagram delivery, as well as congestion control and flow
control. How connections are set up, monitored, and taken
down is discussed. Operations of connection-oriented and connectionless protocols are also explained, with some further
exploration of some protocols that operate at this layer.
Chapter 10, ‘‘The Network Layer,’’ looks at the Network layer and
explains how it interfaces with the Data Link and Transport layers in
communication processes.
Chapter 11, ‘‘The Data Link Layer,’’ discusses the Data Link layer and
how it is used to allow for direct communication between network
nodes over a physical channel. Covered are topics such as one-to-one
communication as well as one-to-many. We cover concerns that are
experienced in a LAN, as well as some of the mechanisms that are in

xxv

Page xxv

Edwards f04.tex

xxvi

V3 - 03/27/2009

Introduction

place to recover from problems. In addition to the operations of this
layer, we discuss the use of Layer 2 switches and bridges in a LAN.

Part III: Network Design and Implementation
The third part of the book takes the information that was covered in the first
two parts and uses it to show provide practical insight into how thought
processes work in network design.
Chapter 12, ‘‘Design Methodologies,’’ covers every facet of networking
design, from inception to rollout. More of a guide that can be followed,
the information that is provided will allow you to understand
(and possibly develop) design concepts for a given network.
Chapter 13, ‘‘Implementation,’’ expands on the information in
Chapter 12 and walks you through the process of implementing
your design. At the end of the chapter is an exercise that will
allow you to test all that you covered in this part of the book.

Part IV: Managing and Maintaining the Network
The last part of the book wraps up our journey to learning networking and
covers the important tasks of securing, managing, and troubleshooting issues
within a given network.
Chapter 14, ‘‘Network Security,’’ details the security concerns that those who manage networks need to be aware of
and what you can do to assist in preventing attacks.
Chapter 15, ‘‘Network Management,’’ considers the extra functionality
that allows nodes to be configured and managed and also allows
for traffic monitoring and analysis. The chapter explains the Simple
Network Management Protocol (SNMP), along with the structure
and content of the management database. Special consideration
is given to network operations, including software, staffing and
support types, and network management and monitoring tools.
Chapter 16, ‘‘Troubleshooting’’ details the top troubleshooting strategies
for any network. The chapter covers the frequent issues that may
arise and outlines some troubleshooting strategies. It also gives an
overview of the troubleshooting process from beginning to end.
This book also includes the following four appendixes:
Appendix A, ‘‘Additional Exercises’’ contains 265 additional questions,
broken down by the chapters in which the answers can be found.

11:10am

Page xxvi

Edwards

f04.tex

V3 - 03/27/2009

11:10am

Introduction xxvii

Appendix B, ‘‘Exercise Answers’’ provides an answer to all of the
questions that were asked throughout the book. It’s up to you (or
your instructor) how these can be used. We suggest you try to answer
the questions before peeking . . . they are really quite simple.
Appendix C, ‘‘Glossary’’ provides gives definitions for the
technical terms that are used throughout the book.
Appendix D, ‘‘Acronyms’’ contains a multitude of common networking
abbreviations and acronyms.

Who Should Read This Book
This book is a self-study guide that is geared toward individuals who have a
background in information technology and want to migrate into a networking
career, and individuals who are working for a certification or a degree in a
networking field of study. Some of these career fields include
Computer engineering
Network sales and marketing
Networking engineering
Networking support
Network field service engineering
Network planning
Network design
Network administration
Network security
Network operations
The reader is assumed to be at least casually familiar with computers and
information technology. It is not necessary to understand any networking
concepts, as we cover networks from very basic concepts to more advanced
protocols and standards that mandate today’s technology, as well as future
growth.
There is no attempt on our part to provide a complete, from-the-ground-up
tutorial that will make you a professional in networking. That would be a task
requiring several volumes of work. Our focus was to provide you with the
information you need to have some experience for any popular standard in
use in networking today.
The readers of this book can expect to learn everything they need to
understand the concepts of networking. We have also provided addresses of

Page xxvii

Edwards

f04.tex

V3 - 03/27/2009

11:10am

xxviii Introduction

websites you can explore to better understand the specifics of a standard that
you have an interest in learning more about. Upon completion of this guide,
you will have a knowledge of the more popular technologies out there and in
the process you will learn about why things work and get some insight into
the reasons why things in networking are the way it is.

N O T E If you are interested, we have provided two course syllabi on our website
(www.wiley.com/compbooks). One syllabus is formatted for a quarter and the
other will fit with an 18-month course schedule.

A Few Words from the Authors
We hope that you enjoy reading this book as much as we enjoyed writing it.
We attempted to tie it all together, while providing details to some current
and up and coming practices that you will come across at some point in your
career.
As you start reading the book, you will
notice that we have included a few extras
throughout each chapter. Some of these will
ACRONYM ALERT
show up as an Acronym Alert or a RanVMS — Virtual memory system
dom Bonus Definition. Here are a couple of
examples:
Don’t get confused when you
come across these. The definiRANDOM BONUS DEFINITION
tions and acronyms are random
10BASE5 — A baseband Ethernet system
and do not necessarily apply
operating at 10 Mbps over thick coaxial
to the subject in the particular
cable.
chapter. We did this on purpose. One reason is that it helps
break the monotony that one
may experience when reading through these darn technical books. The other
reason is that it will hopefully help you to remember the terms as you progress
through the book.
Another extra that we have included are our pop quizzes, which do apply to
material that has been covered in that particular chapter. Here is an example:
At the end of each chapter
are the answers to the pop
POP QUIZ
quiz questions in that particuName 10 issues that you might have on the
lar chapter. This should serve
LAN.
as a quick reference for you as
you progress through the book.
Additionally, each chapter will

Page xxviii

Edwards

f04.tex

V3 - 03/27/2009

11:10am

Introduction

have questions that pertain to information contained within the chapter. The
answers to these questions are in Appendix B, but try to answer them without
looking — you have more to gain that way.
We tried to spice up this book with some jokes and remarks that will
hopefully make this enjoyable as well as informative. There are also some
secret bonuses that we won’t mention here (don’t want to ruin the surprise).

Contact the Authors
We welcome your feedback, both on the usefulness (or not) of this, the second
edition of this book, as well as any additions or corrections that should be
made in future editions. Good network-related stories, jokes, and puns are
always welcome. Please feel free to contact us:
NetworkingST@gmail.com

xxix

Page xxix

Edwards

f04.tex

V3 - 03/27/2009

11:10am

Page xxx

Edwards

p01.tex

V3 - 03/27/2009

12:20pm

Part

I
Networking Nuts and Bolts

In This Part
Chapter 1: Introduction to Networking
Chapter 2: LANs, MANs, and WANs
Chapter 3: Network Hardware and Transmission Media
Chapter 4: Operating Systems and Networking Software
Chapter 5: The TCP/IP Protocol Suite
Chapter 6: Ethernet Concepts
Chapter 7: Not To Be Forgotten

Page 1

Edwards

p01.tex

V3 - 03/27/2009

12:20pm

Page 2

Edwards

c01.tex

V3 - 03/27/2009

10:41am

CHAPTER

1

Introduction
to Networking
What, exactly, is the Internet? Basically it is a global network exchanging digitized
data in such a way that any computer, anywhere, that is equipped with a node
called a ‘‘modem’’ can make a noise like a duck choking on a kazoo.
— Dave Barry

Most of us would be lost without data networks.1 Just a few short years ago,
when computers were first starting to make their way into the business world,
data sharing would normally have to be done by copying and then carrying
the data from one PC to the next.2 Today, the data is transferred from one
user to the next in a fraction of a second. The growth that networking has
undergone is remarkable. And it doesn’t stop there. Every day there are new
standards being proposed, new innovations being developed, and updates
and changes to these being addressed.
Advances in technology are a fact of life. What needs to be considered is that
any advance that requires the movement of data from one point to the next will
need the services of a network to do so. This is why the world of networking
has grown so much (and will continue to do so). With users transferring large
amounts of data and the amount of that data growing at a exponential rate,
there seems to be no end to the opportunities networks offer.
This chapter provides an introduction to networking. The intention is to
provide you with a good foundation before we dive into the ‘‘nitty-gritty’’ of
networking. In this chapter, we cover the history of networking, the TCP/IP
and OSI reference models, standards organizations, as well as some discussions
and definitions. The approach we took with the first chapter will hopefully be
1 As

a matter of fact, everyone would be affected in one way or another.
sneakernet.

2 A.k.a.

3

Page 3

Edwards

4

Part I

c01.tex

V3 - 03/27/2009

Networking Nuts and Bolts

■

an enjoyable read, as well as set the tone for the rest of this book. We tried to
make this an interesting base chapter, splitting up the boring parts as much as
possible.
So, without further ado, welcome to our introduction to networking.

1.1

Networking: A Brief Introduction

Main Entry: net·work·ing3
Function: noun
1: the exchange of information or services among individuals, groups, or
institutions; specifically: the cultivation of productive relationships
for employment or business
2: the establishment or use of a computer network

A data network is a group of computers connected to one another by
communication paths, as well as the standards that allow communication.
A network can connect to other networks, allowing virtually worldwide
communication between two endpoints. Many networks share information
among one another, creating larger networks. Figure 1-1 is an example of a
segment of a network.

Workgroup A

Email

FTP Server
Workgroup B

Radius Server

Workgroup B
Workgroup B

Figure 1-1 A computer network sharing applications as well as hardware
3 Dictionary.com

Unabridged (v 1.1). Random House, Inc., accessed April 18, 2008.

10:41am

Page 4

Edwards

Chapter 1

■

c01.tex

V3 - 03/27/2009

10:41am

Introduction to Networking

Many things are shared on a network. Corporate business is conducted
nearly exclusively on the network. Networks allow users to share applications that are stored on servers in the network (e-mail applications,
word-processing applications, databases, and many others). They allow communication between end users. Data can be shared between companies or
individuals for business or personal purposes. Many websites provide opportunities that would have not existed if networks had never been developed.
Not to mention the entire file sharing that is enabled by a network. The possibilities are endless, and you can be sure that someone is working on a new,
cutting-edge service even as you read this sentence.
Typically, networks are identified by
their size. They range from small local area
networks (LANs) to larger wide area networks (WANs).4 Many networks remain
ACRONYM ALERT
isolated from others. They are there to
VPN — Virtual private networking
perform tasks that fit the specific needs
of the group or organization the network
supports. These networks have in place networking standards that support the needs of their organization, without regard
to anything outside of the network boundaries. This is due largely to the fact
that upgrading (updating) the network can be a cost that the organization has
not justified. If an organization does not need a high-speed LAN, why spend
the money to upgrade to one?
There are many other networks that have taken advantage of the tremendous
technology breakthroughs in the past 25 years that enable these networks to
share data securely. Vendors can connect to their clients’ LAN to exchange
business data in an instant. Internet service providers (ISPs) provide the
gateway to the Internet for their customers to share information. We discuss
many networking advancements throughout this book.

1.1.1

Internetworking

The ability to share information over dissimilar5 networks is known as internetworking. By using a set of standards, nodes in two (or more) data networks
can share information reliably between one another. In a bridged network,6 the
term does not really apply7 as the data is not shared with multiple segments
and no internetworking protocol is required to transfer the data.
Internetworking was designed for the specific purpose of providing an
avenue for sharing data among different nodes on the network and among
4

These are both discussed in depth in Chapter 2, ‘‘LANs, MANs, and WANs.’’
By dissimilar, we mean networks that are running with different node types and/or standards.
6 A collection of networks that are interconnected at the data link layer using network bridges.
7 Although there are some people out there who insist the term does apply.
5

5

Page 5

Edwards

6

Part I

■

c01.tex

V3 - 03/27/2009

Networking Nuts and Bolts

different system software and operating systems. Consider how data can be
shared by the medical profession. Lab work can be returned more quickly,
allowing for a more immediate diagnosis. Many hospitals are now allowing
x-rays and other data to be viewed over a network. Remote offices are able to
access this data in an instant, decreasing the time for a diagnosis to a level not
even dreamed of 15 years ago. The possibilities are endless.8
Networking terminology can
be a bit tricky, but it’s really not
as confusing as it may appear
RANDOM BONUS DEFINITION
at first. Following are some of
network application — A process or
the more common terms9 used
software program that runs on a node
to define networks of various
within a network.
purposes.

1.1.1.1

10

An internet

An internet (lowercase i) is a group of distinct networks connected to one
another via a gateway.11 ‘‘An internet’’ is often confused with ‘‘the Internet’’
(uppercase I ), but an internet is not necessarily part of the Internet.
Basically, any network that conforms to the standards defined in the TCP/IP
protocol suite (see Section 1.4) is an internet.

1.1.1.2

The Internet
‘‘A journey of a thousand sites begins with a single click.’’
— Author unknown

The Internet is what most people think of when they hear the term (upperand lowercases aside). The Web, WWW, the Information Super Highway, and
8 As

a matter of fact, there is work ongoing that may allow a surgeon to log in from home and
conduct an operation. Think how many lives can be saved because of this.
9 As well as one that is outdated, but Jim just loves the word.
10 Take a note of this number (not the section, the number). By the end of this book, you will
know the significance of all 1‘s.
11
As with many other networking terms, a gateway can mean many things. We are referring to
a node capable of relaying user application information among networks employing different
architectures and/or protocol suites.
Following are a few other definitions for the term gateway (for those of you who are interested):
(1) An internetworking node operating at the transport layer or above.
(2) An old term for an IP router.
(3) A marketing term for anything that connects anything to anything else.

10:41am

Page 6

Edwards

Chapter 1

■

c01.tex

V3 - 03/27/2009

10:41am

Introduction to Networking

many other terms define the network of networks. The Internet was developed
mainly upon its predecessor, the Advanced Research Projects Agency Network
(ARPANET). In addition to the Web, it encompasses a worldwide collection of
networks, including academic institutions, government organizations, various
public networks, as well as private networks (hopefully with the appropriate
security measures in place).
SOMETHING YOU JUST HAVE TO KNOW
The Internet Protocol (IP) is the dominant standard used in networking to make
sure that information is delivered from a source to a destination. We will talk
about IP throughout this book, so it is not necessary to go into an in-depth
definition at this point. You just have to understand that IP gets the data there.

1.1.1.3 Intranets (Give Me an ‘‘A’’, Remove My ‘‘E’’,
Now Flip the ‘‘R’’ and the ‘‘A’’)
An intranet is an IP-based12 network that
is administered and controlled by a single
entity. An intranet is a controlled network,
with only users who have authorization
ACRONYM ALERT
to be on the network granted access to it
LAN — Local area network
(both remotely and physically onsite). A
corporate LAN is an example of an intranet.
Although intranets are based on (and operate like) the Internet, they are
not widely available to just anyone who needs to access them. Security is in
place (firewalls, encryption and authentication measures, etc.) that will restrict
access to only those who need the access. This allows remote users to access
work applications over the Internet, while preventing unauthorized users from
gaining access.

1.1.1.4

Extranets

An extranet is an intranet that is opened up to allow outside users (e.g., vendors,
suppliers, employees, customers) access to the intranet (or any portion thereof).
The access normally is provided by a server, which clients access over the
Internet. An extranet operates securely to ensure that only authorized users are
12 See!

We told you that you would need to know what IP meant.

7

Page 7

Edwards

8

Part I

■

c01.tex

V3 - 03/27/2009

Networking Nuts and Bolts

entitled access to the intranet. An extranet may comprise any of the following
for security and privacy purposes13 :
Firewall — Network hardware and/or software that captures data
passing through it and determines whether to pass or drop the data.
Firewalls are configurable, and filters can be applied to provide the
appropriate security for the LAN.
Public key certificate — An electronic document that can verify and
authorize an individual by public key cryptography. Public key cryptography uses two keys14 (one public key and one private key) to encrypt
and then decrypt data to ensure that a message can be transported
securely.
Authentication encryption (AE) — A system that
is able to protect both the
secrecy and the integrity
of data communication.
Virtual private network
(VPN) — A network that
is created when one network connects to another
by a secure tunnel.

1.1.1.5

RANDOM BONUS DEFINITION
Tunneling is a method of securing access to
an intranet. Another popular form is
through a web server, where registered
users can be authenticated after logging in
through a web browser login page.

Virtual Private Networks

A virtual private network (VPN) is an extranet that securely connects separate
networks to one another, as well as individuals to networks. VPNs updated15
the use of dedicated lines that could only be used by one entity at a time. VPN
technology is a much more proficient and cost-effective solution than the use
of dedicated lines.
VPN technology uses a public network (normally the Internet) to connect
users and networks to one another in what are known as tunnels. Data integrity
is ensured by the use of security measures as well as tunneling protocols that
set the rules for the tunnel.
VPN tunneling protocols include:
Generic Routing Encapsulation (GRE)
IP Security (IPSec)
13 It’s

important to note that the technologies listed are not exclusive to extranets, but they are
important technologies within extranets.
14 A key is information used to determine an algorithm’s output.
15
Although many organizations now use VPNs (or some other extranet type) for remote access,
some networks still utilize the dedicated lines (both owned and leased) when network access is
required.

10:41am

Page 8

Edwards

Chapter 1

■

c01.tex

V3 - 03/27/2009

10:41am

Introduction to Networking

Layer 2 Tunneling Protocol (L2TP)
Point-to-Point Tunneling Protocol (PPTP)
Tunneling protocols ensure
that the data is encrypted on the
sending end of the tunnel and
is decrypted appropriately at
the receiving end of the tunnel.
In addition to the data encryption, security is established to
ensure that endpoint addresses
are encrypted as well.

1.1.1.6

RANDOM BONUS DEFINITION
network node — Any device that participates in data communication within a
network.

Catenet

The term catenet stands for ‘‘catenated network.’’ A catenet is simply a group
of networks that are connected to one another via a gateway. It is an obsolete
term that was replaced by some more up-to-date terms (i.e., internet) that we
discuss in the pages that follow.
AND NOW, A MOMENT OF THOUGHT
Maybe someone will propose a standard to replace the word internet
(lowercase i) with catenet and save us all that darn confusion. I mean, it really
would make sense, right? However, should this ever happen, I would bet $20
that it wouldn’t be long before ‘‘the Internet’’ became ‘‘the Catenet’’ and then
we would be right back where we were before.

What it boils down to is that it would be nice to see the term catenet return.
It’s kind of catchy.

1.1.1.7

Area Networks

Chapter 2, ‘‘LANs, MANs, and WANs,’’ discusses area networks in depth.
However, for those who may not have heard these terms, it is appropriate to
have a brief introduction to area networks in this first chapter.
An area network is simply a network that spans a specific geographic area and
serves a specific purpose. Any time you communicate over a network (wired
or wireless), you are using an area network (or even various area networks
and network types). In a nutshell, a LAN, a WAN, and a MAN are basically all
the same. The differences are the geographical area that each covers, as well
as some of the communication protocols that are in use.

9

Page 9

Edwards

10

Part I

■

c01.tex

V3 - 03/27/2009

Networking Nuts and Bolts

The main three area networks
POP QUIZ
you will probably hear about
are the local area network, the
What is a public key certificate?
metropolitan area network, and
the wide area network. There
are a few other area network
terms in use at the time of this writing, but they are not referred to as often as
the aforementioned. These less common area networks are the personal area
network (PAN), the campus area network (CAN), and the global area network
(GAN).16
1.1.1.7.1 Campus Area Networks

A network that spans a limited geographic area specific to academics is
considered a campus area network (CAN). A CAN is nothing more than a
MAN that connects university buildings and provides services for the staff of
the university and its students.
Some CANs provide additional services such as classroom updates, labs,
e-mail, and other necessary services for the students via iPod, cell phone, and
other wireless technologies. You may or may not ever have to be involved
in a CAN, but at least now you can share your CAN knowledge should the
opportunity present itself.17
1.1.1.7.2 Global Area Networks

A global area network (GAN) is any network that connects two or more WANS
and covers an unlimited geographical area. The entire network connected
together would be considered a GAN. GANs are becoming increasingly
popular as so many companies are opening offices and operating business on
a global scale.
1.1.1.7.3 Local Area Network

A local area network (LAN) is a data network that covers a small geographical
area, typically ranging from just a few PCs to an area about the size of an
office building or a group of buildings. Unlike WANs, LANs don’t require a
leased line to operate. LANs also maintain higher data rates than do some of
the larger area networks, due mainly to the smaller area of coverage.
Nodes that are members of a LAN communicate with other LAN nodes by
sharing some form of channel (e.g., a wireless access point, twisted cable, fiber
optic cable). PC users on a LAN often use a shared server to access and work
with certain applications used by the organization.
16 In

the near future, you might see this one used a lot more. The use of the word global has
increased over the past few years, so it stands to reason that a GAN is right around the corner.
17 Or you can just sit on your CAN, er, knowledge and keep it to yourself.

10:41am

Page 10

Edwards c01.tex

Chapter 1

■

V3 - 03/27/2009

10:41am

Introduction to Networking

The three major LAN technologies in use today are Token Ring (discussed
in Chapter 7, ‘‘Not to Be Forgotten’’), Ethernet18 (discussed in Chapter 6, ‘‘Ethernet Concepts’’), and Fiber Distributed Data Interface (FDDI), also discussed
in Chapter 7.
1.1.1.7.4 Metropolitan Area Networks

A metropolitan area network (MAN) is a network that physically covers an
area larger than a LAN and smaller than a WAN. The network is normally
maintained by a single operating entity, such as government offices, healthcare
systems, and any other type of large organization or corporation.
MANs allow communication over a large geographical area, utilizing protocols such as ATM, FDDI, Fast Ethernet, or Gigabit Ethernet.19 This is a
better solution than communication between LANs over a WAN, which relies
on routing to decipher and allow communication of different protocol types
between various area networks. Communication over a WAN is also slower
and more expensive than what is offered by a MAN. MANs also provide
control of the transmission of data from endpoint to endpoint, whereas the
WAN solution requires that you rely on the service provider for a portion of
the data flow control.
1.1.1.7.5 Personal Area Networks

A personal area network (PAN) is a network that is established for an
individual user within a range of around 30 feet — for instance, a person has
a PDA or cell phone and connects to a PC or other node for the purposes of
exchanging data. This is done wirelessly, although wired PANs are feasible
in this day and age. A pure wireless PAN is termed a WPAN, although most
PANs would likely be made predominately of wireless devices. Although
a PAN or WPAN might be considered a LAN or WLAN, the defined area
outlined by the terms certainly does help in isolating network segments.
Some examples of devices that might make up part of a PAN include:
iPhone
Personal digital assistants (PDAs)
Cellular phones

18 Ethernet is by far the most popular and widely used LAN technology.

As a matter of fact, many
LANs are now migrating to Ethernet when they begin replacing legacy nodes in their LANs.
Chapter 6, Ethernet Concepts, is dedicated to this technology.
19 Although many MANs still utilize a lot of these various protocols (e.g., FDDI, ATM),
Ethernet-based MANs are rapidly becoming the preferred standard. Most new MANs are
Ethernet-based, and many MANs are migrating to the Ethernet-based solution as their MAN
standard.

11

Page 11

Edwards

12

Part I

■

c01.tex

V3 - 03/27/2009

Networking Nuts and Bolts

Video gaming systems
Pagers
Personal computers or laptops
Printers
Most portable peripherals
1.1.1.7.6 Wide Area Networks

A wide area network (WAN) is a network that covers a large geographical
area.20 Most people think of a WAN as a public shared network, which is partly
the case, but a lot of privately owned as well as leased WANs are currently in
existence.21 A WAN links other area networks to one another, providing a way
to transmit data to and from users in other places. If you think about it, the
WAN is the king of the area networks (although this might not hold true for
much longer, as the GAN is quickly gaining speed to become the big daddy of
them all).
WANs use networking protocols (e.g., TCP/IP) to deliver data from endpoint to endpoint. A WAN also ensures that addressing of endpoints is
maintained so it knows where data needs to go to reach its intended destination. Some communication protocols that are used on WANs to handle the
transmission of data include:
Asynchronous Transfer Mode (ATM)
Frame relay
Packet over SONET (POS)22
X.2523
1.1.1.7.7 Wireless Local Area Networks

A wireless local area network (WLAN) is an LAN without wires. WLANs use
modulation technologies that are based on radio wave technology to allow
communication with other wireless nodes within a limited geographical area.
Many businesses now offer WLANs for use by their customers (many at
no charge). Additionally, many cities in the United States are implementing
WLANS throughout their city to allow free access to users within the wireless
area.
20

You can consider a network a WAN if the network boundaries exceed the size of a large
metropolitan area. But hey, one man’s MAN is another man’s WAN.
21 These will not be going away. As a matter of fact, no one knows what the future holds. The
possibilities seem endless.
22 Here is another fun acronym to consider. Instead of Packet over SONET (POS), why not SONET
under Packet (SUP)? Then when you greet your fellow networking professionals you could say,
‘‘Hey! What’s SUP?’’
23 X.25 is an oldie but goodie. It has long been replaced by other protocols. Still, it was one of the
earliest WAN protocols and it deserved a mention.

10:41am

Page 12

Edwards

Chapter 1

1.1.2

c01.tex

■

V3 - 03/27/2009

10:41am

Introduction to Networking

Network Relationships and Topologies24

Network relationships refer to
the communication that takes
place between two nodes over
RANDOM BONUS DEFINITION
a network. When a relationship
packet — The encapsulated data that is
is formed, the nodes are able
transmitted and received at the Network
to utilize resources between one
layer (see Section 1.4.2.5).
another in order to share data.
There are two network relationship types that define the foundation of any network. A peer-to-peer network relationship is where both nodes
treat each others as equals, whereas a client/server network relationship is one
in which one node (the server) handles storing and sharing information and
the other node (the client) accesses the stored data.
The manner is which nodes in a network connect to a communication line in
order to exchange data is an example of a physical topology. Another topology
type would be a logical topology, which defines the way data is passed from
endpoint to endpoint throughout the network. The logical topology does not
give any regard to the way the nodes are physically laid out. Its concern is to
get the data where it is supposed to go.

1.1.2.1

Network Relationship Types

The main difference between the two network relationship types are whether you
want to have every user share resources
with each other or have a central node that
ACRONYM ALERT
handles all the processing while serving the
TCP — Transmission Control Protocol
needs of the clients. This means that pretty
much everything else is the same between
the relationships. They both use the same protocols and physical connections
to the network. Which one is appropriate for an organization depends on the
needs, wants, and demands of the users of the network (cost factors, data
speed concerns, etc.).
1.1.2.1.1 Client/Server Network Relationship

In a client/server25 network relationship, one node acts as a server and the
other nodes are clients that utilize the resources of the server to access an
24 Relationships

and Topologies (RAT). Now, that acronym has a certain ring to it. Or maybe we
should have written this heading to read Network Relationships or Topologies (ROT). The former
has a better ring, in our opinion, so RAT it is!
25 A client/server network relationship is different from a client/server database system. In both
cases, the server provides the data requested by a client, but in a database system, the client node
has to use its own resources to format and view the data retrieved.

13

Page 13

Edwards

14

Part I

■

c01.tex

V3 - 03/27/2009

Networking Nuts and Bolts

application or service. In a client/server network relationship, the server
stores data (e.g., e-mail applications, encryption and authorization services,
printers, VPN network access, and many more) that is used by the users of
the organizational LAN. Most servers are Unix based, or a derivative of Unix,
such as Linux or SunOS, all of which are discussed in depth in Chapter 4,
‘‘Operating Systems and Networking Software.’’ The users interface with the
network through a PC or Mac (or whatever device is necessary at that time26 ).
The PCs will have an application that contains the information necessary to
connect to and share data with the server. Figure 1-2 shows an example of the
client/server relationship.

PC–A

PC–B

PC–C

PC–D

Server Farm
Scanner (all)
Printer (all)
Modem (all)
Fax Machine (all)
Documents (A only)
Documents (B only)
Documents (D only)
Warehouse database (shared)
Production Software (shared)
Accounting (D only)
Payroll (C only)
Invoices (C only)
Employee records (C only)

Figure 1-2 A client/server network relationship

No clients share resources with any other client in the client/server network
relationship. They are simply users of the resources that are made available by
26 For the remainder of the book, when a reference is made to a network user, it is assumed that
the user is a PC end user. Otherwise, we will specify the type of user that is being referenced.
Don’t worry, Mac fans. Chapter 4, ‘‘Operating Systems and Networking Software’’ talks about
the Mac OS.

10:41am

Page 14

Edwards

Chapter 1

c01.tex

■

V3 - 03/27/2009

10:41am

Introduction to Networking

the server. The servers maintain and provide shared resources to a specified
number27 of clients.
Advantages of a client/server network relationship include:
It is a secure way to share data over a network. Because all the
accessed resources are on the server, the server is able to control
and maintain the security of sessions. Also, instead of multiple
nodes in various locations, the server is a single entity and can be
secured away from unauthorized visitors.
Because most servers have more built-in redundancy than a single
user’s PC, the servers are very reliable in doing their job. Normally,
there are backup drives (or other servers) that can be failed over28
to if there is a problem with the primary drive or server.
It is easier to back up data that is on the server than to do so with
many nodes. Most organizations perform backups at night when
the server is not as busy. Having only one node to back up makes it a
very simple, time-saving process.
Servers are fast because they have to serve multiple end users at the
same time. The performance standards set for a server are far higher than
the standards for a PC.
Of course, it’s not all peaches and cream in client/server land. Disadvantages
of a client/server network relationship include:
Administrators of the
server have to be trained
and experienced. There
is a lot to know, and the
potential for failure is very
high without a trained
professional (therefore,
be prepared to pay).

POP QUIZ
Encapsulated data that is transmitted
and received at the Network layer is
.
called a

Servers require more physical resources in order to do the job.
This makes the price to operate a bit higher than in a peer-to-peer
environment.
1.1.2.1.2 Peer-to-Peer Network Relationship

A peer-to-peer network relationship is exactly that: all the users are peers
(equals) and they share resources that are necessary to be shared. Each
27 The

total number would depend on the capabilities of both the server hardware and the
software that it is running on the node.
28 In a redundant configuration, a failover occurs when the primary has a failure and the
backup has to take over as the primary. A failover is transparent to the end users.

15

Page 15

Edwards c01.tex

16

Part I

■

V3 - 03/27/2009

Networking Nuts and Bolts

computer is required to determine what is to be shared and then ensures that
resources are made available to the nodes that need to access the resources.
Figure 1-3 shows an example of how this works.

PC–A
Modem (shared)
Fax Machine (shared)
Documents (private)

PC–B

PC–C

Warehouse database (shared)
Payroll (private)
Production Software (shared)
Invoices (private)
Documents (private)
Employee records (private)

PC–D
Scanner (shared)
Printer (shared)
Documents (private)
Accounting (private)

Figure 1-3 A peer-to-peer network relationship

Note that in the example, PC-C does not have any shared resources, but
it may have a need to use some of the shared resources in the peer-to-peer
network. Therefore, PC-C will be a part of the peer-to-peer topology as a user
of the other resources made available by the other peers.
Some examples of shared resources include:
Printers
Modems
Scanners
Data files
Applications
Storage devices
A peer can share any of these in any combination that makes the best use
of resources to meet the needs of the users in the network. One computer
can provide access to the office printer and scanner, while another computer
can have the modem connected to it. By sharing resources, you save the
expense of having to have one of everything for every computer in
the organization. Security for the shared resources is the responsibility of the
peer that controls them. Each node will implement and maintain security
policies for the resources and ultimately ensures that only those that have a
need can use the resources. Each peer in a peer-to-peer network is responsible
for knowing how to reach another peer, what resources are shared where, and
what security policies are in place.
Advantages of a peer-to-peer network relationship include:
It is cheaper to implement and maintain. You don’t have to buy multiple peripherals for each computer. You also don’t have the cost of

10:41am

Page 16

Edwards

Chapter 1

c01.tex

■

V3 - 03/27/2009

10:41am

Introduction to Networking

purchasing and maintaining a server. Because each peer uses its own
resources, there is no stress on only one node to do all the serving.
A peer-to-peer network does not require a special operating
system. A peer-to-peer network can be built on operating systems that
are currently running on most PCs.
There are more redundancy options available in a peer-to-peer
network. Because multiple clients are sharing resources, it is a
good idea to design a way to have a process failover to a backup
peer should the master peer have a failure.
A peer-to-peer network is easier to maintain than a client/server
network, and the job of keeping up with the network can be assigned to
multiple people.29
Disadvantages of a peer-to-peer network relationship include:
If a lot of people are trying to use a shared resource, computer performance may be adversely affected.
Because multiple peers are performing different tasks, it is harder
to back up data in a peer-to-peer network.
Security is not as good as in a client/server network. Because each peer
is responsible for maintaining security for the resources it controls, the
potential exists that an end user may accidentally or maliciously change
the security parameters, causing a security lapse on that particular node.
Also, each node is physically available to multiple people (possibly
even people who work in the same building but whom you don’t
know). In a client/server environment, the administrator maintains
security and the server is physically set apart from the clients.

1.1.2.2

Network Topology Types

A network topology is basically the way all the nodes in the network are
connected. There are five primary topologies (bus, mesh, ring, star, and tree)
that are installed in various networks. When designing a network, knowing
which topology to use is determined by several factors:
Is speed a concern?
How reliable does the network need to be?
How much money are you willing to spend to set it up?
How much are you willing to spend to maintain the network?
29 And

where exactly does the buck stop?

17

Page 17

Edwards c01.tex

18

Part I

■

V3 - 03/27/2009

Networking Nuts and Bolts

Data is carried in the network by a detailed cabling scheme. How the
network performs depends on whether the cabling is set up correctly.30 Miss a
port here or there and you can really cause a network some problems. If there is
a cable that is longer than specifications, you are going to have other problems.
Once you complete this section, you will come to realize that networking is
more than just ‘‘plugging it in.’’
1.1.2.2.1 Bus Topology

The bus topology is probably the easiest one to understand and to implement.
It is simply a topology in which all the nodes are connected to a single shared
cable (called a bus). The cable is terminated at each end to prevent an open
loop condition. Figure 1-4 shows an example of a bus topology.

Figure 1-4 A bus topology

As with any of the topology types, the bus topology has benefits as well as
drawbacks. The advantages of a bus topology include:
It’s easy to install and maintain.
Adding new nodes is rather simple.
Less cabling is required than with some of the other topology types.
It’s inexpensive to implement.
The disadvantages include:
If the cable breaks at any point, network access is lost to all nodes on the
segment.
It can be expensive to maintain over a period of time.
Data communication is slower than with some of the other topologies.
30 When

designing a network, the placement of the cabling is the first thing that you need to
consider and then you expand from that. Of course, wireless networking is an option, but you
still begin planning the wireless network by determining where the access points should be.

10:41am

Page 18

Edwards c01.tex

Chapter 1

■

V3 - 03/27/2009

10:41am

Introduction to Networking

The network segment traffic flow is affected each time a node is added.
There is a limit to the number of nodes that can be added to the segment.
When a node that is connected to a shared bus needs to pass data on to
the network, it has to have a mechanism for detecting whether other nodes
are transmitting data at the same time. It must do this to prevent a collision
on the bus (see Figure 1-5) or have a set of rules to follow when a collision
occurs. In the example, you see that node C is trying to send data to node D.
At the same time, node A is sending data to node E. Because there is no way to
determine whether the other node was passing data, a collision occurs on the
bus. This is not the worst part — because there was no mechanism within the
bus topology to detect collisions, both of the sending nodes assume that the
data reached the intended recipients and they relax, thinking they successfully
sent the data.
Node A

Node C

Collision
Data Destined for Node E

Data Destined for Node D

Node D

Node E

Figure 1-5 The dreaded collision

Collision avoidance can be handled in the following ways in a bus topology:
Carrier Sense Multiple
RANDOM BONUS DEFINITION
Access with Collision
Detection (CSMA/CD)
physical port — A physical interface that
protocol31 — This is a
resides on a network node. Not to be
method of determinconfused with a TCP/UDP port.
ing if another node is
sending data by listening on the bus first. If it senses that the channel is being used by
another node, the node will delay transmitting its data until the channel is available. CSMA is used to avoid collisions, while CD will detect
31 Protocols

are discussed in Section 1.1.3.

19

Page 19

Edwards

20

Part I

■

c01.tex

V3 - 03/27/2009

Networking Nuts and Bolts

when a collision occurs and will stop transmitting data. Once a set
period of time has lapsed, the sending node will send the data again.
Take note that if CSMA is used without the CD, each sending node
will send the entire datagram,32 even when a collision occurs.
A bus master — A bus
master is an application
RANDOM BONUS DEFINITION
running on one of the
nodes within the segTCP/IP port — A number in the data
packet header that maps to a process
ment or a separate node
running on a node. Not to be confused with
known as an input/output
a physical port.
(I/O) controller. The
bus master is the master node and all other
nodes are referred to as slave nodes. The master controls the transmission of data to and from all nodes within the bus topology.
1.1.2.2.2 Mesh Topology

There are two types of mesh topologies that can be used. A full mesh topology
(Figure 1-6) is a configuration where all the nodes within the network segment
are connected to one another. A partial mesh topology (Figure 1-7) is where
some nodes are connected to all the others, and some only connect to the ones
they need to communicate with.

Figure 1-6 A full mesh topology

As with almost any topology, there are some advantages and some disadvantages to the mesh topology. One advantage of the mesh topology is that
you have a lot of redundancy. If one node is down, the others are virtually
unaffected. There is always a route around broken or blocked paths.
32 A

datagram is a self-contained entity of data that is transmitted from one endpoint to another
within a network. Layer 3 packets and Layer 2 frames are two examples of datagrams. As a
matter of fact, many network professionals use the three terms interchangeably.

10:41am

Page 20

Edwards

Chapter 1

c01.tex

■

V3 - 03/27/2009

10:41am

Introduction to Networking

Figure 1-7 A partial mesh topology

One major disadvantage of the mesh
topology is that it is expensive to implement. Also, as the network grows, so does
the complexity of the mesh topology. In
Figure 1-6, there are four nodes within
the mesh topology. Imagine what a nightmare it would be to maintain a mesh that
included 100 nodes.

ACRONYM ALERT
FTP — File Transfer Protocol

1.1.2.2.3 Star Topology

The star network is one of the more popular network types used by organizational LANs. In the star topology, all nodes in the network connect to a central
node that handles the passing of datagrams between the nodes. Figure 1-8
shows an example of the star topology.

Figure 1-8 A star topology

The central node receives a datagram and then broadcasts the data to all the
nodes it connects to. The connecting nodes can communicate with each other

21

Page 21

Edwards

22

Part I

■

c01.tex

V3 - 03/27/2009

Networking Nuts and Bolts

by sending data to and receiving data from the central node. Should one of the
connecting nodes go offline, the central hub will discontinue communication
to the one node only and the other connecting nodes will continue to operate.
The advantages of a star topology include:
It allows for direct communication between two nodes.
It’s simple to implement and maintain
It helps to narrow down problematic network segments.
It’s easy to troubleshoot and allows for quick recovery.33
The disadvantages include:
If the central node fails, all the other nodes are affected.
If there is an increase in network traffic, the central node may become
‘‘sluggish,’’ affecting the performance of some, if not all, of the connecting nodes.
Scalability within the network is limited to the capabilities of the central
node.
1.1.2.2.4 Ring Topology

The ring topology can be a bit confusing, as the term ring defines the logical
topology rather than the physical topology. As shown in Figure 1-9, the
ring passes data logically from station to station until the data reaches its
destination.

Figure 1-9 A ring (logical) topology
33

When the problematic link is discovered, all you have to do is pull out the cable to prevent the issue from propagating to the rest of the nodes within the star.

10:41am

Page 22

Edwards c01.tex

Chapter 1

■

V3 - 03/27/2009

10:41am

Introduction to Networking

Each node handles each datagram that is passed, verifying whether the
datagram is destined for it and, if not, passing it along to the next node. In
the ring topology, there is a single path from one node to the next. Should
there be a break along the way, all nodes on the ring will no longer be able to
communicate on the network. To overcome this, many ring topology networks
employ a dual ring, with data passing in the opposite direction on a redundant
ring (see Figure 1-10).

Figure 1-10 A dual-ring topology

Advantages of a ring topology include:
There’s no need to have a mechanism to ensure collision-free datagram
passing.
It can expand to cover a greater number of nodes than some of the other
topology types.
It’s fairly simple to maintain.
Disadvantages of a ring topology include:
A failure with one node on the ring may cause an outage to all connected
nodes.
Any maintenance (e.g., adding a node, making a change to a node,
removing a node) would affect all the nodes that connect to the ring.
Some of the hardware required to implement a ring is more
expensive than Ethernet network cards and nodes.

23

Page 23

Edwards

24

Part I

■

c01.tex

V3 - 03/27/2009

Networking Nuts and Bolts

Under normal traffic load, a ring is much slower than other topologies.
There are not many of this type of network, as most networks are migrating to Ethernet.
1.1.2.2.5 Hierarchical Topology (a.k.a. Tree Topology)

A hierarchical34 topology is very similar to a star topology. Like the star
topology, the hierarchical topology has a central node that connects multiple
nodes to one another. However, in the hierarchical topology, each node could
potentially act as a central node to a group of other nodes. Figure 1-11 shows
the physical layout of a hierarchical topology.

Figure 1-11 A hierarchical topology

Notice how a hierarchical topology is similar to an organizational structure.
The mainframe computer would be the single node at the top of the chart, and
then the lower levels would be other minicomputers and PCs. The hierarchical
topology is quite effective in smaller areas, where a central mainframe can
connect to different minicomputers, and the minicomputers can provide a
central connection for the PCs in the departments they serve.

1.1.3

Protocols

Simply put, a protocol is a standard (or set of standards) that governs the rules
for setting up a data connection, communicating between endpoints once the
connection is set, and transferring data between those endpoints. There are
34

Jim used to have a colleague who could never get the pronunciation right for the word
‘‘hierarchical.’’ He would pronounce the word ‘‘harr-arrr-cul-cul.’’ No matter how hard he tried,
he never could get the word down. It was pretty funny.

10:41am

Page 24

Edwards c01.tex

Chapter 1

■

V3 - 03/27/2009

10:41am

Introduction to Networking

protocols set for both hardware
POP QUIZ
and software, and sometimes for
the combination of the two.
What is the difference between a physical
Network protocols vary in
port and a TCP port?
purpose and complexity. They
are usually used to detect the
physical properties of both the
sending and the target nodes, as well as whether the target node is available.
Once the connection endpoints are determined, a protocol will handle the
initial communication35 between the endpoints as well as the rules for the
connection. The protocol will identify how each end will know where a data
stream starts and stops, what format it will be sent and received in, and what
to do with the data if there are any problems with the transfer.
The Internet would not be what it is if it were not for the protocols, especially
the Internet Protocol (IP) and the Transmission Control Protocol (TCP), used
in combination with each other and referred to as TCP/IP or the TCP/IP
protocol suite.
TCP/IP and many other protocols are discussed throughout this book, but
here is a short list of a few of the more common protocols:
File Transfer Protocol (FTP) — FTP is used to transfer large
amounts of data from one node to another. The FTP protocol
uses an FTP server to serve files to an FTP client.
Hypertext Transfer Protocol (HTTP) — HTTP is a communications protocol that allows for data transmissions within data
networks as well as the World Wide Web (WWW). HTTP uses
a server (e.g., a website) to serve the clients (end users) data the clients
have requested via a web browser.
Hypertext Transfer Protocol over Secure Socket Layer (HTTPS) —
HTTPS is an enhancement to HTTP that allows secure sessions over SSL.
These sessions provide adequate security for private transactions on the
WWW.
Internet Message Access Protocol version 4 (IMAP4) — IMAP4 is a protocol that allows a client to connect to and retrieve e-mail from an e-mail
server.
Internet Protocol (IP) — IP is a standard that allows for the
transfer of data between nodes that are connected on a network.
Each node within an IP network has a unique address that
identifies it for the purpose of locating and sharing data between
nodes. The latest version of IP that has been released is IPv6.
35

The initial conversation between the two endpoints is commonly referred to as a handshake.

25

Page 25

Edwards

26

Part I

■

c01.tex

V3 - 03/27/2009

Networking Nuts and Bolts

Post Office Protocol version 3 (POP3) — POP3 is a protocol that
allows an e-mail client to connect to an e-mail server and retrieve mail
that is destined for that client.
Simple Mail Transfer Protocol (SMTP) — SMTP is a protocol that
allows a network user to send and receive e-mail.
Simple Network Management Protocol (SNMP) — SNMP is a protocol
that allows for the sharing of management data on a network. SNMP
allows network administrators the ability to quickly access network
nodes to monitor performance, troubleshoot, baseline, and ensure that
the network is capable of addressing the needs of the organization.
Transmission Control Protocol (TCP) — TCP is a protocol that connects
end users with one another and ensures the integrity of the exchanged
data.
Trivial File Transfer Protocol (TFTP) — TFTP is a protocol that is a simpler form of FTP.
User Datagram Protocol (UDP) — UDP is a protocol that connects
end users to one another and transfers datagrams, but does not ensure
the integrity of the datagrams.

1.1.3.1

Transmission Control Protocol

The Transmission Control Protocol (TCP) ensures that data is transmitted
from endpoint to endpoint in a reliable manner. TCP operates at the Transport
layer of the OSI reference model (more on this in Section 1.4). TCP is normally
associated with the TCP/IP protocol suite; however, it is its own entity. It is
a protocol that can adapt to a variety of data delivery standards, providing
reliable data delivery.
TCP is the reliable36 transport protocol that controls the flow of data
between hosts. TCP divides messages into smaller segments and ensures the
data arrives error-free and is presented by the target node in the correct order.
TCP manages the flow of data and makes adjustments to the size and the
speed in which the data is transported. TCP is used by most of today’s more
popular networking services and applications, including the World Wide Web
(WWW), e-mail, and Secure Shell (SSH).

36 The key word here is ‘‘reliable.’’ This does not imply that TCP can provide the quickest delivery

available. TCP is designed to offer reliable and accurate delivery, but it does not guarantee timely
delivery and is not used when speed is needed to transmit data. The Real-time Transport Protocol
(RTP) is normally used in these instances.

10:41am

Page 26

Edwards

Chapter 1

c01.tex

■

V3 - 03/27/2009

10:41am

Introduction to Networking

TCP is a connection-oriented protocol. This means that there is a connection
between two endpoints before any data is sent. A connection-oriented protocol
also ensures that once the data arrives at a destination, it is put back together
in the proper order. A connection-oriented protocol cannot promise that data
won’t get dropped, but if it is received, it will be sequenced appropriately.

1.1.3.2

User Datagram Protocol

The User Datagram Protocol (UDP) provides a method for transmitting datagrams between endpoints, but no guarantee of the delivery is made. This
means that a datagram may be duplicated, can go missing, and may not
arrive in the order in which it was sent. This also means that UDP is a faster
transmission standard than TCP.
UDP is preferred in situations where you need data to be transmitted
quickly. There is simply more processing power to get the data to the destination because there is no error checking. UDP supports broadcasting37 and
multicasting,38 so messages can get to destinations within a network segment
as well as to everyone within the network.
UDP is a connectionless protocol, which means there is no guarantee that
the intended destination is available. There is no checking the communication
line prior to transmitting data, it is just transmitted.

1.1.3.3

Internet Protocol

The Internet Protocol (IP) is the
protocol that defines how data is
POP QUIZ
transmitted between two nodes.
Because IP does not establish a connection
Datagrams are forwarded to a
before sending data to an endpoint, it would
destination endpoint based on
protocol.
be considered a
the IP address that is assigned
to the endpoint. When data is
transmitted, the data is encapsulated into datagrams and multiple datagrams may be required to transmit
a single message. Each datagram is treated as its own entity without regard
to any of the other datagrams that make up the message. Each datagram can
choose whatever path it wishes to take to reach a destination. That is IP’s job:
to get the datagram to the destination by the quickest route possible.39
37 Sending

data to everyone connected to the network segment.
data to a select group of nodes.
39 It is TCP’s job to put them back together again.
38 Sending

27

Page 27

Edwards

28

Part I

1.2

■

c01.tex

V3 - 03/27/2009

Networking Nuts and Bolts

History of Networking

On April 3, 1860, the Pony Express officially opened for business. Covering
250 miles in each 24-hour period, the riders would travel at full gallop from
one Pony Express station to the next. At each stop, they would change horses,
exchange mail, and head on to the next stop. After 100 miles or so, the
rider would be relieved by a fresh rider to continue the journey. What an
accomplishment this was. Only 15 years prior to that, it would take six months
to get a message from the east coast to the west coast. The Pony Express could
do it in about 11 days. The Pony Express dissolved in October 1861, when the
first transcontinental telegraph was transmitted.
Now look where we are today. In milliseconds, we can send a letter from
Hong Kong to New York, or talk over the Internet with a loved one on the
other side of the planet. We can get trip directions, listen to a radio station
anywhere in the world, work, and play games — all at the same time. It is
amazing how far communication has come.
It might surprise you to know that the concept of connecting nodes to one
another was developed as a way for research organizations and educational
institutions to share resources. There was one significant event that occurred
that opened the doors for a lot of various research, some of which eventually
introduced the network concept. What exactly was this event? It was the race
to space.
The Soviet Union launched the Sputnik satellite on October 4, 1957. This
alarmed many American citizens and was an embarrassment to many people
in the United States because of a few failed attempts prior to that date. The
launch of the Sputnik satellite is said to have ushered in the Space Age, but
that is not all it changed. It changed the attitude of those who were involved in
the United States space program, as well as the attitude of U.S. citizens. After
Sputnik launched, funds began flooding to research agencies and institutions.
The National Defense Education Act was signed to promote studies in math,
science, and foreign languages. One of the agencies formed was the Advanced
Research Projects Agency (ARPA) in 1958.
ARPA was formed as an agency that would be tasked by the United States
Department of Defense (DoD) to research and develop projects. ARPA was
not required to focus on only projects of military concern, and it was quickly
determined that a focus on computers would be a worthwhile investment. In
1962, ARPA chose Dr. J.C.R. Licklider to lead the computer research effort.
WHAT’S IN A WORD?
If you think that the whole catenet/internet/Internet terming conventions seem
a little confusing, you haven’t seen anything yet. Check this out:
(continued)

10:41am

Page 28

Edwards

Chapter 1

c01.tex

■

V3 - 03/27/2009

10:41am

Introduction to Networking

WHAT’S IN A WORD? (continued)
The Advanced Research Projects Agency (ARPA) was formed in 1958. In 1972,
ARPA was replaced by the Defense Advanced Research Projects Agency
(DARPA). DARPA did the same job that ARPA did, but DARPA was established
as a separate defense agency (still under the Secretary of Defense).
In 1993, DARPA became ARPA and was put back as it was when it was first
formed. In 1996, the name was officially changed to DARPA again.

Licklider realized even before his appointment the potential of connecting
nodes to one another to share resources. He had developed what he called
a galactic network concept, and he was able to convince other researchers
(including those who took over when he left) how important his concept
was. He outlined his plan to accomplish this concept and the very first large
network research team was formed. This team, known as the ARPA community,
was a group of universities across the United States. It is important to note
that Licklider left his position before his concepts became a reality, but his
successors moved ahead in their development.
ARPA formed a subgroup
called the Information Processing Techniques Office (IPTO)
POP QUIZ
to focus on research pertainWhat is the difference between a WAN and
ing to anything related to coma LAN?
puting. It was funding from
ARPA/IPTO that assisted in
the ARPA community of educational and scientific institutions to investigate time and resource sharing
possibilities.
Many people today still feel that the Internet was developed to provide a
fallback mechanism in the event of a nuclear attack. This is probably due to the
fact that there was so much funding poured into development after the launch
of the Sputnik satellite. The official reason that was given for the concept of
networking nodes together was simply to share files and resources among
investigative agencies and groups.
In 1968, ARPA allowed contractors to bid on the plan they had been working
on, and BBN Technologies was brought in. In 1969, ARPANET was born. The
original ARPANET was a network with several small computers referred
to as interface message processors (IMPs), which were nodes that performed
packet-switching and were used to connect to each other by modems and to
users on host computers.40 The IMPs were configured with 24 Kb41 of memory,
40 Don’t

think of these hosts as PCs. These hosts were huge computers, sometimes occupying a
whole floor of a building.
41
Kb = kilobits

29

Page 29

Edwards c01.tex

30

Part I

■

V3 - 03/27/2009

Networking Nuts and Bolts

supported up to four host computers, and were able to connect to a maximum
of six other IMPs. The IMPs communicated with one another over leased
communication lines. The original ARPANET was made up of four IMPs that
were established at the following locations:
Stanford Research Institute
University of California, Los Angeles
University of California, Santa Barbara
University of Utah
BBN Technologies developed the first
communications protocol, known as the
BBN Report 1822, which later became known
as the 1822 protocol. The 1822 protocol simACRONYM ALERT
ply specified the manner in which a host
DoS — Denial of service
communicated with the IMP. The 1822 protocol predated the OSI reference model (see
Section 1.4) and did not really follow the layering process we use today.42
The 1822 protocol was eventually replaced by the Network Control Protocol
(NCP), which incorporated a transport function. The NCP remained the main
communication protocol until 1983, when it was replaced by the TCP/IP protocol suite. The TCP/IP protocol suite was more resilient than the NCP, and
its introduction was the birth of communication networks as we have known
them to date.
Eventually, ARPA got out of the networking business to focus on research
in other areas. The Defense Department retained the military portion of
the ARPANET and named it the MILNET. The remainder of ARPANET
remained with research and educational organizations, and BBN Technologies
continued to maintain these networks. Because of the split of ARPANET,
many of the resources available to the institutions and organizations were
severed in the interest of security required by the MILNET. In response to this,
the National Science Foundation funded the development of the Computer
Science Network (CSNET), which provided access to shared resources for these
groups. Eventually, the network grew and was transformed into the National
Science Foundation Network (NSFNET), which was developed originally to
allow researchers access to five supercomputers at the following locations:
Cornell University
Pittsburgh Supercomputing Center
42 It

can be said that the 1822 protocol used the physical, data link, and network layers as the
host system packaged data and sent it to the address of the IMP (directly connected). The IMP,
in turn, routed the data to the destination IMP, which sent it to the destination host.

10:41am

Page 30

Edwards

Chapter 1

c01.tex

■

V3 - 03/27/2009

10:41am

Introduction to Networking

Princeton University
University of Illinois
University of California, San Diego
The NSFNET used the TCP/IP protocol suite as a communications protocol
and was completely compatible with the ARPANET. In the early 1990s, more
and more organizations started accessing what was now called the Internet,
but permissions had to be obtained from the NSFNET to use many of the
services that were offered. The main supercomputer centers maintained and
monitored the Internet’s growth.
Today networks are defined by the way they get information from point to
point. The nodes used and the standards deployed are integral parts of any
network, defining the very basis for that network’s existence. Networks are
commonplace and growing on a global level. Only the future can tell what
new advances will be made for this global communication vehicle.
INTERNET TIMELINE TRIVIA
1957: The Advanced Research Projects Agency (AARPA) is formed.
1961: The Massachusetts Institute of Technology (MIT) began researching
data-sharing potential. There are fewer than 9,500 computers in the world.
1966: ARPANET is under development, packet-switching technology is
launched.
1969: ARPANET is launched.
1971: The number of nodes on the ARPANET is 15.
1973: London and Norway join ARPANET. Global communications are
launched.
1974: TCP is launched. Data communication speeds increase and the reliability
of data transmission improves.
1975: The first ARPANET mailing list is launched. TCP tests are run successfully
from the U.S. mainland to Hawaii as well as to the U.K., via satellite links.
1976: Unix is developed.
1978: TCP and IP split into two separate protocols.
1982: TCP/IP becomes the standard used by the Department of Defense
for data communication within the U.S. military’s network.
1984: The number of nodes on the Internet is over 1,000. Domain Name Service is launched.
(continued)

31

Page 31

Edwards

32

Part I

■

c01.tex

V3 - 03/27/2009

Networking Nuts and Bolts

INTERNET TIMELINE TRIVIA (continued)
1987: The number of nodes on the Internet is over 10,000.
1988: The Internet experiences its first Internet worm.
1989: The number of nodes on the Internet is over 100,000.
1990: ARPANET is disbanded. The first commercial Internet service provider
(ISP) is launched.
1991: The first Internet connection is made (at 9600 baud). The World Wide
Web is launched.
1992: The number of nodes on the Internet is over 1,000,000.
1994: The WWW becomes the most popular service on the Internet.
Some radio stations start broadcasting over the Internet.
1995: Internet streaming technology is introduced.
1996: Web browser software vendors begin a ‘‘browser war.’’
1997: Over 70,000 mailing lists are now registered.
1998: The 2,000,000th domain name is registered.
2000: The first major denial-of-service (DoS) attack is launched. Most major
websites are affected.
2002: Blogs become cool.
2003: Flash mobs are born. Flash mobs are groups of people who gather
online and plan a meeting in a public place. Once they assemble, they
perform a predetermined action, ranging from pillow fights to zombie
walks. The participants leave as soon as the meeting is over. (Wikipedia has
a good article about flash mobs: www.wikipedia.org/wiki/Flash mob.)
2005: The Microsoft Network (MSN) reports that there are over 200 million
active Hotmail accounts.
2006: Joost is launched, allowing for the sharing of TV shows and video using
peer-to-peer technology.
2008: Online search engine Technorati reported that they are
now tracking and indexing over 112 million online blogs.

1.3

Standards and Standards Organizations

As we have discussed already, the standards that are put in place to ensure
that data communication can be shared between nodes on a network are
an essential part of the network. Without a standard way of doing things,

10:41am

Page 32

Edwards c01.tex

Chapter 1

■

V3 - 03/27/2009

10:41am

Introduction to Networking

networks would not be able to operate nearly as efficient as they do today.43
So it is fair to say that based on what we have discussed so far, we can all be
in agreement that standards are required in order for data communication to
be shared on a network. Standards serve the following purposes:
Set up and maintain rules to be followed in the network
Define how network hardware interfaces operate
Maintain all communication protocols that are in use in a network
Offer the ability of utilizing the hardware and software available from
multiple vendors and ensure that these are interoperable with like
resources from other vendors
Standards begin when an individual or organization has an idea. A proposal
is put forth and a committee reviews it to determine if the proposal has any
merit. If the proposal is accepted, the idea will be transferred to a development
committee, which will outline the scope of the proposed standard and submit a
draft to a committee that will vote on whether the standard is to be approved.
If the standard is passed for approval, the final draft is written and then
published as a new standard.
There are three main types of networking-related standards. It important
that you understand the differences, as it is virtually a guarantee that you will
need to know this at some point.
De facto standards — A de facto standard is a standard that began
as a proprietary standard and then grew to a standard that is used
by pretty much everyone. As a matter of fact, it is widely assumed
that many proprietary standards are developed with the hopes
that they will become de facto standards.44 A de facto standard is
similar to an open standard in that it is universally used by multiple
vendors, but it is never approved as a formal open standard.
Proprietary standards — A proprietary standard is a standard that is
developed and owned by a specific vendor. When PCs first started coming out, most vendors tried to avoid admitting the importance of a cooperative standard that could be used between different vendors. The
technology was starting to boom, and corporate confidentiality was a
huge concern, so it was important to keep their standards to themselves.
As a matter of fact, it really made sense that having control of a standard
43 That

is assuming that they would work at all without standards.
would they do this? To become the industry leader for whatever the standard covers.
Think about it this way. If you want to purchase a computer that supports the widget standard, you might have more faith in the company that introduced and has supported the standard for years, as opposed to purchasing a PC from ‘‘Mom and Pop’s PC shop,’’ which only
recently started supporting the widget standard.

44 Why

33

Page 33

Edwards

34

Part I

■

c01.tex

V3 - 03/27/2009

Networking Nuts and Bolts

as it would be beneficial to the future of the company. To take this even
further, companies saw no real value in supporting the proprietary standard of the competition (why have to pay them for the rights to use the
standard?), so instead they developed something close to what the competition had, and then encouraged the consumer to move to what they
had to offer, as they did ‘‘xyz’’45 more than the competitor. Proprietary
standards still exist, but they are not as common as they once were.
Open standards — An open standard
is a standard that is used by almost
everyone. Most vendors involved in
ACRONYM ALERT
networking resources now realize
IEEE — Institute of Electrical and Electronics
that they can be just as competitive
Engineers
while developing cooperative standards that are agreed upon by other
vendors. This quickly became evident as consumer demand grew. Consumers wanted to be able to choose from multiple vendors, and expected
the nodes to communicate well with one another. There are some companies that still prefer to work with mostly proprietary standards, but
there is a larger customer base for devices that use open standards.
This section discusses some of the standards organizations and what purpose
each one serves. These organizations develop formal standards for the area
of networking they are applicable to. Most standards committees operate as
nonprofit organizations and are made up of researchers, educators, specific
vendors, and industry professionals. In turn, vendors model the development
of their products based on the agreed standard.

1.3.1

American National Standards Institute

The American National Standards Institute (ANSI) is the
organization responsible for
POP QUIZ
ensuring that guidelines are
The three types of standards are
established for every type of
,
business you can imagine. From
, and
.
construction standards to agricultural standards, ANSI is
responsible for outlining and
accrediting these standards. The mission of ANSI is to ensure that standards
are defined and followed in order to protect and ensure global competitiveness
45 This

could be anything from a true advance over the competitor to a ‘‘prettier’’ package.

10:41am

Page 34

Edwards

Chapter 1

c01.tex

■

V3 - 03/27/2009

10:41am

Introduction to Networking

for American business and ultimately improve life standards for the American
consumer.
ANSI is the organization that represents the United States in working with
the global community on issues relating to two important global standards
organizations. These are:
International Organization for Standardization (ISO)
International Electrotechnical Commission (IEC)
It is important to note that ANSI is not the developer of standards; rather, it
oversees the development of standards by accrediting the standards once they
have been set up and proposed by what are known as Standards Development
Organizations (SDOs). It is the responsibility of the SDOs to develop and
maintain standards that represent the users for their group.46
Examples of some of the SDOs that have had standards accredited by
ANSI47 :
American Dental AssoRANDOM BONUS DEFINITION
ciation (ADA)
North American Die Casting Association (NADCA)
Standards Australia (SAI)
Institute of Electrical
and Electronics Engineers (IEEE)

working group — A group formed by
interested members of an organization. The
working group can have open meetings, as
well as communication through Internet
forums and mailing lists. The working
group works on issues relating to standards
and standards development.

Chinese Standards (SPC)

1.3.2

International Organization for Standardization

Founded in 1947, the International Organization for Standardization (ISO)48
is an organization that is tasked with standardizing international standards
for various interests. Based in Switzerland, the ISO is made up of members
46 By

‘‘group,’’ we mean the individuals outside of the SDO for whom the developing standards
will apply.
47 This list is provided as an example of the broad range of communities that are ANSI accredited.
That being said, some of these have nothing to do with networking. If you are interested in
further reading, you can go to the ANSI website (www.ansi.org), or there is a search engine
you can use to locate standards and SDOs (www.nssn.org).
48 You might wonder why the acronym is not IOS for the International Organization for
Standardization. Being an international organization, the acronym would be different depending
on which country you were in (English would be IOS, but the French acronym would be OIN,
which stands for Organisation Internationale de Normalisation). The forming members of the
organization agreed upon ISO, which came from the Greek word isos, meaning ‘‘equal.’’ This
provided a globally standard acronym for the organization.

35

Page 35

Edwards

36

Part I

■

c01.tex

V3 - 03/27/2009

Networking Nuts and Bolts

from 157 nations. In addition to the development of international standards,
the ISO also is responsible for publishing an assortment of technical reports,
specifications, and guides. Following is a list of some of the available ISO
standards:
ISO/IEC 9541 –Information Technology — Font information interchange
ISO 9000 — Quality management system in production environments
ISO 9141 — Network interconnection of computers in a vehicle
ISO 15930 — Portable Document Format (PDF)
The preceding is only a short example of the many standards maintained
by the ISO. For further reading, visit the ISO website at www.iso.org.

1.3.3

International Electrotechnical Commission

The International Electrotechnical Commission (IEC) is responsible for standards that relate to electrotechnology (electronics and related technology). The
strict standards developed by the IEC are used by its members as references
when standardizing electrotechnical resources and contracts. Products that are
manufactured to these standards can be used regardless of where in the world
you live. The IEC is credited for promoting trade and technical efficiency on
a global scale. This ensures that the end user can operate the IEC-supported
device without having to understand the complexities that may be involved
in the technology itself.
In addition to international standards, the IEC also produces various publications that outline specifications and guidelines for areas that may not be
considered standards. Many of these publications are revisions to existing
standards or draft standards that are under review.

1.3.4

Telecommunications Industry Association

The Telecommunications Industry Association (TIA) develops standards that
apply to telecommunications technologies. TIA has over 70 formulation
groups, each of which manages
different subcommittees comRANDOM BONUS DEFINITION
posed of industry professionals, manufacturers, service probirds of a feather (BoF) — A BoF is an
informal discussion group that consists of
viders, and even government
members who share a common interest or
representatives.
concern.
These subcommittees and formulation groups devise and develop standards that are submitted to ANSI for accreditation. TIA committees

10:41am

Page 36

Edwards

Chapter 1

c01.tex

■

V3 - 03/27/2009

10:41am

Introduction to Networking

write and maintain standards and specifications for the telecommunications
industry. TIA also participates within various international telecommunications groups representing the interests of the United States on a global
forum.

1.3.5

Electronic Industries Alliance

The Electronic Industries Alliance (EIA) is an association made up of technical
and electronic manufacturers from the United States that cooperatively work
with each other to ensure that the development and competitiveness of these
companies are represented on a global scale. The issues the EIA addresses are
of interest to the common good of these companies as a whole, ensuring that
the companies are able to achieve the success they deserve. The EIA focuses
on the following areas:
Cyber security
The environment
Information technology reform
Telecommunications reform
Global competitiveness
Global trade and market access

1.3.6

International Telecommunication Union

Dedicated to bringing worldwide communication to everyone, the International Telecommunication Union (ITU) is an organization that works to
facilitate telecommunications and data network development and continued
growth on a global scale. The ITU is striving to enable individuals everywhere
to have access to benefits that are available with the information community
and the global economy.
In 2007, the ITU launched the Global
Cybersecurity Agenda (GCA), envisioning
the future assurance of cybersecurity as
well as cyber peace throughout the InterACRONYM ALERT
net. Another goal of the ITU is to strengthen
RIP — Routing Information Protocol
communications to assist in disaster recovery and prevention efforts in major countries as well as developing countries that lack resources and economies to
support the Information Age.

37

Page 37

Edwards c01.tex

38

Part I

■

1.3.7

V3 - 03/27/2009

Networking Nuts and Bolts

IEEE

Originally, IEEE was the acronym for the Institute of Electrical and Electronics
Engineers. Over time, the scope and mission of the IEEE grew into other related
fields, and now the name of the organization is simply IEEE (that’s I-triple-E).
The IEEE develops49 global standards applicable to information technology,
telecommunications, power generation, and other related services. The IEEE
has developed and maintains more than 900 standards that are active and in
use. Additionally, more than 400 draft standards are in development.
The IEEE membership is made up of scientists, engineers, and other leaders
in the fields of computer science, electronics, engineering, and related professions. Membership in the IEEE provides access to the latest developments
in technology, assists in career development, provides access to technical
information, and many other benefits.
In additional to the standards that are developed and maintained by the
IEEE, the organization publishes almost a third of the world’s technical
literature for the fields of computer science, electrical engineering, and electronics. They also maintain an online digital library, sponsor conferences, offer
educational and special-purpose grants, and bestow recognition awards.
One of the largest family of standards maintained by the IEEE is IEEE 802.
The IEEE 802 organization is made up of 22 working groups (see Section 1.3.7.1)
that work to develop standards applicable to LAN, MAN, and some WAN
technologies. This section introduces some of the IEEE LAN standards. For
more information about the IEEE, go to their website, www.ieee.org.

1.3.7.1

IEEE 802 Working Groups

A working group is a team of professionals who are brought together to work
on new research activities. Usually these are formed when an individual or a
group presents a suggestion for a resolution to a current standard or on the
behalf of a new technology that is being mainstreamed. Working groups are
often referred to as a task force, task group, study group, advisory group, and
many others. Following is a list of IEEE 802 working groups and their current
status:
Active groups
802.1 Higher Layer LAN Protocols Working Group
802.3 Ethernet Working Group
802.11 Wireless LAN Working Group
49 As a

matter of fact, at the time of this writing, IEEE touted that they were the leading developer
of international standards.

10:41am

Page 38

Edwards

Chapter 1

c01.tex

■

V3 - 03/27/2009

10:41am

Introduction to Networking

802.15 Wireless Personal Area Network (WPAN) Working Group
802.16 Broadband Wireless Access Working Group
802.17 Resilient Packet Ring Working Group
802.18 Radio Regulatory Technical Advisory Group
802.19 Coexistence Technical Advisory Group
802.20 Mobile Broadband Wireless Access (MBWA) Working Group
802.21 Media Independent Handoff Working Group
802.22 Wireless Regional Area Networks
Inactive groups50
802.2 Logical Link Control Working Group
802.5 Token Ring Working Group
Disbanded groups
802.4 Token Bus Working Group
802.6 Metropolitan Area Network Working Group
802.7 Broadband TAG
802.8 Fiber Optic TAG
802.9 Integrated Services LAN Working Group
802.10 Security Working Group
802.12 Demand Priority Working Group
802.14 Cable Modem Working Group
QOS/FC Executive Committee Study Group
The remainder of this section lists some of the standards that have been
developed by the IEEE working groups that deal with subject matter common
in most LANs and MANs.51 These working groups are IEEE 802.1, IEEE 802.3,
IEEE 802.5, and IEEE 802.11.

1.3.7.2

IEEE 802.1

IEEE 802.1 is responsible for the development of numerous standards, as well as
providing recommendations for the following areas: 802 LAN architecture, 802

50 ‘‘Inactive’’

does not mean the technology is not out there; it just means there are no updates
being worked on at this time.
51 These are also the main working groups within the IEEE 802 family that sets standards for the
material covered in this book.

39

Page 39

Edwards

40

Part I

■

c01.tex

V3 - 03/27/2009

Networking Nuts and Bolts

MAN architecture, 802 WAN architecture, 802 overall network management,
protocol layers above the MAC and LLC sublayers (see Section 1.4), and 802
Security. Following is a list of IEEE 802.1 standards:
IEEE 802.1AB — This standard defines how to use the Link Layer
Discovery Protocol (LLDP) as well as identifying node access points for
network and device management.
IEEE 802.1AD — This standard sets the rules used by service providers
to use bridges, so they can basically provide the equivalent of a separate
catenet to their customers.
IEEE 802.1AE — This standard defines the MAC security guidelines for
the purpose of data security.
IEEE 802.1B — This standard defines the rules for remote management
of IEEE 802 LANs.52
IEEE 802.1D — Of all the 802.1 standards, this is the one that is the
most well known. It is also the most used standard and outlines the
rules followed by LAN bridges and switches.
IEEE 802.1E — This standard outlines the rules for using multicast to
reliably transfer large amounts of data to multiple network nodes.
IEEE 802.1F53 — This standard outlines some common definitions used
for system management information common through the series of IEEE
802 standards.
IEEE 802.1G — This standard outlines the rules that allow bridges in
LANs to communication using WAN technology.
IEEE 802.1H — This is more of a recommendation than a standard.
It provides a way for end stations and bridges in an Ethernet LAN
to communicate with end stations and bridges in other LANs that use a
non-native encapsulation type.
IEEE 802.1Q — This standard outlines the requirements and rules for
nodes operating in an virtual LAN (VLAN). Like the 802.1D standard,
this is one of the more widely used and implemented 802.1 standards.
IEEE 802.1X — This standard outlines the rules that allow a way of
authenticating devices attached to a LAN port at the Data Link layer (see
Section 1.4).
52 The

Simple Network Management Protocol (SNMP) is the de facto standard, used by pretty
much everyone. Because of this, the IEEE 802.1B standard is not used very often.
53 SNMP has pretty much taken over. 802.1F has joined 802.1B on the not used often list.

10:41am

Page 40

Edwards

Chapter 1

1.3.7.3

c01.tex

■

V3 - 03/27/2009

10:41am

Introduction to Networking

IEEE 802.3

IEEE 802.3 is the standard for Ethernet-based LANs. It defines the rules for
the Media Access Control (MAC) sublayer and the Physical sublayer of the
Data Link layer (Layer 2 of the OSI reference model, which is discussed in
Section 1.4) in an Ethernet LAN. IEEE 802.3 is one document maintained by
the IEEE 802.3 working group — the IEEE 802.3 standard. Supplements to the
standards are identified by letter designations at the end (for instance, 802.3a,
802.3c, etc.). The following is a list of some of the supplements that have been
part of the 802.3 standard:
IEEE 802.3a — Thin coaxial cable, 10BASE2
IEEE 802.3c — Specifications for repeaters
IEEE 802.3d — Fiber optic inter-repeater link
IEEE 802.3i — UTP cable, 10BASE-T
IEEE 802.3j — Fiber optic LAN, 10BASE-F
IEEE 802.3u — Fast Ethernet, 100BASE-T
IEEE 802.3x — Full duplex operation and flow control
IEEE 802.3z — Gigabit Ethernet over optical fiber
IEEE 802.3ab — Gigabit Ethernet over UTP cable, 1000BASE-T
IEEE 802.3ac — Frame extensions for VLAN-tagging
IEEE 802.3ad — Link aggregation
IEEE 802.3ae — 10 Gbit/s Ethernet over fiber
IEEE 802.3af — Power over Ethernet
IEEE 802.3ah — Ethernet in the First Mile
IEEE 802.3ak — Ethernet over Twinaxial
IEEE 802.3an — 10GBASE-T
IEEE 802.3ap — Backplane Ethernet
IEEE 802.3aq — 10GBASE-LRM
IEEE 802.3as — Frame expansion

1.3.7.4

IEEE 802.5

IEEE 802.5 is the standard for Token Ring–based LANs. I t defines the rules
for the Media Access Control (MAC) sublayer and the physical sublayer of
the Data Link layer (Layer 2 of the OSI reference model, which is discussed

41

Page 41

Edwards c01.tex

42

Part I

■

V3 - 03/27/2009

Networking Nuts and Bolts

in Section 1.4) in an Token Ring LAN. IEEE 802.5 is one document that was
maintained by the IEEE 802.5 working group (now inactive) — the IEEE 802.5
standard. Supplements to the standards are identified by letter designations
at the end (for instance, 802.5c, 802.5j, etc.). The following is a list of some of
the supplements that have been part of the 802.5 standard:
IEEE 802.5c — Dual-ring redundant configuration
IEEE 802.5j — Optical fiber media
IEEE 802.5r — Dedicated Token Ring/full duplex operation
IEEE 802.5t — 100 Mb/s High Speed Token Ring
IEEE 802.5v — Gigabit Token Ring

1.3.7.5

IEEE 802.11

IEEE 802.11 is the standard for wireless LAN technology. All the supplements
to 802.11 follow the basic protocol, with the difference being the frequency,
speed, and distance supported. The original 802.11 standard supported an
operating frequency of 2.4 Ghz.54 The maximum supported data rate is 2
Mbit/s, with an indoor range of 20 meters and an outdoor range of 100
meters.55
IEEE 802.11a — The 802.11a standard supports an operating frequency
of 5 GHz. The maximum data rate for 802.11a is 54 Mbit/s and the average data rate is approximately 23 Mbit/s. 802.11a reaches a maximum
indoor range of 35 meters and an outdoor range of 120 meters.
IEEE 802.11b — The 802.11b standard supports an operating frequency
of 2.4 GHz. The maximum data rate for 802.11b is 11 Mbit/s. 802.11b
reaches a maximum indoor range of 38 meters and an outdoor range of
140 meters.
IEEE 802.11g — The 802.11g standard supports an operating frequency
of 2.4 GHz. The maximum data rate for 802.11g is 54 Mbit/s. 802.11g
reaches a maximum indoor range of 38 meters and an outdoor range of
140 meters.
IEEE 802.11n — The 802.11n standard supports an operating frequency
of 2.4GHz and 5 GHz. The maximum data rate for 802.11n is 248 Mbit/s.
802.11n reaches a maximum indoor range of 70 meters and an outdoor
range of 250 meters.
54 In

this section, operating frequencies are listed in accordance with the industrial, scientific, and
medical (ISM) radio bands.
55 Any guesses on why the outdoor range is higher? Two words: NO WALLS.

10:41am

Page 42

Edwards

Chapter 1

c01.tex

■

V3 - 03/27/2009

10:41am

Introduction to Networking

IEEE 802.11y — The 802.11y standard supports an operating frequency of 3.7 GHz. The maximum data rate for
802.11y is 54 Mbit/s. 802.11y reaches a maximum indoor
range of 50 meters and an outdoor range of 5000 meters.

1.3.8

Internet Society (ISOC)

The Internet Society (ISOC) was formed in 1992 as an organization dedicated
to structuring the development process of Internet standards. ISOC maintains
a global focus, striving to ensure that the ongoing development and growth of
the Internet provides benefits to users all over the world.
ISOC has more than 27,000 members split into groups and chapters throughout the world. The main offices are in Washington, D.C., and Geneva,
Switzerland. ISOC has several organizations that assist in its purpose, including the Internet Architecture Board (IAB), the Internet Research Task Force
(IRTF), and others. There are three main goals that ISOC works to achieve.
They support the Internet Engineering Task Force (IETF) in standards development. They also work with organizations, institutions, and other groups
to form public policy to promote global equality for all global users of the
Internet. Finally, ISOC is dedicated to technical education by providing training, educational grants for experts in the field in developing countries, and
conferences pertaining to issues that affect the Internet.
More information can be found on the ISOC website: www.isoc.org.

1.3.9

Internet Engineering Task Force

The Internet Engineering Task
Force (IETF) develops and maintains the standards pertaining
RANDOM BONUS DEFINITION
to the TCP/IP protocol suite.
IP address — An address assigned to
Membership is open to anynetwork nodes in order to transmit data at
one, and the committees are
the Network layer.
composed solely of volunteers
(although sometimes employers and sponsors may fund
research). The IETF is a task force within ISOC.
The IETF has both working groups and birds of a feather (BoF) discussion
groups. Regardless of the group type, each has a charter that explains the goals
of the group. Decisions are determined by an open consensus, rather than a
vote. Once a BoF or working group completes its goals, the group dissolves56
56 Some

working groups have it written into their charter that the working group can continue to
take on new tasks that pertain to the working group.

43

Page 43

Edwards

44

Part I

■

c01.tex

V3 - 03/27/2009

Networking Nuts and Bolts

and the members usually go on to other tasks. Following are some important
terms that pertain to the standards process within the IETF:
Internet Architecture Board (IAB) — The IAB is a committee within the
IETF. It is responsible for defining and managing the rules for the Internet’s architecture. As an IETF committee, the IAB provides oversight
and direction to the IETF and is an advisory group for the ISOC.
Internet Assigned Numbers Authority (IANA) — The IANA is
responsible for three very important Internet technical functions.
The first function is the assignment of protocol name and number
registers for many Internet protocols. The second function is maintaining the top-level domain names (a.k.a. the DNS root), the .int
domain, the .ARPA domain, as well as maintaining the Internationalized Domain Name (IDN) registry. The third service provided
by the IANA is the coordination of IP addresses and Autonomous
System (AS) numbering used for routing data on the Internet.
Internet Engineering Steering Group (IESG) — The IESG manages
the activities of the IETF and is also responsible for reviewing and monitoring Internet standards development and, ultimately, the approval of
the standards.
Internet-Drafts — Internet-Drafts are documents that are being worked
on by the IETF or one of its working groups, BoFs, members, etc.
Internet-Drafts are not approved standards and should not be treated
as such. An Internet-Draft must have some revision or edit every six
months, or it must be either removed or transformed into an approved
standard. An Internet-Draft is also referred to as a draft standard (DS).
Request for Comments (RFCs) — RFCs are documents that provide
new technology information, updates to standards, better ways of doing
things, R and D, and other miscellaneous information57 dealing with network technologies. The IETF reviews RFCs and takes up some of ideas
and proposals in the RFCs as an Internet standard. Some people confuse RFCs with Internet standards, but they are not the same thing. If
the IETF decides to adopt an RFC for consideration to be a standard,
it starts the RFC on a standards track. Initially, the RFC will be a proposed standard (PS). If the RFC makes it past the approval process, it
then becomes a draft standard (DS). Finally, if the RFC gets approval
through the draft process, it becomes an Internet standard (STD).
57 You

can even find some funny RFCs, such as RFC 1438, ‘‘Internet Engineering Task Force
Statements Of Boredom (SOBs), or RFC 1097, ‘‘TELNET Subliminal-Message Option.’’ There are
quite a few out there; see how many you can find. Read a couple and then write to Jim or Rich
and tell them which one is your favorite. Or better yet, write your own and submit it. See if it
gets published.

10:41am

Page 44

Edwards

Chapter 1

c01.tex

■

V3 - 03/27/2009

10:41am

Introduction to Networking

Interested in reading more? You can get more information about the IETF
on the IETF website (www.ietf.org).

1.4

An Introduction to the OSI Reference Model

In 1977, ANSI began work on what eventually became known as the OSI reference model.58 A working group was formed, and the proposal was submitted
to the ISO to begin working on a networking suite to develop a layer model
for network architecture in an attempt to standardize. ISO and the International Telecommunication Union –Telecommunication Standardization Sector
(ITU-T) participated in a joint effort to standardize networking. The joint effort
became known as the Open Systems Interconnection (OSI). OSI was an effort
to establish some commonality among communication protocols. Through the
efforts of the OSI, the OSI protocol suite and the OSI reference model were
born.
Since its inception, the OSI
reference model has been the
model that most networking
RANDOM BONUS DEFINITION
professionals first learn about.59
MAC address — The physical (hardware or
It still remains an excellent moadaptor) address that identifies a network
del to learn networking archinode
tecture from. It’s important to
note that the reference model is
only a guide and not the rules
for networking. It serves as a tool for vendors to follow if they want their
product to be available for use in multivendor environments. It is important
to note that many of the protocols on the market today are modeled after the
TCP/IP reference model (see Section 1.6), and may not fit into any particular
layer of the OSI reference model.
The OSI reference model is a standard reference model for data communication between network nodes. From a user’s perspective, it is used as a
reference to define and understand a network. From a vendor’s perspective, it
is used when developing a product that you expect to be able to operate with
products from other vendors.
The OSI reference model divides data communication into seven layers, as
shown in Figure 1-12. The lower three layers are used to pass data between
58 The

OSI reference model is also known as the OSI Basic Reference Model, the seven-layer
model, and the OSI model. For the purposes of standardization, we will refer to this as the
OSI reference model throughout this book. This does not infer that the other names are not
appropriate, only that it is preferred by the authors.
59 The OSI reference model has been largely superseded by publications that have been developed
since it first came out.

45

Page 45

Edwards

46

Part I

■

c01.tex

V3 - 03/27/2009

Networking Nuts and Bolts

network nodes, whereas the upper four layers are used when user data is
passed between end users.
Layer 7

Application

Layer 6

Presentation

Layer 5

Session

Layer 4

Transport

Layer 3

Network

Layer 2

Data Link

Layer 1

Physical

Figure 1-12 The OSI reference model

1.4.1 All People Seem to Need Data Processing—
A Mnemonic Device
You might think that this is silly, but no self-respecting self-teaching guide
would hold back from sharing information that might be of a benefit to the
reader. You need to know the layers of the reference model and what each
layer does. It will not only make you sound like you know what you’re doing,
it will also help you understand what others are talking about. It is also about
an 80 percent certainty that you are going to be asked to name the layers, so
here is a quick tip on how you can remember them. Simply take the first letter
of each name in the model, in order, and replace it with a word that fits into a
sentence. For instance:
Application–Presentation–Session–Transport–Network–
Data link–Physical
becomes
All–People–Seem–To–Need–Data–Processing
You can also do this in reverse order:
Physical–Data link–Network–Transport–Session–Presentation–
Application

10:41am

Page 46

Edwards

Chapter 1

c01.tex

■

V3 - 03/27/2009

10:41am

Introduction to Networking

becomes
Please–Do–Not–Throw–Sausage–Pizza–Away60
Figure 1-13 has an example of these two
mnemonic devices, set next to the layers
in the OSI model. Many other mnemonic
devices have been made up for the purposes of memorizing the layers, and you’re
certainly welcome to create your own. Hey,
if it works, don’t knock it!
All

Application

Away

People

Presentation

Pizza

Seem

Session

Sausage

To

Transport

Throw

Need

Network

Not

Data

Data Link

Do

Processing

Physical

Please

ACRONYM ALERT
OSPF — Open Shortest Path First

Figure 1-13 Using a mnemonic device as a memory aid

1.4.2

A Layered Approach

The OSI reference model is a systematic approach to outlining the services
of protocols that define network architecture. Each layer within the model
works with the layers above and/or below them to serve a data transmission
purpose. In most networks, the theory of the OSI model may not represent the
entire network, and that is why it is a reference model, not a required set of
rules.
The OSI reference model breaks down the services within a network into
seven layers. Each layer represents protocols that perform a certain purpose
or method for allowing data communication within the network. Data is
transmitted from a user on the network to another user. It is an application
that begins and ends the network connection process. As shown in Figure 1-14,
60

Jim actually once interviewed an individual who when asked to name the layers of the OSI
model actually said, ‘‘Please do not throw sausage pizza away’’ out loud to remember the layer
names. His intention wasn’t to say it out loud, but he did. He also ended up getting the job.

47

Page 47

Edwards

48

Part I

■

c01.tex

V3 - 03/27/2009

Networking Nuts and Bolts

data flows from Layer 7 to Layer 1, is transmitted to the destination, where
it travels up the layers to the end user. So what exactly is going on in these
layers? Let’s talk about that for a while.

Computer

Computer

Application

Application

Presentation

Presentation

Session

Session

Transport

Transport

Network

Network

Data Link

Data Link

Physical

Physical

Figure 1-14 A complete, end-to-end network connection

1.4.2.1

Layer 7 — The Application Layer

The name application might confuse you at first. The Application layer contains
the operating systems that enable application programs to interface with the
network. This layer serves application processes that the network uses, but not
the applications that interface with the user. Let’s look at a couple of examples.
Example 1: Sending an e-mail — The Application layer defines
the protocols used in an e-mail transmission, but not the interface
that the end user has to initiate in order to send the e-mail.
Example 2: Initiating an FTP session — The Application layer defines
the protocol used for a file transfer, but the end user has to initiate
an interface with an FTP application to perform the file transfer.
Keep in mind that the OSI reference model is for the architecture of networks
and network nodes. Therefore, the Presentation layer does not define end users
and the interfaces they have with a PC (and the applications running on the

10:41am

Page 48

Edwards

Chapter 1

c01.tex

■

V3 - 03/27/2009

10:41am

Introduction to Networking

PC). Not only does the Application layer serve the applications process, it also
sends service requests to the Presentation layer. Examples of some common,
and a few uncommon, Application layer protocols and services include:
Association Control Service Element (ACSE)
Common Management Information Protocol (CMIP)
Common Management Information Service (CMIS)
CMIP over TCP/IP (CMOT)
Dynamic Host Configuration Protocol (DHCP)
File Transfer Access and Management (FTAM)
File Transfer Protocol (FTP)
Hypertext Transfer Protocol (HTTP)
Internet Relay Chat (IRC)
Network File System (NFS)
Post Office Protocol 3 (POP3)
Remote Operation Service Element (ROSE)
Reliable Transfer Service Element (RTSE)
Simple Mail Transfer Protocol (SMTP)
Simple Network Management Protocol (SNMP)
Telecommunications Network (Telnet)
Virtual Terminal Protocol (VSP)
X.400 –Message Handling Service Protocols
X.500 –Directory Access Service Protocol (DAP)

1.4.2.2

Layer 6 — The Presentation Layer

The Presentation layer responds to service requests from the Application layer,
and sends service requests to the Session layer. The Presentation layer also is
responsible for accepting data from the lower layers and then presenting the
data to the Application layer, and, ultimately, to the destination. The following
functions operate at the Presentation layer:
Encryption services
Decryption services
Data compression services
Data decompression services
Translation services

49

Page 49

Edwards

50

Part I

■

c01.tex

V3 - 03/27/2009

Networking Nuts and Bolts

The Presentation layer takes care of translating data from lower layers so the
data is understood at the Application layer. This saves the Application layer
the headache of having to translate the data itself. The translation also occurs
at the Presentation layer when data is being passed down the stack from the
Application layer. Note that the Presentation layer is not always needed61 and
that the Application layer may actually work with the Session layer and keep
the Presentation layer out of the loop. Here are some examples of the data
formats that are defined at the Presentation layer:
American Standard Code for Information Interchange (ASCII)
Binary
Extended Binary Coded Decimal Interchange Code (EBCDIC)
Joint Photographic Experts Group (JPEG)
Musical Instrument Digital Interface (MIDI)

1.4.2.3

Layer 5 — The Session Layer

The Session layer is responsible for setting up communication between nodes.
The Session layer responds to service requests from the Presentation layer62 as
well as sending service requests to the Transport layer. The Session layer may
also provide access control services, authentication, data synchronization, and
other services.
The Session layer establishes a communication session, manages the session,
and then terminates the session between endpoints. The Session layer is able to
gather data streams that are coming from multiple originators and can ensure
that the data is synchronized correctly for the destination.63
Here are some examples of the data formats defined at the Session layer:
Network Basic Input/Output System (NetBIOS)
Network File System (NFS)
Secure Shell (SSH)
Structured Query Language (SQL)

1.4.2.4

Layer 4 — The Transport Layer

The Transport layer takes care of getting data from endpoint to endpoint. As
long as there is an open communications path, the Transport layer can do its
job. The Transport layer receives requests from the Session layer and sends
61

This is due to the fact that encryption/decryption and compression/decompression are not
always used.
62
As mentioned previously, the session layer can also respond to the application layer if the
presentation layer is not necessary for a session.
63 Imagine how much fun we would all have if the destination had to just figure it out on its own.

10:41am

Page 50

Edwards c01.tex

Chapter 1

■

V3 - 03/27/2009

10:41am

Introduction to Networking

requests on to the Network layer. The Transport layer ensures end-to-end
delivery of data, allowing communication to occur between various endpoint
nodes within a network.
The Transport layer utilizes various standards to ensure that data arrives in
the right order and that its integrity is maintained. To do this, several functions
occur at the Transport layer, including:
Ensuring that a connection is established
Disassembling and then reassembling large data streams
Flow control
Error recovery
Data sequencing
The Transport layer is similar to a delivery service, such as the U.S. Postal
Service, UPS, or Fed-Ex. They sort, separate, and distribute packages, and have
different priorities and classifications. Without caring what is in the package,
they get the package where it is supposed to go.64
Some examples of Transport layer protocols include:
AppleTalk Transaction Protocol (ATP)
Transmission Control Protocol (TCP)
User Datagram Protocol (UDP)
Sequenced Packet Exchange (SPX)

1.4.2.5

Layer 3 — The Network Layer

The Network layer is responsible for exchanging data between nodes across
several data paths. The Network layer uses nodes called routers to route
packets from endpoint to endpoint. The Network layer allows the packet to
pass through various network topologies, choosing from multiple paths until
it reaches its destination.
The Network layer is able to transfer variable amounts of data between
endpoints over one or more networks. The Network layer breaks data into
smaller packets and then reassembles the data once it arrives at its destination.
The Network layer is also responsible for identifying when an error in data
transmission occurs.
IP is the most well-known and widely used Network layer protocol. Remember, IP is connectionless and is not required to regulate and ensure reliable
data delivery. It does, however, identify errors in transmission, ensuring that
bad packets are dropped. Also, it is IP that fragments data into packets that
the next node on the network can support.
64

Hopefully in the condition it is expected to arrive in.

51

Page 51

Edwards

52

Part I

■

c01.tex

V3 - 03/27/2009

Networking Nuts and Bolts

Some examples of Network layer protocols include:
Internet Protocol (IP)
Internetwork Packet Exchange protocol (IPX)
Routing Information Protocol (RIP)
Internet Control Message Protocol (ICMP)
Address Resolution Protocol (ARP)
Reverse Address Resolution Protocol (RARP)
Open Shortest Path First (OSPF)
Internet Group Management Protocol (IGMP)

1.4.2.6

Layer 2 — The Data Link Layer

For the most part, LAN communication is handled at the Data Link layer and
the Physical layer. At the Data Link layer, network nodes known as switches
or bridges pass frames between nodes in the LAN. Data communication at the
Data Link layer can be between two nodes (point-to-point) or between a single
endpoint node to many endpoint nodes (point-to-multipoint).
The Data Link layer ensures
data delivery between nodes,
using the physical addresses
RANDOM BONUS DEFINITION
of the nodes. It is important
multiplexing — The act of combining
that considerations are made
multiple data streams into a single signal
for the physical topology of the
and then transmitting the data over a
network segment for the data
shared medium. Also known as muxing.
link traffic. The Data Link layer
provides for data flow control,
which is used to prevent a node
from receiving more data than it can handle at any particular time. The Data
Link layer also provides for error notification to the upper layers when a data
transmission error occurs.
Some examples of Data Link layer protocols include:
High-level Data Link Control (HDLC)
Serial Line Internet Protocol (SLIP)
Point-to-Point Protocol (PPP)
The IEEE divides the Data Link layer into two sublayers: the Logical Link
Control (LLC) sublayer and the Media Access Control (MAC) sublayer. The
LLC sublayer is referred to as the upper sublayer of the Data Link layer, whereas
the MAC sublayer is the lower sublayer. The LLC sublayer multiplexes and

10:41am

Page 52

Edwards

Chapter 1

c01.tex

■

V3 - 03/27/2009

10:41am

Introduction to Networking

demultiplexes data transmitted over the MAC sublayer. The IEEE standard
that encompasses the LLC sublayer is IEEE 802.2. The MAC sublayer acts as an
interface between the LLC sublayer and the Physical layer. The MAC sublayer
makes it possible for network nodes to communication within a multipoint
network (such as a LAN or a MAN), by providing address and access control
services.

1.4.2.7

Layer 1 — The Physical Layer

The Physical layer serves the Data Link layer. The Physical layer provides a
way for the data to be transmitted in a network. Data is converted into a signal
which is passed to an endpoint over a physical connection. The Physical layer
is responsible for the procedures, mechanics, and the electricity required for
operating.
Examples of network nodes that are Physical layer nodes include network
adaptors (NIC cards), network hubs, and modems.

1.5 TCP/IP, Please (and Don’t Be Stingy
with the IP)
TCP/IP is the main protocol used by the Internet and most other network
types. If you are a node that connects directly to the Internet, then you will use
the TCP/IP protocol to communicate with other nodes. Earlier you learned
that TCP and IP are two separate protocols that work with one another.
TCP handles breaking down data into small packages, known as packets, and
then puts the data back together when the data arrives at its destination. IP
knows how to get the data there. In this section, we introduce TCP/IP. In
Chapter 2, ‘‘The TCP/IP Protocol Suite,’’ we will discuss it more in depth.
This introduction is required, however, because you will need to have a basic
understanding for some of the material covered in Chapters 2 through 4.
A network is simply nodes
that are connected to one
another to pass data. For data
POP QUIZ
to arrive intact and at the right
What is ARPANET? (Note: If you don’t
destination, you must have the
know the answer to this one, go back and
protocols that can make sure this
reread Section 1.2. The next paragraph is
happens. This combination of
where that information starts to come in
protocols is the TCP/IP protohandy.)
col suite. TCP/IP was brought
about to standardize communications protocols, as there were
a lot of proprietary protocols when networking was in its infancy.

53

Page 53

Edwards c01.tex

54

Part I

■

V3 - 03/27/2009

Networking Nuts and Bolts

If you are reading this, that
POP QUIZ
means you remember what
ARPANET was. This is imporName the four IMPs that made up the
tant, because you probably
original ARPANET.
remember when those supercomputers from different geographical areas first talked to
each other. Well, the ARPANET protocols that made that happen are what is
now known as TCP/IP. The name TCP/IP somewhat implies that these two
protocols are what makes TCP/IP what it is. Actually, TCP/IP is a collection of
several protocols that work with one another to accomplish data transmission.
TCP/IP has its own reference model (see Section 1.5.3) that basically follows
the OSI reference model. The protocols that make up TCP/IP use the TCP/IP
reference model to map out where they are to function.
Over the years, other protocols have been used to provide upper-layer
functionality to transmit data. There are still a few of these out there, but most
people support and utilize the TCP/IP protocol. Why use TCP/IP? The answer
is simple: because everyone uses TCP/IP. Besides the fact that everyone uses
it in some fashion or another, there are several other reasons why TCP/IP has
grown into the ‘‘method of choice.’’ Some of these are:
Routing — TCP/IP was designed to route data from node to node of
networks of variable sizes and complexities. TCP/IP is not worried
about the status of nodes in the network; it is concerned about the
networks that it should know about. Various protocols within the
TCP/IP protocol suite manage data flow between networks.
Addressing — And guess what is built into TCP/IP? That’s right, IP.
IP provides a way for a node to identify other nodes within a network
and deliver data to any endpoint node it has been made aware of.
Name resolution — TCP/IP provides a way to map an IP address
(10.10.10.10) to an actual name (networkz.org). Can you imagine
how tough it would be to remember the IP addresses of all the
websites you needed to know about? Name resolution really helps.
Doesn’t discount the lower layers — Although TCP/IP operates
at the upper layers (Layer 3 and above), it does have the ability to
operate at the lower levels as well. This means that for most LANs and
WLANs, and some MANs and WANs, TCP/IP is able to work with
multiple networks of these types and connect them to each other.
Open standards — TCP/IP was mainstreamed to enable different
nodes to communicate with one another. The open standards that
TCP/IP contains are available to anyone. These standards are
determined through the RFC process discussed in Section 1.3.9.

10:41am

Page 54

Edwards

Chapter 1

c01.tex

■

V3 - 03/27/2009

10:41am

Introduction to Networking

Talking endpoint to endpoint — TCP/IP provides a way for
one endpoint to speak directly with another endpoint, regardless
of any nodes that are in between. It is as if the endpoints were
directly connected to one another, even when they are not physically connected to the same local network. Thanks to TCP/IP,
both the originating and the destination nodes can exchange
connection acknowledgements directly with one another.
Application support — TCP/IP provides protocols that provide a commonality among end user applications. Often when an application that
utilizes TCP/IP is developed, many of the functions required for the
application are already common with any node supporting TCP/IP.
There are some basic Network layer services provided by any network.
All user applications that utilize TCP/IP rely on these standard services to
assist in data transport. The first of these standards is that TCP/IP supports
connectionless datagram delivery. The TCP/IP network is able to route data
from node to node based on the address of the source and destination nodes,
but is not concerned about the order in which the data is sent. Having
connectionless datagram delivery gives TCP/IP the flexibility to support a
wide range of hardware through the network. The other basic service that is
used by TCP/IP applications is a reliable transport service. Endpoints establish
a connection prior to exchanging data. This allows a temporary connection
to appear, from a user’s perspective, as a direct connection. The connection
remains while the endpoints exchange data (regardless of the amount of data
that is transported).

1.5.1

TCP/IP Applications

End users are able to navigate networks by using applications based on the
TCP/IP protocol suite. They are able to do so without having any understanding of exactly what it takes to get information shared with destination
nodes. The only details the average user needs to know is how the actual
interface works. Users rely on the software and technology to get the data to
an endpoint.
Numerous TCP/IP-based applications are in deployment within networks
worldwide. The following list contains some of the more popular applications
that are widely used today:
Electronic mail (e-mail)
File transfer
IP address allocation
Remote login
Web browser

55

Page 55

Edwards

56

Part I

■

1.5.2

c01.tex

V3 - 03/27/2009

Networking Nuts and Bolts

TCP/IP Utilities

In addition to application support, TCP/IP also provides some helpful utilities
that are available in any node that supports TCP/IP. These utilities provide a
variety of information that can be used to help maintain the network. These
utilities will be discussed in detail throughout the book. It is important to
be aware of these, and no good networking introduction would be complete
without a summary of the utilities and the purpose they serve. There are three
main categories of TCP/IP utilities:
Diagnostic utilities — These utilities assist in troubleshooting issues
within the network.
General purpose utilities — These utilities are used to connect
to other TCP/IP nodes to perform a specific action, to exchange
data, or to allow remote management and related services.
Services utilities — These utilities are software applications
that are offered by a TCP/IP-based server to TCP/IP clients.
Table 1-1 contains a list of some commonly used TCP/IP utilities.
Table 1-1 TCP/IP utilities
DIAGNOSTIC UTILITIES

GENERAL PURPOSE
UTILITIES

SERVICES
UTILITIES

Address Resolution Protocol
(ARP)

File Transfer Protocol
(FTP)

TCP/IP print server

IPConfig

Line Printer Daemon
(LPD)

Web server

Line Printer Daemon (LPD)

Remote Copy Protocol
(RCP)

File Transfer Protocol
server

netstat

Remote Shell (RSH)

E-mail server

nslookup

Telnet

ping

Trivial File Transfer
Protocol (TFTP)

route
tracert (Windows)
Traceroute (other operating
systems, such as Linux, Unix,
and others)

10:41am

Page 56

Edwards c01.tex

Chapter 1

1.5.3

■

V3 - 03/27/2009

10:41am

Introduction to Networking

The TCP/IP Reference Model

The TCP/IP reference model,
the specification established by
POP QUIZ
DARPA65 to set the rules for
ARPANET (and now maintainWhat is the Post Office Protocol?
ed by the IETF), was developed
long before the OSI reference
model. Rather than the seven-layer OSI reference model, the TCP/IP reference
model has only five66 layers, as shown in Figure 1-15.

Layer 5

Application

Layer 4

Transport

Layer 3

Network

Layer 2

Data Link

Layer 1

Physical

Figure 1-15 The TCP/IP reference model

An important thing to note is that the TCP/IP reference model, although
represented in layers, does not really operate in a layered manner as the OSI
reference model does. There is not a lot of agreement where the layers really
fall, though you will often hear about the upper and lower layers in the TCP/IP
reference model. The main point is that regardless of whether you follow the
OSI reference model or the TCP/IP reference model, the functionality of the
network is, for the most part, the same.
As mentioned previously, Chapter 2 discusses the TCP/IP reference model
in depth. For the purposes of this introductory chapter, it is important to have
only an introduction to the model. The TCP/IP reference model layers are:
Application layer (Layer 5) — The Application layer in the
TCP/IP reference model assumes most of the functions performed by the Session and Presentation layers of the OSI reference
model. All upper-layer protocols are handled at this layer.
65 At

least we think it was DARPA . . . or was it ARPA? Okay, enough funning around — it was
DARPA at the time.
66 A lot of people don’t consider the physical layer to be part of the TCP/IP reference model. For
the purposes of this book, we have decided to include the physical layer. We don’t want you to
be confused in the future when someone mentions the four-layer TCP/IP model.

57

Page 57

Edwards c01.tex

58

Part I

■

V3 - 03/27/2009

Networking Nuts and Bolts

Transport layer (Layer 4) — The Transport layer functions the same
in both reference models. The two major protocols that operate
at this layer are TCP and UDP. TCP is a connection-oriented protocol and therefore provides reliable delivery. UDP, on the other
hand, is connectionless and provides unreliable data delivery.
Network layer or Internet layer (Layer 3) — This layer performs the
same functions as Layer 3 of the OSI reference model. The network layer
is responsible for routing a packet from a source to a destination. It can
do this within a LAN as well as over multiple LANs, MANs, and WANs.
Data Link layer (Layer 2) — This layer is often combined with
the Physical layer and is referred to as the host to Network layer.
The TCP/IP reference model largely ignores these lower layers.
All it cares about it that there is a connection to pass data on.
Physical layer (Layer 1) — This layer is often combined with the Data
Link layer and is largely ignored as well, although it does provide the
connections to get data passed to a destination. Make no mistake, however: If the Physical layer isn’t working, you will miss it real quick. It’s
like that old saying, ‘‘You don’t know what you’ve got until it’s gone.’’

1.6

Chapter Exercises

1. The network used exclusively by the University of Texas is an example
area network.
of a
2. What are the names of the layers in the OSI reference model?
Layer 7
Layer 6
Layer 5
Layer 4
Layer 3
Layer 2
Layer 1
3. List at least five applications and/or utilities that use TCP/IP.

10:41am

Page 58

Edwards c01.tex

Chapter 1

■

V3 - 03/27/2009

10:41am

Introduction to Networking

4. What are the two types of network relationships?

5. Explain the difference between a client/server network relationship
and a client/server database system.

6. What is the 1822 protocol?

7. What are the three types of standards? Do a search on the Internet
to see if you can find at least one of each standard type.

8. The 802.11n standard supports an operating frequency of
and
. The maximum data rate for
802.11n is
. 802.11n reaches a maximum indoor
and an outdoor range of 250 meters.
range of 7
9. T or F: The application layer of the OSI model concerns itself
with the application/user interface on a PC.
10. In this chapter, we listed seven reasons why TCP/IP has grown
to be the method of choice. What are these seven reasons?

59

Page 59

Edwards c01.tex

60

Part I

1.7

■

V3 - 03/27/2009

Networking Nuts and Bolts

Pop Quiz Answers

1. What is a public key certificate?
Public key certificates are electronic documents that can verify and
authorize an individual by public key cryptography. In public key
cryptography, two keys (one public key and one private key) are used
to encrypt and then decrypt data to ensure that a message can be transported securely.
2. Encapsulated data that is transmitted and received at the network layer
is called a packet.
3. What is the difference between a physical port and a TCP port?
A physical port is an interface that resides on a network node. A TCP/IP
port is a number that is in the data packet header that maps to a process
running on a node.
4. Because IP does not establish a connection before sending data to
an endpoint, it would be considered a connectionless protocol.
5. What is the difference between a WAN and a LAN?
The main difference between a LAN and a WAN is the size of
the geographical area that is covered. A LAN covers a small
geographical area whereas a WAN covers a large geographical area.
6. The three types of standards are called a de facto standard, a proprietary
standard, and an open standard.
7. What is ARPANET?
ARPANET stands for the Advanced Research Projects Agency Network
and was the first packet-switching network ever. The Internet was developed from the ARPANET.
8. Name the four IMPs that made up the original ARPANET.
Stanford Research Institute
University of California, Los Angeles

10:41am

Page 60

Edwards

Chapter 1

c01.tex

■

V3 - 03/27/2009

10:41am

Introduction to Networking

University of California, Santa Barbara
University of Utah
9. What is the Post Office Protocol?
Post Office Protocol (POP) is a protocol that allows an e-mail client to
connect to an e-mail server and retrieve mail that is destined for that
client.

61

Page 61

Edwards

c01.tex

V3 - 03/27/2009

10:41am

Page 62

Edwards

c02.tex

V3 - 03/27/2009

11:58am

CHAPTER

2
LANs, MANs, and WANs
This is my LAN; that is your LAN; we are joined at the MAN, but I am also connected
to a WAN . . . from sea to shining sea.
— The authors

Digital data communications has changed rapidly and continues to evolve due
to the demand of many types of ‘‘data consumers.’’ High-speed data communications is no longer the preferred network of only large companies; everyday
consumers use these networks for various forms of communication — voice,
text, video, and teleconferencing. The past decade has seen a convergence of a
wide range of services utilizing the public network simply referred to as the
Internet.
The term Internet covers a wide range of network devices and services offered
by a wide range of companies commonly referred to as the telecommunications
industry. This chapter discusses local area networks (LANs), metropolitan
area networks (MANs), and wide area networks (WANs). The topics will
be discussed in this order, but it is not meant to imply that this was the
evolutionary process in networking technology. In reality, it is perhaps more
like WANs, LANs, and then MANs. However, there have been areas of overlap
where the evolution of all three occurred simultaneously.
The quote above is trying to give a sense of the relationship between LAN,
MAN, and WAN. Some LAN networks are a personal thing, like my LAN
at home. It is mine, all mine, and not to be shared with others.1 Strategically
speaking, a LAN is owned by a person or small group, but it is fairly local
1 Rich gets

a little over-possessive at times. He is a giving soul and does go out of his way to share
with others, but his LAN is his LAN.

63

Page 63

Edwards

64

Part I

■

c02.tex

V3 - 03/27/2009

Networking Nuts and Bolts

geographically no matter how many network nodes it may have. MANs may
comprise many LAN networks spread about a geographical region whereas
WANs can be global. However, the purpose of the MAN or WAN is so that
users on LANs, no matter where they may be located geographically, can
communicate with each other in the sharing of data and network resources.

2.1

Local Area Networks

A LAN may consist of computers, printers, storage devices, and other shared
devices or services available to a group of users within a ‘‘local’’ geographical
area. These devices are interconnected
either via copper wire, optical wire (fiber),
or wireless media. Information passing over
the LAN is controlled by a set of network
ACRONYM ALERT
protocols that allows for the orderly sharing
PAN — Personal area network
of data between applications and devices,
even though these may come from many
different companies and manufacturers.

2.1.1

LAN Standards

As discussed in Chapter 1, the IEEE recognized that standards had to be
developed in order for LAN devices from differing manufacturers to be able
to communicate with one another. The IEEE 802 Overview and Architecture
standard heading described how these devices are to be interconnected on
both LANs and MANs.
For the purposes of this chapter, the standards that will be primarily
discussed as far as LAN networks go are:
802.2 Logical Link Control
802.3 CSMA/CD Access Method and Physical Layer Specifications
802.5 Token Ring Access Method and Physical Layer Specifications

2.1.1.1

802.2 Logical Link Control

The lower two layers of the Open Systems Interconnection (OSI) reference
model, Data Link and Physical, are addressed within the IEEE 802.2 standard. It further divides the Data Link layer into two sublayers, Logical Link
Control (LLC) and Media Access Control (MAC). This allows for ease in
mapping between different LAN Physical layers throughout the 802 family of
LAN/MAN standards.

11:58am

Page 64

Edwards

c02.tex

Chapter 2

■

V3 - 03/27/2009

11:58am

LANs, MANs, and WANs

The 802.2 implementation
RANDOM BONUS DEFINITION
uses a strategy of having the
LLC sublayer as a common
hop count — A measure of the number of
interface between the upper layrouters that a packet has passed through.
ers and the Physical layer no
matter what type of media is
being used in the construction
of the LAN. Figure 2-1 shows the LLC structure.
Destination Service Access
Point
DSAP
8 bits

Source Service Access
Point
SSAP
8 bits

Control
8 to 16 bits

Data
Variable Length

Figure 2-1 The IEEE 802.2 LLC structure

Destination service access point — The type of service that is
to receive the packet based on assigned SAP numbers, which
are independent from the type of network being used.
Source service access point — The type of service sending the packet
based on assigned SAP numbers, which are independent from the network type being used.
Control — Used for flow control and contains the send and receive
sequence numbers ensuring packets are being received in the proper
sequence.
Data — A variable length field containing the information being carried
within the packet.
The Media Access Control sublayer provides addressing and channel control. The MAC address, considered the physical address of the device, is a
unique value that allows multiple devices to share the same LAN no matter
what the physical medium being used for its implementation. Examples of
shared medium networks are those utilizing bus, ring, or wireless topologies.
Figure 2-2 illustrates the format of the 48-bit MAC address.
As illustrated, the address is split into two sections. The most significant
three octets make up the portion of the address that is referred to as the
organizationally unique identifier (OUI). These identify the organization that
issued the identifier. The NIC specific portion of the address assigned and
the serialization of the assigned numbers are under the control of the organization that owns the assigned OUI. With 24 bits of address, an organization
can assign 16,777,216 unique addresses to devices they have manufactured.
Assigned OUI addresses are maintained by the IEEE and can be found at
http://standards.ieee.org/regauth/oui/oui.txt.

65

Page 65

Edwards

66

Part I

■

V3 - 03/27/2009

Networking Nuts and Bolts

Organizationally Unique Identifier
(OUI)

Octet 1

c02.tex

Octet 2

Bit 1

Network Interface Controller
(NIC) specific

Octet 3

Bit 2

Bit 3

Octet 4

Bit 4

Octet 5

Bit 5

Bit 6

Octet 6

Bit 7

0: OUI unique address
1: locally assigned address

Bit 8

0: unicast
1: multicast

Figure 2-2 The IEEE 802 MAC address format

Bit B8 determines if the packet is either a unicast addressed packet, meaning
it is directed to a single network node address, or broadcast, which is directed
to all network nodes within a subnet.
MAC addresses are usually
written with either hyphens
or colons separating the hexaPOP QUIZ
decimal numbers representing
What are the two sublayers of the Data Link
each of the octets. A MAC
layer?
address annotated with the
use of hyphens would look
like 00-04-54-AA-B1-C2. If using
colons, it would be presented as 00:04:54:AA:B1:C2.
There is provision for network administrators to locally assign MAC
addresses to network interface controllers. If the NIC has been manufactured
to allow modification of the factory-assigned MAC address, the administrator
can set the bit to indicate that the MAC address has been locally assigned.
The NIC portion of the address can be a number for the interface that is of
administrator’s choosing. Locally assigned addresses do not contain values
representative of assigned OUI values. An example of a typical locally assigned
MAC address would be:
02-00-00-01-00--F4

2.1.1.2

802.3 CSMA/CD Access Method and Physical Layer

The IEEE 802.3 standard contains a group of standards that addresses the
unique characteristics of the network Physical layer being used on the network.

11:58am

Page 66

Edwards

c02.tex

Chapter 2

■

V3 - 03/27/2009

11:58am

LANs, MANs, and WANs

These standards were evolutionary and were issued as new types of media
with differing characteristics were developed.
This standard defined the MAC structure for CSMA/CD,2 as shown in
Figure 2-3.

Start Frame
Delimiter

Destination
Address

Source
Address

Length

802.2 LLC Structure

Frame Check
Sequence

Figure 2-3 The CSMA/CD MAC structure

When first introduced, IEEE
802.3 dealt with the use of
data networking on a bus
RANDOM BONUS DEFINITION
type network architecture using
multicast address — A method of
thick coax cable. This coax
identifying a set of one or more stations as
cable carried the designation of
the destination for transmitted data.
10BASE5 and was more commonly referred to as thicknet.
This type of cabling was rigid
and difficult to work with. It required a transceiver that would tap3 the cable
to form a node on the network. A cable constructed with a 15-pin D style
connector was needed to connect the transceiver to the device residing on a
node of the network.
To circumvent the difficulties with 10BASE5 cabling, a new standard was
developed, IEEE 802.3a, which still is bus network architecture but utilized
thin coax, commonly referred to as thinnet.4 The cable used was referred to
as 10BASE2, with RG-58 coax cable being the popular choice. RG-58 cable
being thinner offered more flexibility over the RG-8 cable that was used in a
10BASE5 network. The network was formed by using lengths of RG-58 cable
terminated with a BNC connector on each end. A BNC T connector formed the
network node at the back of each workstation. The network was terminated
on each end with a 50 ohm terminator. Figure 2-4 illustrates a simple 10BASE2
network with three workstations connected to the network using a BNC T
connector to connect to the network interface card.
2 Carrier

Sense Multiple Access with Collision Detection is necessary in a bus architecture where
any workstation may transmit randomly at any given time. The bus segment these workstations
reside on is sometimes referred to as a collision domain.
3 This type of tap was also referred to as a vampire tap since it had a pointed probe that pierced
the protective layer of the cable insulation to strike the ‘‘vein’’ at its core, which was the center
copper conductor. The bits would be allowed to flow like the life’s blood of the network was
being sucked out. OK, getting a little too dramatic with the class B horror movie genre references.
4 Thinnet was also referred to as cheapernet since the cost factor was a mere fraction of the cost
of 10BASE5 cabling, being more readily available at many electrical supply houses. There really
is something to that supply and demand theory that I learned in my economics classes.

67

Page 67

Edwards

68

Part I

■

50 ohm
terminator

c02.tex

V3 - 03/27/2009

Networking Nuts and Bolts
T
Conn

T
Conn

T
Conn

A

B

C

50 ohm
terminator

Figure 2-4 A simple 10BASE2 network

The BNC T connector on workstation B has a coax cable connected to it going
to workstation A and another going to workstation C. Workstations A and C,
having only one cable connected to their BNC T connector and being at each end
POP QUIZ
of the network, require that
MAC addresses are represented with
the open connection on each
hexadecimal numbers, separated by a colon
BNC T connector be terminated
.
or a
with a 50 ohm BNC terminator.
Although this is an improvement over 10BASE5 cabling, the
one drawback is that workstations not on the end of the network required two
cables to be terminated at the workstation’s BNC T connector.
Bus-based network architectures have inherent problems with cabling
that don’t exist in star-based networks. The development of IEEE 802.3i
(a bus network that allows for wiring to have the appearance that it is
physically a star-based topology while maintaining the CSMA/CD bus
network architecture) provided for network cabling that uses unshielded
twisted pair (UTP) and is commonly referred to as 10BASE-T. This allows
for the use of Category 5 cable, which contained four twisted pairs contained within an unshielded jacket. Each end of the cable is terminated with
an RJ-45 plug for short lengths of cable. Larger installations may terminate at wall jacks for workstation areas and to a patch panel at a central
location. Since these appear to be spokes out to the workstations, the central location would require a device to concentrate these network nodes
on a CSMA/CD network. The devices that accomplish this are appropriately called hubs. Figure 2-5 shows a hub and workstations in a CSMA/CD
network.

11:58am

Page 68

Edwards

c02.tex

Chapter 2

■

V3 - 03/27/2009

11:58am

LANs, MANs, and WANs

Hub

Figure 2-5 A CSMA/CD network using UTP cabling and a hub

Each workstation can be located at varying lengths from the hub. The
maximum length of cable between a workstation and a hub is 100 meters.5
This topology allows for the easy reconfiguration of the workstation. If a
workstation is removed, there are no special considerations as there are with
a 10BASE2 network topology.
The maximum transmission
speed of the IEEE 802.3 networks discussed in this section
RANDOM BONUS DEFINITION
is 10 Mbps. Subsequent stannibble — A 4-bit unit of data (half of a byte).
dards have been added to the
IEEE 802.3 standard that provide for 100 Mbps Fast Ethernet and 1Gbits/s over twisted
pair wire.
A QUICK REMEDIAL LESSON
Mega represents a million of something. In decimal number notation, it is
1,000,000. This number can be represented in shorthand notation as 1M.
(continued)

5

Meters are a metric measurement of distance. A quick calculation would be there are roughly 3
feet to the meter. Therefore, 100 meters is about 300 feet. But to be more precise, it’s 328.08 feet.

69

Page 69

Edwards

70

Part I

■

c02.tex

V3 - 03/27/2009

Networking Nuts and Bolts

A QUICK REMEDIAL LESSON (continued)
Giga represents a billion of something. In decimal number notation, it is
1,000,000,000. This number can be represented in shorthand notation as 1G.
Now, we have millions and billions of bits, but what exactly is a bit, you ask?
It is a single binary number represented by a 1 or a 0. Even if the value is 0, it
still requires a signal on the wire, so this is one place where exactly zero does
truly represent something.
Ten million bits per second (10 Mbps) is 10 million binary numbers having a
value of either 0 or 1 being sent over some medium in a one-second interval.
With giga rapidly becoming the new standard in Ethernet transmission speed,
which is the equivalent of a billion bits per second (bps) hitting the wire, data
that is normally referenced in bytes containing 8 bits of data would equate to
125 MBps (125,000,00 bytes per second) as the maximum number of bytes that
can be sent within a second. Note that lowercase ‘‘b’’ signifies bits and that
uppercase ‘‘B’’ signifies bytes in the notation used to reference these
quantities. Make sure you keep your bits and bytes straight because you can be
off by a factor of 8 in your calculations — usually not a problem when you
overestimate but you can really feel some heat if you underestimate a
network’s throughput capability.

2.1.1.3

802.5 Token Ring Access Method and Physical Layer

The IEEE 802.5 standard defines a Token Ring protocol that is much different
from that of a CSMA/CD protocol. With CSMA/CD, multiple workstations
can transmit onto the wire at the same time, potentially causing collisions.
When a collision occurs, they remedy the situation by backing off and retransmitting. With Token Ring, only one workstation is permitted to transmit onto
the wire, that being the workstation currently in possession of the token.
Transmission onto the wire
is sequential in a fixed pattern. After a workstation posPOP QUIZ
sessing the token has completed
What is the maximum length of a cable
its transmission onto the wire,
between a workstation and a hub?
it passes the token to the next
workstation. This is an advantage over CSMA/CD when the
network has fewer workstations. As the number of workstations increases, the
advantage is lost and the chattier CSMA/CD finally wins out.
When Token Ring was first introduced by IBM, it possessed a speed of
4 Mbps, thus not offering any advantage over CSMA/CD networks. With the
introduction of 16 Mbps Token Ring, it was a toss-up between it and CSMA/CD

11:58am

Page 70

Edwards c02.tex

Chapter 2

■

V3 - 03/27/2009

11:58am

LANs, MANs, and WANs

networks far as performance when the total number of workstations is lower.
Figure 2-6 illustrates the IEEE 802.5 frame structure.

Starting Frame
Delimiter

Access

Frame

Destination

Source

Route

802.2 LLC

Frame Check

Ending

Frame

Control

Control

Address

Address

Information

Structure

Sequence

Delimiter

Status

Figure 2-6 The IEEE 802.5 Token Ring frame structure

There are two minor differences between the IBM and IEEE 802.5 standards
for Token Ring:
The number of nodes on a ring is up to 260 nodes per IBM specification,
and the IEEE 802.5 standard limits it to a maximum of 250 nodes.
IBM allows up to 8 fields for route designation when source routing is
employed, whereas the IEEE 802.5 standard allows for a maximum of 14
fields.
The frame format for IBM/IEEE 802.5 is as follows:
The Starting Frame Delimiter and Ending Delimiter fields are
each a single byte with deliberate breaches in certain positions
of the Manchester Code6 so that the start or end of a frame can
never be recognized from any other portion of data on the wire.
Access Control is a single-byte field serving to signal control
and maintenance functions. The fourth bit position in this field
is the token bit. If it is set to 1, this frame is a token and only
consists of the Starting Frame Delimiter, Access Control, and
Ending Delimiter. A token frame is only 3 bytes long.
Frame Control is a single-byte field that indicates if the frame is control
information or data.
The Destination Address field contains either 2 or 6 bytes of addressing
information, depending on whether the frame is addressed to a single
node or a group of nodes.
The Source Address field contains either 2 or 6 bytes of addressing
information that indicates the address of the sending node.
The Route Information field is present only when source routing
has been enabled. It defines routing control, a route descriptor,
and type of routing information contained within the packet.
6 The

Manchester Code is Phase Encoding used within telecommunications where each data bit
has a minimum of one voltage transition within a fixed time slot, making it self clocking since
the clocking signal can be extracted directly from the encoded data stream.

71

Page 71

Edwards

72

Part I

■

c02.tex

V3 - 03/27/2009

Networking Nuts and Bolts

With source routing enabled there is a minimum of two fields
that will be present. The 2-byte route designator field defines
a ring number and the bridge number that the frame is to pass
through. The last route designator will contain the ring number
of the receiving node and a bridge number that is set to zero.
802.2 LLC Information Structure is a variable-length field that, surprise,
contains 802.2 LLC information.
Frame Check Sequence is a 4-byte field containing the checksum
information verifying the integrity of the frame starting from
the Frame Control field through the 802.2 LLC/Data field.
Ending Delimiter is an 8-bit field that indicates the end of the frame.
Frame Status is a 1-byte field indicating that the intended recipient has
received the frame.

Token

Figure 2-7 The token-passing sequence

Figure 2-7 is a logical visualization of a Token Ring network. The token is
a frame type that is transmitted sequentially around the ring network. When
a workstation needs to transmit on the ring, it keeps the token and modifies
it with address and data information, and then transmits it onto the ring. The
receiving station the data frame was intended for accepts the frame and sets a

11:58am

Page 72

Edwards

c02.tex

Chapter 2

■

V3 - 03/27/2009

11:58am

LANs, MANs, and WANs

flag in the frame to acknowledge proper receipt of the frame. The receiving station then retransmits the frame with the flag set back onto the ring network. On
receipt of the frame with the flag set, the transmitting workstation transmits a
new token frame onto the ring network and forwards it, allowing any of the following sequential workstations an opportunity to transmit onto the network.
In a Token Ring network, one of the workstations becomes the active ring
monitor. Any workstation can be an active monitor, but only one workstation
at a time. It is the role of the active monitor to detect data frames that have
traveled around the ring more than once. Once a frame that traveled around
the ring more than once is detected, the active monitor will remove the frame
from the network and discard it. If the active monitor determines that a token
frame is missing from the ring network, it purges the ring network of any
frames and then transmits a new token onto the ring network. The active
monitor workstation is responsible for the timing and clocking on the ring
network. All workstations on the ring network use the timing from the active
monitor to ensure that the same timing is being used to receive and send data.
A workstation becomes an active monitor by an election process when
the absence of a ring monitor is detected. Upon detection of this message, a
workstation transmits a claim token onto the ring network. Any subsequent
workstation with a higher address that wishes to participate as the active
monitor initiates a new claim token and transmits it onto the ring network.
Through this election process the workstation with the highest address and
participating in the claim token process is elected as the active monitor.
Although Token Ring is a logical ring, its topology appears as a star-based
network. This is accomplished by cabling and connectors designed by IBM.
The cabling consists of IBM type 1 shielded twisted pair (STP) cable and a
unique connector design which is bulky, giving it a distinct space disadvantage
compared to other cable connectors. To complete the ring, these connectors
are plugged into a media access unit (MAU), as illustrated in Figure 2-8.
The cable is constructed with a receive pair and a transmit pair. When the
Token Ring connector is inserted into the MAU,7 the receive pair is connected to
the transmit pair of the preceding workstation. The transmit pair is connected
to the receive pair of the following workstation, and the MAU completes
the ring. Multiple MAU units can be combined to form a larger single ring
network, as needed.

2.1.1.4

The Collision Domain Battle

Both IEEE 802.3 and Ethernet are CSMA/CD network standards; however,
the two are not fully compatible with each other. Although both 802.3 and
7 MAU

(media access unit) allows multiple units connected in a star topology to form a logical
Token Ring. These devices are sometimes referred to as a ‘‘ring in a box.’’

73

Page 73

Edwards c02.tex

74

Part I

■

V3 - 03/27/2009

Networking Nuts and Bolts

MAU

Figure 2-8 A Token Ring network using MAUs

Ethernet devices can coexist within the same LAN network, there are important
differences. The major difference between IEEE 802.3 and Ethernet is the frame
format. For them to coexist in the same LAN, the network software must be
able to differentiate between the different frame types.
Figure 2-9 illustrates the IEEE 802.3 frame.

7 Bytes

Preamble

1 Byte

6 Bytes

Start Frame Destination
Delimiter
Address

6 Bytes

2 Bytes

1 Byte

1 Byte

1 or 2
Bytes

Source
Address

Length

Destination
Service
Access
Point

Source
Service
Access
Point

Control

Variable
Length

4 Bytes

Information
Frame
(Data and
Check
Padding) Sequence

Figure 2-9 The 802.3 frame structure

The IEEE 802.3 frame contains the following fields:
Preamble — A 7-byte binary pattern used to establish frame synchronization.
Start Frame Delimiter — A single byte used to denote the start of a
frame.
Destination Address — The address the frame is being sent to. Although the standard allows this field to be anywhere between 2 to 6
bytes in length, the implementation in common use consists of 6 bytes.
Source Address — This field contains the address of the device sending
the frame. The standard allows this to be anywhere between 2 to 6 bytes
in length, but most implementations use 6 bytes in defining this field.

11:58am

Page 74

Edwards c02.tex

Chapter 2

■

V3 - 03/27/2009

11:58am

LANs, MANs, and WANs

Length — A 2-byte field used to denote the size of the IEEE 802.2 structure, including header and data.
Destination Service Access Point — A 1-byte field that indicates which
network protocol the receiving device should use in interpreting the
frame.
Source Service Access Point — A 1-byte field indicating which network protocol was used to create the frame. Normally this field contains the same information as the Destination Service Access Point.
Control — This field may be either 2 or 6 bytes long, where
the length of the field is indicated by the first 2 bits of the field.
It is used for indicating various commands such as exchange
identification, test, connect, disconnect or frame rejection.
An information field containing data and any number of required
padding bytes.
Data — A variable length field that contains the actual
information that is being transmitted within the frame.
Pad Bytes — An optional field that contains no information
but is added to ensure that the frame meets the minimum length
requirement.
Frame Check Sequence — A 4-byte field that contains the checksum of
the fields starting with the Destination Address through the Data field.
Figure 2-10 illustrates the Ethernet frame.

7 Bytes

1 Byte

6 Bytes

6 Bytes

2 Bytes

Variable Length

Preamble

Start Frame
Delimiter

Destination
Address

Source
Address

Type

Information
(Data and Padding)

4 Bytes

Frame Check
Sequence

Figure 2-10 The Ethernet frame

The Ethernet Frame contains the following fields:
Preamble — A 7-byte binary pattern used to establish frame synchronization.
Start Frame Delimiter — A single byte used to denote the start of a
frame.
Destination Address — The address the frame is being sent to.
Although the standard allows this field to be anywhere between 2 to 6
bytes in length, the implementation in common use consists of 6 bytes.

75

Page 75

Edwards c02.tex

76

Part I

■

V3 - 03/27/2009

Networking Nuts and Bolts

Source Address — This field contains the address of the device sending
the frame. The standard allows this to be anywhere between 2 to 6 bytes
in length, but most implementations use 6 bytes in defining this field.
Type — This is a 2-byte field that indicates the network protocol or
the protocol service contained within the frame.
Information — This is a variable length field that contains the
actual data being carried by the frame and any number of
bytes of padding to ensure the minimum frame size.
Frame Check Sequence — A 4-byte field that contains the checksum of
the fields starting with the Destination Address through the Data field.
The key difference between the IEEE 802.3 frame and the Ethernet frame is
Ethernet’s Type field. The IEEE 802.3 frame uses the IEEE 802.2 Source Service
Access Point and Destination Service Access Point fields to indicate which
network the frame is coming from and which network it is going to.
A list of registered Ethernet types can be found at http://standards.ieee
.org/regauth/ethertype/eth.txt.

2.1.1.5

The Most Common Wireless Standards

As covered in Chapter 1, the IEEE 802.11 is a group of standards defining
the operation of network communications using radio frequencies. These
standards are loosely interchanged with
the term Wi-Fi, but do have some differences with the standards of the Wi-Fi
Alliance. With the proliferation of wireless
ACRONYM ALERT
network products into the marketplace, the
STE — Spanning tree explorer
Wi-Fi Alliance is in the process of certifying these products before amendments to
the 802.11 are completed. Today’s wireless
products are being sold under the following
standards:
802.11 — This is the legacy base standard for wireless networking
802.11a — This standard’s advantage is the use of the less crowded
5 GHz band, but its chief disadvantage is that its signals are more
easily absorbed and dampen the signal quality as the signal travels
through solid objects along its path.
802.11b — Introduced in 1999, this standard uses the 2.4 GHz broadcast
band providing a typical data rate of 4.5 Mbps with a maximum data
rate of 11 Mbps. Its major disadvantage is that it can receive interference
from other devices that also share the 2.4 GHz frequency band such as
microwaves, cordless telephones, and a wide variety of Bluetooth

11:58am

Page 76

Edwards

c02.tex

Chapter 2

■

V3 - 03/27/2009

11:58am

LANs, MANs, and WANs

devices. The substantial increase of data rate throughput and the
reduction of product cost have led to the rapid acceptance of this
standard as the definitive standard for wireless LAN networks.
802.11g — Consumer demand for higher data rate products led to
the introduction of products that supported the older IEEE 802.11a
and b standards as well as this standard, which made these products
capable of supporting all three standards within a single device.
However, an 802.11g standard wireless LAN network can reduce
the overall speed of the network if one device participating in the
wireless network is only capable of supporting the IEEE 802.11b
standard. As with 802.11b, this standard also falls prey to the same
interference from other devices sharing the same frequency band.
802.11-2007 — This is a standard that was released to be all-inclusive
of the amendments to 802.11 since its introduction. To date this is the
most conclusive standard document that defines wireless LAN network
operation.
802.11n — With a proposed release date of 2009, this is an amendment
that will add additional features to the 802.11 standard and will include
multiple input/multiple output (MIMO) technology. MIMO will
use multiple antennas for both transmission and receiving, which
would offer significant increases in range and data rate throughput
without the need for increased bandwidth of transmission power.
Although it is still in draft, many vendors are beginning to sell
products labeled under the 802.11n standard. To avoid any interoperability problems between differing vendors, it is recommended to
purchase routers and access points from the same manufacturer.
The standards listed above are not all-inclusive of the IEEE 802.11 standard.
They are the most commonly known and discussed standards when there is
a discussion on wireless LAN networks. Additional information can be found
at the IEEE 802.11 group’s website at http://ieee802.org/11/.

2.1.2

LAN Topologies

Chapter 1 presented a variety of network topologies. In this chapter, we will
attempt to provide further information concerning the implementation and
use of these topologies in the creation of a LAN.
Figure 2-11 illustrates a very basic network map. The purpose is to demonstrate that even a simple network can and probably will use a variety of media,
protocols, and network devices. The media shown on this network topology
is a combination of wired systems, which include both ring and bus network
topologies, along with a network segment that is connected using wireless
network technology. Users are connected to the network either hard-wired to
a bus or ring LAN segment or through a wireless LAN access point.

77

Page 77

Edwards

78

Part I

■

c02.tex

V3 - 03/27/2009

Networking Nuts and Bolts

Figure 2-11 A sample LAN’s topological map

The network allows for the access of users to network resources such as
mainframe computers, network storage devices, network printers, and other
shared resources connected to the network. The LAN segment illustrated
in this figure has no access to the outside world via the Internet and is
self-contained. Most of today’s LAN networks ultimately do connect to the
Internet and will be discussed further in the ‘‘Metropolitan Area Networks’’
and ‘‘Wide Area Networks’’ sections of this chapter. So the focus of this section
is solely on the LAN. This is the section that deals with ‘‘this is my LAN and
that is your LAN’’ area of networking.
A LAN can contain a single network segment of any media type, or it may be
a collection of two or more of the network media currently in use today. So, if a
LAN is a combination of different media types, how do they interconnect? This
is where devices called gateways, bridges, and routers come into play. They

11:58am

Page 78

Edwards

c02.tex

Chapter 2

■

V3 - 03/27/2009

11:58am

LANs, MANs, and WANs

are depicted in Figure 2-11 as boxes between LAN segments. How you plan to
implement your network and the networking address schemes that are to be
used will determine which type of these devices would need to be used for these
network nodes. These devices will be covered in depth in Chapter 3, ‘‘Network
Hardware and Transmission Media.’’ For the purpose of this chapter, it will be
generally accepted that these devices do allow for communications between
LAN segments with different media and network protocols.

2.1.2.1

Token Ring Network Topologies

Wired Token Ring networks are still around, but the number of new installations is declining as more new network installations opt toward wired bus
network implementations. The need to discuss the wired Token Ring network
architecture is due to the fact that there are a number of these networks still
deployed in the field today even though they are considered legacy8 networks.
The original design of a Token Ring network was literally a ring where each
node of the network was daisy-chained to the next node until the network
came back around to the first node in the ring. There was a ring-in (RI) port
and a ring-out (RO) port, with the RO of one station connecting to the RI of the
next upstream station on the ring. This would continue until all the network
nodes had been connected. The major disadvantage of this network design
was that the disruption or disconnection of any one node on the ring brought
the whole network down. Newer Token Ring networks were designed using
hubs or media access units (MAUs), which allowed for ease in cabling while
maintaining the logical ring of the Token Ring network architecture. Figure 212 illustrates the construction of a Token Ring network with two nodes with
the use of a two-port MAU.

MAU

Figure 2-12 A simple Token Ring network

Obviously, a network of this construction has a limited use. To overcome
this limitation, an eight-port MAU was designed with the ability to extend the
Token Ring by daisy-chaining multiple eight-port MAU units together using
the RI and RO ports on the eight-port MAU. Figure 2-13 illustrates this more
complex Token Ring network.
8 A legacy network is one that is installed and operational although its technology has been super-

seded by other network technologies. Networks in large organizations are mostly evolutionary.
It is not uncommon to find some networks still operational although they are no longer sold and
supported by the original manufacturer. A lot of companies work on the ‘‘if it ain’t broke, don’t
fix it’’ mentality when it comes to their internal LAN networks.

79

Page 79

Edwards

80

Part I

■

c02.tex

V3 - 03/27/2009

Networking Nuts and Bolts

2 Port Hub

RO

RI

8 Port MAU

8 Port MAU

RO

RI

RI

8 Port MAU

RO

Figure 2-13 A typical Token Ring network

Up to a maximum of 33 MAU units can be interconnected to form the ring
network. The distance between MAU units is determined by the cable used to
interconnect them. With the use of Type
1 cable, MAU units can be placed up to
a maximum of 100 meters apart. If greater
distances are needed, a repeater is required.
ACRONYM ALERT
Repeaters used for copper wire network
UDP — User Datagram Protocol
segments can increase this distance up to
740 meters. If even greater distances are
required, the network segment can be further extended up to four kilometers
with the use of a fiber optic repeater and fiber optic cable.
Workstations and hubs connected to the MAU by cable are referred to as
lobes. Normally a lobe connects a workstation to a MAU, but if multiple
workstations in the same area need to be connected to the ring network, this is
accomplished with the use of a lobe access unit (LAU). A LAU unit splits the
lobe into two or more lobes. A LAU can be placed at the end of a cable to allow
for the connection of multiple workstations in that area. Although LAU units
sound as if they are the same as MAU units, there is a major difference. Unlike
a MAU, a LAU cannot be used to create a standalone ring. So LAU units are
basically used as hubs.
Although the difference between LAU and MAU units has become obscured
because some manufacturers market products called LAU units, in reality they
are functionally MAU units. However, the primary use of both MAU and

11:58am

Page 80

Edwards

c02.tex

Chapter 2

■

V3 - 03/27/2009

11:58am

LANs, MANs, and WANs

LAU units is in maintaining the functioning of the ring network as devices are
disconnected from the network.
A MAU or LAU allows a lobe on the ring to be opened for the insertion
of a new workstation, and it closes the ring when a workstation is removed
from the network. This allows for flexibility of network construction and any
necessary network reconfiguration without the problem of interruption of ring
network function.
2.1.2.1.1 Token Ring Cabling

The physical layout of a Token Ring network depends not only on the
placement of MAU, LAU, and hub units, but also on the cabling being used in
its construction. It has been previously mentioned that the cable construction
can be either STP or UTP cable.
STP Token Ring cable, also known
as IBM Type 1 cable, is constructed with twisted pair wires that are shielded.
The use of this cable allows for Token Ring lobe connections to be a maximum
of 100 meters apart. STP cables are terminated with either DB9 connectors or
patch connectors. Generally, patch connectors are used to connect to MAU
units, whereas DB9 male connectors are used to connect to workstations or
LAU units. DB9 female connectors are used to daisy-chain one LAU unit to
another.
The signals carried by the cable are transmit and receive. Two shielded
pairs are needed for these differential9 signals. Table 2-1 lists the DB9 pin
assignments.
2.1.2.1.1.1 Shielded Twisted Pair Cable

Table 2-1 DB9 Pin Assignments
SIGNAL

PIN

Receive +

1

Receive −

6

Transmit +

9

Transmit −

5

2.1.2.1.1.2 Unshielded Twisted Pair Cable UTP Token Ring cable, also

known as IBM Type 3 cable, is constructed with unshielded twisted pair wire
similar to telephone cable. These cables are terminated with RJ-45 modular
9 Differential

Manchester encoding is used for the transmission and reception of data in the use
of either STP or UTP Token Ring cabling. The balanced signals for both the send and receive data
signals allow for data integrity and greater noise immunity.

81

Page 81

Edwards

82

Part I

■

c02.tex

V3 - 03/27/2009

Networking Nuts and Bolts

plugs. This style of Token Ring cabling is dependent on the operating environment the network segment is in and the speed of the LAN itself. This cabling
is used to form lobe segments that do not exceed 45 meters. Typically these
cables10 are constructed using 10BASE-T UTP cable terminated on each end
with RJ-45 plugs. The RJ-45 pin assignments are listed in Table 2-2.
Table 2-2 RJ-45 Pin Assignments
SIGNAL

PIN

WIRE COLOR

Receive +

4

White with orange stripe

Receive −

5

Orange with white stripe

Transmit +

6

White with blue stripe

Transmit −

3

Blue with white stripe

2.1.2.1.1.3 Other Variations of Token Ring Cabling For special environments
or applications, IBM also uses cabling that consists of Type 2, Type 5, Type 6,
Type 8, and Type 9 cables.

Type 2 — Consists of two STPs as can be found in Type 1
cable and four UTPs as can be found in Type 3 cable.
Type 5 — Consists of multimode fiber optic cable used to extend
the Token Ring network and to interconnect optical repeaters.
Type 6 — Consists of two STPs. It is considered a low cost, short
distance cable with a maximum length of 45 meters and is often used for
MAU-to-MAU connection.
Type 8 — Consists of two parallel pairs. The wires in this cable are
untwisted and have a maximum length of 50 meters. The primary
purpose of this wire is in installations requiring the cable to run under
carpeting.
Type 9 — A lower cost alternative to Type 1 cable with a maximum
length of 65 meters. It consists of two pairs of STPs.
2.1.2.1.2 High-Speed Token Ring

There have been efforts made to push the speed of Token Ring networks
beyond the standard 16 Mbps. High-speed Token Ring has not been fully
deployed with the decline in newer Token Ring installations. However, it is
10 Although

these cables appear to be similar to those used for Ethernet 10BASE-T patch cables,
they are not the same. Ethernet 10BASE-T cables are constructed to use pins 1 and 2, and 3 and
6, for their twisted pair combinations.

11:58am

Page 82

Edwards c02.tex

Chapter 2

■

V3 - 03/27/2009

11:58am

LANs, MANs, and WANs

worth mentioning since there is a high likelihood of it being encountered in
the remaining legacy Token Ring networks.
32 Mbps Token Ring — Both IBM and other vendors of Token Ring
components and devices attempted to push Token Ring operation to a
higher speed.
Token Ring switches — These are in the form of switching bridges
capable of speeding up how messages travel between network rings.
Fiber distributed data interface (FDDI) — Although closely
related to Token Ring, it is not officially considered as part of the
Token Ring family. They both use a token-passing protocol.

2.1.2.2

Bus Networks Topologies

Bus networks initially were designed as a physical bus allowing devices to be
connected to nodes along the bus. Figure 2-14 shows a typical bus network.

Bus network

Figure 2-14 A typical bus network

In this illustration, workstations are connected to the bus with the use
of transceivers. With 10BASE5 cabling being used to form the bus network,
external transceivers were typically used to connect a workstation to the
network. In later bus implementations using 10BASE2 cabling in the form of
RG-58 coax cable to form the bus network, the transceiver was integrated into
the network adapter card that was installed within the workstation.
The transceiver not only converted the digital data generated by the workstation into the appropriate data signals, it performed other functions useful
to both 802.3 and Ethernet LAN networks.
Collision detection — Provided by circuitry designed to detect
collisions on the bus network. If a collision is detected, the transceiver
notifies the transmitting function that a collision has occurred and
then broadcasts a jamming signal on the network to notify other
systems connected to the bus network. The LAN is then allowed
to settle before the resumption of transmissions on to the bus.

83

Page 83

Edwards

84

Part I

■

c02.tex

V3 - 03/27/2009

Networking Nuts and Bolts

Heartbeat — Generation of a short signal to inform the main
adapter that the transmission is successful and collision free.
Although specified in the 802.3 standard and the Ethernet
standard, it is rarely used because many adapters confuse this
signal with the signal that signifies a collision has occurred.
Jabber — The function that allows the transceiver to cease transmission if the frame being transmitted exceeds the specified
limit of 1518 bytes. This helps prevent a malfunctioning system
or adapter from flooding the LAN with inappropriate data.
Monitor — This function monitors LAN traffic by prohibiting transmit functions while receive and collision functions
are enabled. It does not generate any traffic onto the LAN.
A bus network created using 10BASE5 or thick coax cable can have a
maximum overall segment length of 500 meters. Each node on the segment is
created with the use of a transceiver. Nodes on a thick coax cable are to be
spaced no closer than 2.5 meters with a maximum number of 100 nodes per
segment. The impedance for thick coax is 50 ohms. With the use of repeaters,
the overall length of the combined segments is not to exceed 2,500 meters.
Generally, bus networks that
are formed by using 10BASE2
cabling use adapters that have
RANDOM BONUS DEFINITION
the transceiver function built in.
twisted pair — A communications medium
The network is formed using
consisting of two copper conductors twisted
a BNC coax T connector contogether.
nected to the workstation’s BNC
coax connector. Workstations
are then daisy-chained together
using lengths of coax cable terminated at both ends with coax plugs. These
interconnecting cables should not be less than 0.5 meters in length with a
maximum of 30 nodes and a total length of 185 meters per network segment.
The BNC T connector on each end of the network segment requires a 50 ohm
terminator to be attached to the open end of the T connector to maintain
the cable impedance. This is essential to maintain signal integrity and the
dampening of any signal reflections on the cable. With the use of repeaters,
the overall length of the combined segments is not to exceed 925 meters.
The maximum frame size for both IEEE 802.3 and Ethernet frames is 1518
bytes. 802.3 provides for a maximum data segment size of 1460 bytes while
Ethernet allows for a maximum data size of 1500 bytes. The original speed for
Ethernet was 10 Mbps.
There are two other implementations of logical bus networks: the star
topology and the tree topology.

11:58am

Page 84

Edwards

c02.tex

Chapter 2

■

V3 - 03/27/2009

11:58am

LANs, MANs, and WANs

2.1.2.2.1 Star Network Topology

A star topology is implemented with the use of hubs and UTP cables terminated
with RJ-45 plugs. Hubs maintain the logic of the bus network while the UTP
cables radiate out in a star pattern. Figure 2-15 illustrates a star network formed
with the use of a single hub and UTP cables that are no longer than 100 meters
in length.

Figure 2-15 A star network

The simplicity of this type of network is the ease in which devices may be
added or removed from the network. The only limiting factor for this type of
network with a single hub is the number of ports contained on the hub. This
type of network is only useful for a small self-contained work group with no
requirement of connecting to other network segments located elsewhere.
2.1.2.2.2 Tree Network Topologies

Tree network topologies consist of network segments connected by hubs
and other devices in various combinations to create the network. Network
segments can either be geographically close or remote. Many networks fall into
the tree network architecture. This is especially true for very large networks
with many nodes. Figure 2-16 illustrates a simple logical diagram showing a
series of user nodes.
This could be considered a top level drawing where the later drawings show
more detail of how the segments are to be connected and the media that make
up the network segments. Figure 2-17 illustrates what one of the network
segments might look like. It is a combination of devices using both wired and
wireless media to connect nodes within that network segment.

85

Page 85

Edwards

86

Part I

■

c02.tex

V3 - 03/27/2009

Networking Nuts and Bolts

Tree network

Figure 2-16 A logical drawing of tree network topology

Wireless Router
Wireless Access
point
Main Network
Segment

Bridge

Router

Hub

Hub

Figure 2-17 A tree topology network segment

11:58am

Page 86

Edwards c02.tex

Chapter 2

■

V3 - 03/27/2009

11:58am

LANs, MANs, and WANs

Laptop users with wireless enabled laptops can communicate directly from
their laptop to a wireless access point to gain access to the network. Laptops
that are not wireless enabled can be directly connected to the wired network
segment using the network interface card, which is internal to the laptop.
Another option, if needed, is to connect the laptop to a wireless router that is
able to communicate to the wireless access point to gain access to the network.
Workstations on the network segment are connected to the network with the
use of a hub. Separate local network segments are connected with the use of a
bridge. This whole network segment is connected to other network segments
with the use of a router.
2.1.2.2.3 Devices that Make Up a Network

True bus networks11 can still be found, but they are considered legacy networks
by today’s standards. Most newly deployed networks, although they are bus
networks, logically make use of devices to maintain the bus while nodes are
placed in either a star or tree network topology or, in many cases, a combination
of both. The majority of cabling used is 10BASE-T UTP cable connected to the
bus network devices with the use of RJ-45 plugs.
The following devices may be found in a variety of network topologies:
Hubs — Considered to be passive network devices.12 Passive hubs allow
the connection of multiple nodes to the network. They can be standalone or daisy-chained to other hubs to form a larger network segment.
Repeaters — Used to extend network segments beyond the recommended distance over wire cabling by performing signal regeneration to
ensure that data integrity is maintained over the long network segment.
Bridges — Used to divide a network into smaller segments to reduce
the number of network devices contending on the network segment for
network access. The bridge only passes network traffic that is specifically intended for the other network segment that it is connected to.
Ethernet switches — These are more predominately used today in
LAN networks to perform the role of bridges in dividing a network
into smaller segments to reduce network contention between network
devices. A single Ethernet switch is capable of having multiple
network segments contained within it. This is accomplished by
programmable ports, which may be dedicated to virtual LAN (VLAN)
11 The term true bus network refers to networks that are physically constructed as a bus. They
consist of either thick or thin coax cable. These networks use 10BASE5 and 10BASE2 cabling to
form the network segment.
12 Passive network devices such as hubs are designed to maintain the electrical characteristics
of a bus network while physically giving the appearance that they are interconnected in
either a star or tree network topology.

87

Page 87

Edwards

88

Part I

■

c02.tex

V3 - 03/27/2009

Networking Nuts and Bolts

segments on that device. They usually contain multiple ports and
are similar in appearance to hubs but differ in that hubs are not
able to reduce network contention on the network segment they
are being used on. Some Ethernet switches provide the ability to
gang multiple devices together to form a larger network segment.
Routers — Used to connect multiple network segments but differ
vastly from bridge devices. Bridges operate solely on the information
contained within the 802.3 data frame and are not effected by the
routing protocols being run over the network. Routers operate
at the network protocol level and forward network traffic based
upon the network protocol information contained within the data
frame being forwarded from one network segment to another.
Network interface cards (NIC) — A term used predominately to refer
to the cards contained within devices connected to the network. However, the devices that fall under this category are wide and diverse,
from cards meant to fit into a PC slot to other devices intended to connect via a USB port. Some NIC devices fit into a PCMCIA card slot on
a laptop and allow it to gain network access via a wireless link. They
all serve the same purpose: to allow a device to connect to a LAN.
The devices briefly described in this section are covered in further depth in
Chapter 3.
2.1.2.2.4 Bus Network Cabling

This section discusses the following bus wire types: 10BASE5 coax (thicknet),
10BASE2 (thinnet), and 10BASE-T (UTP). The predominant wiring used in
today’s network is 10BASE-T, which is commonly referred to as Ethernet
cabling. The characteristics and limitations of each cable type will be discussed
in this section.
2.1.2.2.4.1 10BASE5 Thicknet This cable type was the initial introduction
to CSMA/CD bus network topology. The network segment is formed using
this thick coax cable, which has a maximum segment length of 500 meters.
Being thick and heavy, the cable is difficult to handle when routing the cable
throughout a building. A network node is formed with the use of what is
commonly referred to as a vampire tap. This device pierces the jacket of the
coax cable to make contact to the center conductor of the coax cable and
provide the signal to the network node with the use of a transceiver. The
physical construction of the transceiver appears the same for both Ethernet
and IEEE 802.3, both using a DB15 connector style. However, where they
differ is in the circuit assignment for each pin. Table 2-3 shows the DB15 pin
assignments for both Ethernet and IEEE 802.3.

11:58am

Page 88

Edwards

c02.tex

Chapter 2

■

V3 - 03/27/2009

11:58am

LANs, MANs, and WANs

Table 2-3 DB15 Pin Assignments
PIN

ETHERNET

IEEE 802.3

1

Ground

Ground control in

2

Collision detected +

Control in A

3

Transmit +

Data out A

4

Ground

Data in

5

Receive +

Data in A

6

Voltage

Common

7

Control

Out A

8

Ground

Control out

9

Collision detected −

Control in B

10

Transmit −

Data out B

11

Ground

Data out

12

Receive −

Data in B

13

Power

14

Power ground

15

Control

Out B

The Ethernet transceiver specifies the pinout for three signals, transmit,
receive, and collision detect, whereas the IEEE 802.3 standard provides for an
added signal of control out (which is not used). Although the pin assignments
are such that a cable manufactured for either standard would work with the
other standard’s transceiver, it is not recommended due to differences used in
signal grounding.
Vampire taps may not be located any closer together than 2.5 meters with
a maximum of 100 taps per network segment. Network segments can be
combined with the use of repeaters to increase the overall combined network
length to 2,500 meters. The characteristic impedance of 10BASE5 cable is
50 ohms.
10BASE2 networks are constructed mostly with
the use of RG-58 coax cable, which has a characteristic impedance of 50 ohms.
This cabling is more desirable for use in network segments due to its lower
cost and greater flexibility than that of 10BASE5 cable. Network nodes are
easily formed with lower cost BNC T connectors, whereas 10BASE5 cabling
requires a more expensive vampire tap transceiver. However, 10BASE5 cable

2.1.2.2.4.2 10BASE2 Thinnet

89

Page 89

Edwards

90

Part I

■

c02.tex

V3 - 03/27/2009

Networking Nuts and Bolts

is capable of far greater network segment length than 10BASE2, which makes
it more suitable for a network backbone. The 10BASE2 network, with its lower
cost and ease of reconfiguration if needed, is more suited for a work group
environment clustered in a smaller geographical area. To properly terminate a
10BASE2 network to maintain the characteristic 50 ohm impedance across the
network and reduce signal reflections on the wire, the last BNC T connector
on each end of the network segment must have a 50 ohm BNC terminating
plug connected to the open tap on that BNC T connector.
The overall segment length for a 10BASE2 cabled network is 185 meters
with a maximum of 30 network nodes per segment. The minimum distance
between network nodes is 0.5 meter. The overall network length that can be
achieved with the use of repeaters for 10BASE2 is 925 meters.
2.1.2.2.4.3 10BASE-T UTP Cabling These days, 10BASE-T cable and Ethernet
UTP cable are simply synonymously called Ethernet cable. Although logically
it is considered as bus topology cable, it is point-to-point between a network
node device and a device that completes the logical bus. Cable construction is
similar to telephone cable, which makes it easily routable through a building.
Similar to telephone cable in larger installation sites, patch panels are used to
terminate cables from differing locations throughout the facility.
Ethernet cables of various lengths terminated with RJ-45 plugs on both ends
are usually referred to as patch cables or straight-through cables. These cables are
used to connect a network node device to a network device that completes the
logical bus. Table 2-4 shows the pinout for an RJ-45 plug on an Ethernet cable.
Table 2-4 RJ-45 Pin Assignments
PIN

SIGNAL

1

Transmit +

2

Transmit −

3

Receive +

4
5
6

Receive −

7
8

It can be seen that a patch cable or straight-through cable carries the same
signal from one end to the other on the same pin if both RJ-45 jacks are wired

11:58am

Page 90

Edwards

c02.tex

Chapter 2

■

V3 - 03/27/2009

11:58am

LANs, MANs, and WANs

exactly alike. However, there is another cable that appears physically identical
but is wired differently, called a crossover cable. These cables do literally just
that — they cross over the transmit signals to the receive signals. The purpose
of these cables is to connect two network devices whose connectors are wired
exactly the same. A simple example of this would be two computers connected
by a crossover cable to use the network cable to transfer files between them.
Many of today’s network devices such as hubs and switches use autosensing, auto-switching ports to sense the cable and dynamically configure
the port to ensure that the transmit signal from another network device is
connected to its receive signal input. This was not always the case, so in order
to expand a network segment, crossover cables were necessary to daisy-chain
multiple hubs together. Figure 2-18 illustrates how hubs can be daisy-chained
to form a larger network segment.
Patch Panel

Hub
First Tier
Hub
Second Tier

Hub
Third Tier

Patch Cable
Crossover Cable

Figure 2-18 Daisy-chaining for an expanded network segment

In Figure 2-18, a local geographical area is serviced by a series of hubs to
allow network devices in that location to gain access to the network. The feed
for this network is from a patch panel over a patch cable to the first tier hub
device. This device with the use of crossover cables is attached to a number of
second tier hubs. In this illustration, one of the second tier hubs is connected
using a crossover cable to a third tier hub, which services some computers
attached to the network. This appears at first to be an unlimited geometric

91

Page 91

Edwards

92

Part I

■

c02.tex

V3 - 03/27/2009

Networking Nuts and Bolts

progression, but in reality it is a bus network, so network devices do contend
for network bandwidth. It can be readily seen that all devices on this network
segment that send traffic to other network segments need to have it pass over
the single cable between the patch panel and the first tier hub device. This is
often referred to as a single point of access.13
Hub manufacturers saw the inconvenience of having two cable types and
began to design and sell hubs with a mechanical switch on one of the ports
so that a patch cable could be used between hubs in place of a crossover
cable. More recent Ethernet port designs have led to the development of a port
device using electronic auto-sensing, auto-switching to configure the port to
match transmit and receive signals no matter if a patch or crossover cable is
connected to the port.
Any segment of the network shown in Figure 2-18 may not have a cable
linking two network devices that exceeds 100 meters. The overall combined
length of the entire segment with the use of hubs and repeaters may not exceed
2,500 meters. For smaller local networks, these lengths are more than adequate.
For much larger installations, special considerations will be required to ensure
data integrity on the network.
2.1.2.2.4.4 So What about Speed and Duplex? The initial speed standard
for CSMA/CD bus networks over UTP cable was 10 Mbps. Since the initial
introduction, devices that can
pass network traffic at 100 Mbps
(100BASE-TX) are now fairly
RANDOM BONUS DEFINITION
common. Many of today’s inping — A utility program used to test for
stallations make use of giganetwork connectivity by using the echo
bit speeds (1000BASE-T), which
request and echo response mechanisms of
sometimes is referred to as gig-E.
ICMP.
These advances in technology
have allowed for the attainment
of greater network speeds without the need for changing the current wiring infrastructure. Devices capable
of any of the speeds listed are able to do so over existing Category 5 cabling.
Duplex is either half-duplex or full-duplex. The difference between the two
is that full-duplex devices are capable of transmitting and receiving at the
same time, whereas half-duplex devices are either in transmit or receive mode
but never both simultaneously.
Since UTP cabling is connected in a point-to-point fashion, the ports connected to each end of the cable must be able to transmit and receive at the same
speed. On some devices, these are only manually configurable. Some devices
13

Single point of access is also a single point of network failure. Depending on the number
of devices in a local area or how critical network availability is to those users, some thought
should be given to network segmentation and redundancy. There will be further discussions and
examples of this throughout this book.

11:58am

Page 92

Edwards

c02.tex

Chapter 2

■

V3 - 03/27/2009

11:58am

LANs, MANs, and WANs

are able to negotiate speed and duplex with their peer port to set the speed
and duplex to be used over the link.14 This mode of operation is referred to as
auto-negotiation.
Careful attention must be paid to the speed and duplex of an interface.
If there is a mismatch between the devices, network performance will be
degraded and full network speed cannot be realized. This is a small detail that’s
often overlooked but has major implications in overall network performance.

2.2

Metropolitan Area Networks

The term metropolitan area network is a bit nebulous and embraces a variety of
differing network scenarios. The common denominator in all these networks is
that they cover areas that are much larger than a conventional LAN is capable
of, as discussed in Chapter 1.
The technological development of fiber optic network devices has facilitated
the growth of both private and public MAN networks. Fiber optics allowed the
network to stretch to over several kilometers, which made extended networks
more feasible. Fiber distributed data interface (FDDI) is used for the backbone
that interconnects distant portions of the MAN. So what exactly is an FDDI?

2.2.1

Fiber Distributed Data Interface

Fiber optic cabling presents several advantages over conventional copper
wiring. It is lighter in weight than copper, weighing in at roughly 10 percent
of a copper cable of the same length. It is capable of driving data signals
much further with less loss and is immune to crosstalk and noise caused by
electromagnetic interference (EMI). Fiber optic cable, being electrically inert,
aids in the elimination of ground loops between sending and receiving nodes.
Since fiber optic cable does not emit any radio frequency interference
(RFI) when data is transmitted on the cable, it cannot be snooped using
radio frequency detectors as copper wire can. The only way data can be
eavesdropped on is by actually breaking the cable and placing a receiver in
the line. Since this action would not go undetected, fiber optic cabling offers
greater security over copper.
All this stuff about fiber optic cable is great, but how is it used in a network,
you ask? Well, knowing you read the section on Token Ring LAN segments,
the authors feel we do not have to review the concept of token passing. If
we are wrong, you should go back and read the Token Ring section about
how a token is passed about a ring. Although the token-passing concept is
14 Link

is a reference to a cable connecting (linking) two network devices’ ports. Many interface
connectors on network devices have an LED indicator to indicate the presence of link. Link on
an interface indicates that the transmit and receive signals are properly connected and the two
devices are capable of communicating over the cable (link).

93

Page 93

Edwards

94

Part I

■

c02.tex

V3 - 03/27/2009

Networking Nuts and Bolts

similar, FDDI is not the same as the IEEE 802.5 Token Ring standard. FDDI
was standardized under ANSI standard X3T9.
From the previous paragraph, you are already aware that FDDI is implemented using token passing over a ring topology consisting of fiber optic
cable. Construction of the network consists of dual rings, a primary ring and
a secondary ring. Both rings are capable of passing data, but usually the
counter-rotating secondary ring, which can carry data in the opposite direction, is reserved to be used as a backup in case of ring failure. Figure 2-19
shows a logical representation of an FDDI network.
Primary Ring
FDDI
Bridge

FDDI
Bridge
Secondary
Ring

Figure 2-19 An FDDI network

Although this network is shown logically as a ring, it is physically deployed
in a star topology similar to that of wired Token Ring networks. FDDI
bridge/concentrators complete the logical ring while also providing the optical
to electrical signal conversion to allow data to be transferred from an optical
network segment to a wired network segment and in the reverse direction.
To facilitate the star physical topology, fiber optic cable is dual strand cable.
There is one fiber optic strand carrying intelligent light information to the
FDDI bridge concentrator while the other strand allows for the transmission of
data from that FDDI concentrator to the next. These fiber optic network cables
are sometime called light pipes.15
15 Don’t

confuse fiber optic data cables with those fiber strands you see at the mall emitting all
those wild colors. Although similar in terms of light being transmitted through an optical fiber, the
quality and construction are far different. After all, it is for the purpose of sending intelligent data.

11:58am

Page 94

Edwards

c02.tex

Chapter 2

■

V3 - 03/27/2009

11:58am

LANs, MANs, and WANs

FDDI networks are capable of transmitting data at 100 Mbps for a maximum
ring circumference of 100 kilometers. If both the primary and secondary rings
are used, an effective data rate of 200 Mbps can be achieved. This is what
makes FDDI the preferred choice for backbones on large LAN networks and
for deploying a MAN over a wide geographical area.
To pass data from either an Ethernet or Token Ring LAN segment
requires a bridge to transform electrical signals into intelligent light impulses.
These bridges fall into two categories, encapsulating bridges and translating
bridges. Encapsulating bridges encapsulate Ethernet frames into FDDI frames,
and translating bridges translate the received frame source and destination
MAC addresses into FDDI addresses. The maximum FDDI frame size is
4500 bytes.
A dual ring FDDI network can connect up to a maximum of 500 stations.
Since FDDI requires a repeater every 2 kilometers, it is unsuitable for a WAN
network deployment. FDDI lends itself easily within existing metropolitan
infrastructures where cabling is routed in hostile environments under streets
and overhead lines. It is impervious to EMI, so no special shielding is required
other than having the fiber jacketed to withstand the environment it is to
be placed in. Since fiber cable depends on a continuous, undistorted fiber to
transmit data without degradation, care must be taken to maintain a minimum bending radius for the type of fiber cable being used, to prevent a
possible crimp in the fiber. A distortion of the fiber can cause light reflections that could render the total cable length unusable for the transmission
of data.
Fault tolerance is built into the dual ring FDDI network. When an interruption on the primary ring is detected, beaconing is used to determine where
the break occurred. Beaconing is also used to monitor the health of the ring
network token-passing process. Each station on the ring is responsible for
checking the token-passing status of the ring. If a fault is detected by a station,
it transmits a beacon onto the ring. The upstream station receives the beacon
and begins to transmit its own beacon. The downstream station ceases beaconing after receiving a beacon from its upstream station. The process keeps
moving to the next upstream station around the ring until the beaconing station
does not detect a beacon from its upstream station. The fault has been isolated
between the beaconing station and its upstream station. The secondary ring
can then be placed into service by allowing for data traffic flow in the opposite
direction. When the beaconing station detects its own beacon being received
on the primary ring, it is notified that the fault has been isolated and repaired.
Upon receipt of its own beacon, the station shuts off beaconing and returns to
normal service.

95

Page 95

Edwards

96

Part I

2.2.2

■

c02.tex

V3 - 03/27/2009

Networking Nuts and Bolts

A MAN Example

Anytown, USA, considers itself a happening place. Not wanting to miss out
on being part of the ‘‘connected’’ age, the city fathers have launched a plan
to provide computer services to all city departments. In order for the local
citizenry to see their tax dollars at work, they decided as part of the overall
project they would provide Internet access to the general populace. The greater
Anytown metropolitan area spans several miles, with some buildings as far as
five miles away from city hall.
The mayor called in the heads of Anytown’s IS department, told them of his
great vision, and asked how they would go about implementing his great plan.
The IS department managers went away scratching their heads and wondered
how they were to pull this one off. The general thought within the group
was that, since the mayor’s vision was pie in the sky, they would draw up a
proposal that would be doable while still maintaining their control over the
administration of Anytown’s information services.
After several weeks of thrashing about among the IS department’s staff,
the plan was devised and drawn up. The big night arrived, and the chief of
Anytown’s IS department wore his Sunday best for the presentation of the
devised plan to the mayor and the city counselors.
When the slide was placed on the overhead projector, the mayor and
counselors saw what is shown in Figure 2-20.
The IS chief’s explanation went as follows. The main departments within
Anytown’s government already had LAN technology deployed within the
areas they were responsible for. General communication and the passing of
data between departments was being done via e-mail. By implementing a
citywide FDDI network, each department’s LAN would be able to send data
directly from station to station over the newly connected LAN networks.
He went on to explain that servers located on each individual LAN would
be centrally located within the IS department at city hall. Each department
location would be connected directly to city hall via high-speed fiber optic
cable, shown as dashed lines on the MAN network diagram.
He went on to further explain that each department currently was responsible for its own Internet access. With the proposed high-speed fiber optic
network, this could be consolidated under the control of
the city hall IS staff. A single
POP QUIZ
high-speed network connection
IEEE 802.5 limits the number of nodes on a
would give Internet access to
nodes.
ring to
not only all city departments
but also the general public. It
was stated that there would be

11:58am

Page 96

Edwards

c02.tex

Chapter 2

■

V3 - 03/27/2009

11:58am

LANs, MANs, and WANs

security precautions put in place to prevent unauthorized access to servers
maintained by the city.

Fire
Department

Police
Department

Library

City Hall

Internet Service Provider
(ISP)

School Department

School

Figure 2-20 Anytown’s MAN

The local telephone company would be contracted to run the dedicated fiber
optic cable from city hall to the remote buildings over their current cableways
and overhead lines. The general public would have access over wireless links

97

Page 97

Edwards

98

Part I

■

c02.tex

V3 - 03/27/2009

Networking Nuts and Bolts

to access points located throughout the city to ensure that all of Anytown’s
citizenry would have equal access to the Internet service provided by the city.
For those without personal computers or unable to connect to the citywide
wireless network, public access computers would be located at schools and
libraries.
With his presentation completed, the IS chief asked if there were any
questions. The mayor seemed pensive at first and then asked, ‘‘Can you
explain why there is only wireless Internet for the public?’’ The IS chief said,
‘‘Yes, sir, I can.’’ He went on to explain that the infrastructure cost to bring a
wired Internet alternative to all of the city inhabitants would drive costs for
the project beyond reach of the city’s budget. Also, some of the expenses for
the FDDI network could be recouped over time from consolidation of common
services utilized by each city department. Providing a citywide wired public
network would be cost-prohibitive. The IS chief went on to explain that
there were already a few Internet providers servicing the Anytown greater
metropolitan area, and those citizens desiring a wired Internet access were
more likely to already be subscribed to their service or would do so in the
future.
The mayor thanked the IS chief for his presentation. The counselors all voted
their approval, and the mayor began drawing up his new campaign speech on
how he was instrumental in getting Anytown connected.
This example of how a MAN might come about is largely tongue-in-cheek.
However, it does demonstrate that the basic definition of a MAN is a network
that covers a wide geographical area that can be either a city or include the
greater metropolitan area of a city. The feasibility of MAN networks would
not be possible without the availability of high-speed networks such as Metro
Ethernet or FDDI optical networks.
The chief piece of information that the student should take from this section
is the awareness that a high-speed data link is required when connecting LAN
networks located some distance apart. When users and services on both ends
of the link are contending for use of the link, the speed at which the link is
able to pass traffic will be the determining factor of the performance of the
interconnected LAN networks over that link. A safe rule of thumb is the more
bandwidth the better. It gives better performance and allows for future growth
and expansion of the connected LAN networks.

2.3

Wide Area Networks

As discussed in Chapter 1, the main use of a WAN is to provide a high-speed
data network between two geographically distant networks. This chapter will
discuss a few WAN telecommunications services most used in the makeup of
a WAN network.

11:58am

Page 98

Edwards

c02.tex

Chapter 2

WAN networks are constructed from a
wide range of service levels that can be
obtained from the telephone companies.
These can range from slow, low-grade analog circuits to high-speed digital signal services. The most widely used and available
WAN standards are POTS, ISDN, and frame
relay.

2.3.1

■

V3 - 03/27/2009

11:58am

LANs, MANs, and WANs

ACRONYM ALERT
QoS — Quality of service

Whose POTS?

POTS stands for plain old telephone service. It refers to the use of voice-grade
telephone lines to form a point-to-point data connection. Because these
voice lines can be found in many places around the world, it is possible to create a WAN connection between two LAN networks that are far
apart. Figure 2-21 illustrates a dialup modem16 connection between two
offices.
This figure shows two LAN networks, one located in Boston and the other
in Santa Fe. This is a manual WAN connection operation. Each modem can be
set to auto-answer so that when another modem dials in, it will answer the call
and allow the connection to be completed. This is a very rudimentary WAN
network. It works and is still the only available WAN-type connection that can
be made from some very rural areas of the country.
The speed of the WAN connection is determined by the type of modem
and the signal quality of the telephone line it is connected to. Customary
speeds that can be attained are between 28.8 and 57.6 Kbps. There are devices
in the marketplace that automate the dialing process. These are considered
to be dial-on-demand routers. These devices reside on the LAN and will
automatically dial a preprogrammed number when they detect that the data
received from the network is destined for a LAN at the other end of the dialup
WAN connection.
With a clear line and the use of compression, some modem-based devices
are capable of throughput of 115 Kbps. As other access technologies have
rolled out, such as DSL and Internet access over cable and fiber to the home,
modem use has fallen off. These newer technologies can provide higher speed
access to the Internet, but they are unable to provide a point-to-point WAN
connection, which some organizations require. Later in this section we will
discuss how these technologies can be used to provide a virtual point-to-point
WAN connection.
16 Modem

takes its name from modulate/demodulate. It is a device able to both modulate and
demodulate a digital signal into an analog signal that can be sent across standard voice-grade
telephone lines.

99

Page 99

Edwards

100

Part I

■

c02.tex

V3 - 03/27/2009

Networking Nuts and Bolts
Boston

POTS
Network

Santa Fe

Figure 2-21 A POTS WAN connection

2.3.2

Integrated Services Digital Network

Integrated services digital network (ISDN) is a set of standards to provide
voice, data, and video transmission over a digital telephone network. It is
similar to a POTS line and
modem in that it is able to
use existing premises wiring to
POP QUIZ
make a called connection to
What is the major difference between
another ISDN subscriber. HowEthernet and IEEE 802.3?
ever, it can only call another
ISDN subscriber, whereas a
POTS setup can call any number

11:58am

Page 100

Edwards

c02.tex

Chapter 2

■

V3 - 03/27/2009

11:58am

LANs, MANs, and WANs

that has an analog telephone connection to it. By integrating analog and digital
signal transmissions using a digital network, ISDN is capable of delivering
an improved data rate over typical modem connections. Unlike POTS, ISDN
service is mostly concentrated in major metropolitan areas.
Taking advantage of LAN-to-LAN connectivity with ISDN providing the
link can best be accomplished with the use of ISDN routers. They are typically
configured for on-demand dialing. When there is data to be sent from one
LAN to a remote LAN, the router will dial the remote ISDN router. When
the remote ISDN router answers the call, data can be sent across the link.
Since most ISDN service usage is typically billed by the number of calls and
total minutes connected, ISDN routers may utilize an idle timer. This timer
determines when there is no traffic being passed across the link. When the idle
time interval has been reached, the call is terminated. These timers need to be
set properly to eliminate excessive dialing and increased telephone charges. It
is recommended that you understand how your local ISDN provider bills for
this service. It could be by connected minutes, number of calls, or a combination
of both. The only advantage that ISDN has over leased lines is that for low
usage data connections it is cheaper than paying for a point-to-point leased
line connection. ISDN is at a cost disadvantage in situations where the line is
up for great periods of time. In those circumstances, it is best to look into using
a leased line.
The two most commonly found ISDN services are:
Basic rate — Provides two B channels of 64 Kbps and a single D channel
of 16 Kbps.
Primary rate — Provides 23 B channels of 64 Kbps and a single D
channel of 64 Kbps for U.S.- and Japan-based subscribers. Subscribers
in Europe and Australia are provided with 30 B channels.
An advantage that ISDN has over other WAN connection types when
connecting to sites located in other countries is the service levels have been
standardized by the International Telegraph and Telephone Consultative
Committee (CCITT), so subscribers with ISDN service around the globe are
able to interconnect to form a WAN network.

2.3.3

Point-to-Point WANs

In reality, all the WAN connections we spoke of in the two previous sections
are also point-to-point WAN connections even though they require a manual
or automated dial from a modem-based router. For the most part, when people
refer to a point-to-point connection in the telecommunications arena, the first
thought that comes to mind is directly connected point-to-point leased line
connections. Figure 2-22 illustrates an organization with three major offices

101

Page 101

Edwards

102

Part I

■

c02.tex

V3 - 03/27/2009

Networking Nuts and Bolts

located in New York, Los Angeles, and Miami. The amount of data traffic
between these locations warrants dedicated point-to-point WAN connections.
The lines in use are considered to be of the T class variety.
New York

Los Angeles
Telephone
Network

Miami

Figure 2-22 A point-to-point WAN network

Organizations do not only use
these lines for data transmission. The lines can also be used
RANDOM BONUS DEFINITION
for telephone, teleconferencing,
preamble — A frame field used to allow a
and other forms of communireceiver to properly synchronize its clock
cations. The most common serbefore decoding incoming data.
vices used for these T class
connections are T1, fractional
T1, and T3. T1 can provide
1.544 Mbps of speed while T3
can deliver 44.736 Mbps.
A full T1 line provides 24 channels, each with 64 Kbps of bandwidth. When
an organization leases a dedicated full T1 line, they are responsible for the
T1 multiplexer equipment located at each endpoint. They can then dedicate
the channels in any manner they choose. An example of this would be 6

11:58am

Page 102

Edwards

c02.tex

Chapter 2

■

V3 - 03/27/2009

11:58am

LANs, MANs, and WANs

channels dedicated to telephone service, 2 channels for teleconferencing, and
the remaining 16 channels dedicated to moving data between locations. For
organizations with demands for more bandwidth, the option would be to
move up to T3 service. These services are point-to-point through the telephone
network, but the service level is guaranteed by the telecommunications company. The lease cost is determined by the required bandwidth and distance
between locations.
Organizations that require guaranteed throughput between organizations
but do not need the speed of a full T1 can purchase a number of channels
split out from an existing trunk circuit. This does provide a cost advantage,
but it has its downside — the organization does not have control over where
that circuit is routed. Cost is determined by the number of channels required
and the distance between the locations. As the number of channels begins to
increase, the cost advantage of fractional T1 is lost.

2.3.4

Frame Relay

So far, we have talked about WAN circuits being directly connected endpoint
to endpoint, although traveling through a switched telephone network. Those
connections were dedicated to creating a full-time fixed bandwidth connection.
Frame relay17 is designed for data traffic that tends to move in bursts. This is
accomplished by using packet switching in a switched cloud provided by the
telecommunications companies.
Because frame relay lends
itself to burst-oriented traffic,
it is not suitable for real-time
POP QUIZ
applications such as telephones
What are the two most common ISDN
or teleconferencing. As informaservices?
tion is moved in packets, the
service is provided as a committed information rate (CIR). It is
listed as a bandwidth number, but that does not necessarily mean you have
continuous access at that bandwidth.
The level of service is measured for frame relay using a formula that includes
committed burst size (CBS) over an interval of time. The basic formula is as
follows:
Time = Committed Burst Size (CBS) / Committed Information Rate (CIR)

17 Frame relay is based on X.25 packet-switching technology, which was developed to move data
signals that were primarily analog, such as voice conversations. X.25 works in Layers 1, 2, and 3
of the OSI model. Frame relay only uses Layers 1 and 2, giving it greater speed that is about a
factor of 20 over X.25. This is accomplished by dropping packets that are found to be in error and
relying on the endpoints to process packet-drop detection and request retransmission of packets.

103

Page 103

Edwards

104

Part I

■

c02.tex

V3 - 03/27/2009

Networking Nuts and Bolts

To illustrate this further, a customer has chosen a service that provides a
CIR of 64 Kbps and a CBS of 256 Kbps. At first glance, it appears that traffic
can burst up to 256 Kbps, but that is not the case. If CBS is divided by CIR, the
resulting value is four seconds. This means the circuit needs to be capable of
moving 256 Kbps in any four-second interval. This is far different from what
most people think burst rate means. So the CIR and CBS need to be carefully
looked at when subscribing to a frame relay service. If the network burst rate
begins to exceed the CBS, network congestion will occur and data traffic will
be affected. When selecting a frame relay service, it is best to have a good
knowledge of the networks to be interconnected over frame relay. Figure 2-23
illustrates how a frame relay network may be implemented.
Boston

Seattle

T1 Link

Frame Relay
Switched Network

T1 Link

T1 Link
T1 Link
New York

Los Angeles

Figure 2-23 A frame relay network

This figure shows an organization with offices in Boston, New York, Seattle,
and Los Angeles. Each has a T1 connection to the frame relay switched
network. In this figure, each office is connected to every other office within
the frame relay switched network using a private virtual circuit (PVC), which
is illustrated by the dashed lines between each of the nodes connected to
the switched network. This does have an advantage over pure point-to-point

11:58am

Page 104

Edwards

c02.tex

Chapter 2

■

V3 - 03/27/2009

11:58am

LANs, MANs, and WANs

WAN implementations, but it is best suited for burst type traffic and not traffic
requiring a continuous guaranteed rate.

2.3.5

Using the Internet for Your WAN

The Internet is a network mesh that covers most of the globe. So it is possible
to connect remote LAN networks over the Internet. However, the Internet is
really a best-attempt-possible service. It is not guaranteed far as performance
and is open to the public, which makes security a major concern. The chief
advantage of using the Internet over other subscriber services is cost. Other
than local Internet access fees, there are no other charges involved such as
can be found when using a dedicated long line solution. Unlike dedicated
point-to-point services, it is inconsequential how these devices connect to the
Internet. The type of connection to the Internet is not a factor in the creation
of the virtual point-to-point connection. Factors that can affect performance
include the speed of the connection and its reliability where connectivity is
concerned. Although electrons move at the speed of light, intelligent electrical
signals are also subject to latency problems the greater the distance is between
two endpoints of a network.
The solution of using virtual private networks (VPN)18 is only viable in
scenarios that require a remote office to connect to a central office. It is not
intended to replace dedicated high-speed point-to-point network connections.
Data integrity and security are maintained and ensured using encryption
and encapsulation of the data packets that are transmitted over the Internet.
Authentication is used to confirm that an endpoint device or user is fully
authorized to send and receive data from the VPN connection. Figure 2-24
illustrates how VPN connections may be used as a substitute for a dedicated
WAN network connection.
A remote office in Boston is
connected to the corporate office
in New York using the InterPOP QUIZ
net to form its VPN tunnel. This
True or false: Virtual private networking is
is a peer-to-peer tunnel where
networking that does not require any
each endpoint knows the other
hardware at all.
and is part of the security as
the peers are known to each
other. Authentication security is
increased with the use of preshared keys (PSK), and other authentication
methods such as certificates and tokens may also be added. Once the VPN
18 For

further information on how to use VPN tunnels, check out Nortel Guide to VPN Routing for
Security and VoIP, by James Edwards, Richard Bramante, and Al Martin (Wiley Publishing, Inc.,
2006).

105

Page 105

Edwards

106

Part I

■

c02.tex

V3 - 03/27/2009

Networking Nuts and Bolts

tunnel is formed, traffic destined for either LAN is passed through the tunnel
as if it were a dedicated link. The end-user workstations only need to be
concerned with the address of the device on the other LAN. The VPN routers
are the only devices that need to be aware of the endpoint address of its peer
VPN routers. So for this purpose, the peer-to-peer tunnel functions as if a
dedicated point-to-point link is in place between the two LAN networks.
Boston

Remote LAN

VPN Router

Peer to Peer
VPN Tunnel

VPN Router

Client
Tunnel

Remote Client

St. Louis

Central LAN

New York
Figure 2-24 A VPN as a WAN

VPN routers are also able to accommodate end-user tunnel connections.
For this example in Figure 2-24, a user in St. Louis is able to connect to
the central office in New York to gain access to the network and use the

11:58am

Page 106

Edwards

c02.tex

Chapter 2

■

V3 - 03/27/2009

11:58am

LANs, MANs, and WANs

services on that network. Since remote users can contact the central office
from almost anywhere, their endpoint addresses would not be previously
known. However, users are required to be authenticated in the same manner
as a peer-to-peer tunnel, which may include multiple forms of authentication
processes. Once authorized, a user is able to access the services they are
authorized to use. Many installations require additional authentication to
access internal servers. Access to the network does not necessarily mean access
to all devices. VPN routers are capable of applying security policies on both
peer-to-peer and end-user client tunnel connections.
The protocols used for VPN tunneling are Point-to-Point Tunneling Protocol
(PPTP), Layer 2 Tunneling Protocol (L2TP), and IPSec (IP Security).

2.4

Chapter Exercises

1. The term modem is short for

.

2. A
is a network where network devices are located within
close proximity to each other.
3. CSMA/CD is an acronym for
with a network using a

and is associated
network topology.

4. Which network topology allows for orderly network access for the stations connected to that network?
5. What two standards define a CSMA/CD network?

6. Name three media types that can be used to connect devices located on a
LAN?

7. The major characteristic of 10BASE-T cable is:
8. A personal computer (PC) requires a
local area network (LAN).

to be connected to a

9. FDDI is an acronym for
, which is often used
.
to construct citywide networks called
10. POTS is an acronym for

.

107

Page 107

Edwards

108

Part I

■

c02.tex

V3 - 03/27/2009

Networking Nuts and Bolts

11. A dialup service that connects to a digital network is

.

12. What technology can be used to create a point-to-point network connection over the Internet?

2.5

Pop Quiz Answers

1. What are the 2 sublayers of the Data Link layer?
Logical Link Control (LLC) and Media Access Control (MAC)
2. MAC addresses are represented with hexadecimal numbers, separated
by a colon or a hyphen.
3. What is the maximum length of a cable between a workstation and a
hub?
100 meters
4. IEEE 802.5 limits the number of nodes on a ring to 250 nodes.
5. What is the major difference between Ethernet and IEEE 802.3?
Frame format
6. What are the two most common ISDN services?
Primary rate and basic rate
7. True or false: Virtual private networking is networking that does not
require any hardware at all.
False

11:58am

Page 108

Edwards c03.tex

V2 - 03/27/2009

7:38pm

CHAPTER

3

Network Hardware and
Transmission Media
Men have become the tools of their tools.
— Henry David Thoreau

Most Internet users don’t understand the hardware and media used to give
them the freedom they enjoy on the WWW. There are a lot of different types
of nodes that serve specific purposes, as well as different transmission media
types that connect network nodes together. The average Internet user is mainly
concerned that they are able to send that important e-mail and have it get
there, or that they are able to download the new episode of Survivor. For the
average user, the Internet simply is there, and that is fine for them.
The same holds true in today’s workplace. Almost every business uses a
network in some form and in some capacity. Even if a worker does not interface
with a computer, they are probably working off a printout that was generated
electronically and often from a database that connects to . . . you got it — a
network. As long as they have what they need to perform the functions they
need to do, they don’t care what it takes to get the data passed from one point
to the next.
The fact that you are reading this book means you have a reason for learning
how data is transmitted. That means you need to know the information in
this chapter intimately.1 In later chapters, when we refer to a router, you
need to recognize that name and know what it does.2 This chapter provides
an explanation for most of the network hardware that is in use in networks
today. Network traffic and traffic patterns, as well as the cables (or lack of)
used to pass the traffic, are also discussed. After reading this chapter, when
1 This

in no way implies that you don’t need to know the rest of the information in this book.
that, if we kept saying ‘‘node’’ through this whole book, we would all get pretty bored
and probably a little confused. Maybe that is why they got rid of the term ‘‘network’’ — people
simple got bored and confused.

2 Besides

109

Page 109

Edwards c03.tex

110

Part I

■

V2 - 03/27/2009

Networking Nuts and Bolts

someone asks you to explain what ‘‘10 half or 100 full’’ means, you will be able
to explain what they mean, define the difference between the two, and list a
few pros and cons of each.

3.1

Stuff You Just Need to Know

There are a few things you need to have
a basic understanding of before we jump
into this chapter. First, you need to know
what bits and bytes are. Even if you know
ACRONYM ALERT
what bits and bytes are, take a quick skim
SNMPv3 — Simple Network Management Protocol
through this section. We also provide an
version 3
overview of network addressing, encapsulation types, and other technologies we will
be discussing throughout this chapter. If everything seems familiar to you,
please feel free to skip to Section 3.2. If further discussion is required for any
of the information in this section, it will be introduced when appropriate.3 If
you decide to skip to Section 3.2 and later get to a point in this chapter where
you are not sure about something, check back to see if it was explained in this
section.

3.1.1

Bits, Bytes, and Binary

A binary number is a system of numbering used in data communications.
Sometimes referred to as the base-2 number system, the binary numeral system
represents numeric values by a 0 or a 1. The numeral system that we are
all most familiar with is the base-10 number system, often referred to as the
decimal numeral system. The decimal numeral system represents numeric values
by a 0, 1, 2, 3, 4, 5, 6, 7, 8, or 9. Table 3-1 shows a comparison of the decimal
and binary systems.
You can see that the decimal representation of the number ten is 10, whereas
the binary representation is 1010. In the binary system, the numbers are
counted just like they are in the decimal system. Numeric symbols count
incrementally one at a time and when the highest symbol is reached (a 1 in
binary, a 9 in decimal), the number resets to 0 and carries one to the left.
For example, if you count from zero through ten in decimal, it looks like
this: 0, 1, 2, 3, 4, 5, 6, 7, 8, 9, 10. When the highest symbol (9) is reached, the
number carries over a 1 symbol to the left and then resets the first symbol to 0.
If you count zero through ten in binary, it looks like this: 0, 1, 10, 11, 100, 101,
110, 111, 1000, 1001, 1010. In binary, when the highest symbol (1) is reached, it
carries a number to the left and resets, just like in decimal.
3

In fact, this is information you are probably familiar with. We won’t dwell too much on this
section; that way we can have more room to talk about the beefier hardware that moves data in
any given network.

7:38pm

Page 110

Edwards c03.tex

Chapter 3

■

V2 - 03/27/2009

7:38pm

Network Hardware and Transmission Media

Table 3-2 shows some examples of converting decimal numbers to binary.
Table 3-1 Decimal Numbers and Their Binary Number Equivalents
DECIMAL

BINARY

0

0000

1

0001

2

0010

3

0011

4

0100

5

0101

6

0110

7

0111

8

1000

9

1001

10

1010

Table 3-2 Decimal/Binary Conversions
DECIMAL
BINARY

128

64

32

16

8

4

2

1

0

0

0

0

0

0

1

1

Starting from the right of the table, you can reference the decimal symbols with the
binary symbol. The decimal number 3 is equal to (2+1). The binary symbols that
correspond with the decimal symbols being referenced are then set to 1 and all others
are set to 0.
DECIMAL
BINARY

128

64

32

16

8

4

2

1

1

0

0

0

1

0

0

1

Starting from the right of the table, you can reference the decimal symbols with the
binary symbol. The decimal number 137 is equal to (128+8+1). The binary symbols
that correspond with the decimal symbols being referenced are then set to 1 and all
others are set to 0.

111

Page 111

Edwards c03.tex

112

Part I

■

V2 - 03/27/2009

Networking Nuts and Bolts

The symbols that are used in the binary system are known as binary digits,
or bits. The single digit in the binary number is 1 bit (which is a 1 or a 0).
For example, binary number 0100 is 4 bits long. The bit is the basic unit of
information in data communication. It is much like a toggle switch with only
two settings, on (1) or off (0). In data communications, the bit is set based on
electrical levels. A 1 is set if voltage is received, and a 0 is set if there is no
voltage.
There are other terms you will come across that you need to understand
when referencing a group of bits. Eight bits are equal to 1 byte, 1,024 bits are
equal to 1 kilobit (Kbit or Kb), 125,000 bytes are equal to 1 megabit (Mb), and
so on (see Table 3-3).
Table 3-3 Grouping of Bits
SI NAME

BINARY VALUE IN BITS

BINARY NAME (IEC)

Kilobit (Kbit)

210

Kibibit (Kbit)

20

Megabit (Mbit)

2

Mebibit (Mibit)

Gigabit (Gbit)

230

Gibibit (Bibit)

Terabit (Tbit)

240

Tebibit (Tibit)

Petabit (Pbit)

250

Pebibit (Pibit)

Exabit (Ebit)

260

Exbibit (Ebit)

Zettabit (Zbit)

270

Zebibit (Zibit)

YottaBit (Ybit)

280

Yobibit (Yibit)

We have already determined that 8 bits are referred to as 1 byte. To continue,
1,024 bytes is equal to 1 kilobyte (KB or kB), 1,048,576 bytes is equal to 1 megabyte
(MB or Mbyte), and so on (see Table 3-4).

3.1.2

Non-human Resources

There is a vast array of resources in use in a network. Anything that is
used within the network to provide data to the end users (e.g., applications,
operating systems, servers, memory, storage devices, etc.) is considered a
network resource. All the hardware and media discussed throughout this

7:38pm

Page 112

Edwards c03.tex

Chapter 3

■

V2 - 03/27/2009

7:38pm

Network Hardware and Transmission Media

chapter are network resources. In this section, we refer to the processing and
storage resources used by the nodes in a network.
Table 3-4 Grouping of Bytes
SI NAME

BINARY VALUE IN BYTES

BINARY NAME (IEC)

Kilobyte (KB, kB)

210

Kibibyte (KiB)

Mebibyte (Mbyte)

220

Mebibyte (MiB)

Gigabyte (Gbyte)

230

Gibibyte (GiB)

Terabyte (Tbyte)

240

Tebibyte (TiB)

Petabyte (Pbyte)

250

Pebibyte (PiB)

Exabyte (Ebyte)

260

Exbibyte (EiB)

Zettabyte (Zbyte)

270

Zebibyte (ZiB)

Yottabyte (Ybyte)

280

Yobibyte (YiB)

Network resources can be classified as volatile or nonvolatile.
vol·a·tile4
adjective
1: readily vaporizable at a relatively low temperature
2: flying or having the power to fly
3: a: lighthearted
b: easily aroused 
c: tending to erupt into violence
4: a: unable to hold the attention fixed because of an inherent lightness
or fickleness of disposition
b: characterized by or subject to rapid or unexpected change
5: difficult to capture or hold permanently
non·vol·a·tile5
1: not volatile:
a: not vaporizing readily
b: of a computer memory : retaining data when power is shut off

4 volatile.

(2008). In Merriam-Webster Online Dictionary. Retrieved May 14, 2008, from

www.merriam-webster.com/dictionary/volatile
5 nonvolatile.

(2008). In Merriam-Webster Online Dictionary. Retrieved May 14, 2008, from

www.merriam-webster.com/dictionary/nonvolatile

113

Page 113

Edwards c03.tex

114

Part I

■

3.1.2.1

V2 - 03/27/2009

Networking Nuts and Bolts

Volatile Memory

Data storage is performed by a
storage device or memory that
is set aside for the storage of
POP QUIZ
data for a nonpermanent period
The decimal number 211 is equal to what
of time. In other words, a device
binary number?
receives and reviews data, processes it, and then moves on to
the next data process. It uses
volatile memory or storage in order to perform this action. Once the data is
no longer needed, it can be removed and new data can take its place. When
power is removed, volatile memory does not retain its data.
3.1.2.1.1 Random Access Memory

Random access memory (RAM)6
is the most well known form
of memory in the data environRANDOM BONUS DEFINITION
ments. It is called random access
data storage density — The quantity of data
memory because it is memory
that can be stored within a data storage
that is available for data storage
medium.
and access, regardless of the order
in which it is stored. Information
stored in RAM is accessible until
it is cleared out or the device it is being used on is shut down.
Computers store OS and system data in RAM when the computer boots
up. The remaining space that is not used by the system software is utilized as
programs are accessed and used on the computer. Data access is quicker with
data that is stored in RAM than any of the other storage devices a computer
may use.
3.1.2.1.2 Dynamic Random Access Memory

Dynamic random access memory (DRAM) is the type of RAM that is used as
the main memory by most PCs. DRAM has to have a little jolt of electricity
every couple of milliseconds in order to operate. DRAM uses a transistor and
a capacitor for each storage cell it contains. Each received bit is stored in a cell.
As the capacitor loses its charge, an electronic charge refreshes the capacitor.
6A

lot of companies are working on a nonvolatile form of RAM. This will speed up the boot-up
and shutdown times of a device, and will save energy as well. As more and more companies
are releasing ‘‘green’’-friendly devices, this technology may debut soon (maybe even before this
book is released).

7:38pm

Page 114

Edwards c03.tex

Chapter 3

■

V2 - 03/27/2009

7:38pm

Network Hardware and Transmission Media

DRAM is considered high density because it is able to store more data
than other memory types. This is because each storage cell only requires one
capacitor and transistor. Examples of DRAM modules include:
Dual inline memory module (DIMM) — Designed for use in
personal computers, miscellaneous workstations, and servers.
Single inline memory module (SIMM) — Used in personal computers
prior to the late 1990s.
Single inline pin package (SIPP) — Used in older computers that had the
Intel 80286 processor.
Synchronous dynamic random access memory (SDRAM) — DRAM
with a serial interface, which allows the memory to accept
new instructions while it is still processing previous instructions. Used in computers, workstations, and servers.
3.1.2.1.3 Static Random Access Memory

Static random access memory
(SRAM) uses electronic circuitry
to store bits in memory. SRAM
POP QUIZ
does not need to be charged,
The binary number 01011100 is equal to
as there are no capacitors being
what decimal number?
used to store the bits. SRAM
cells maintain one of two states,
either a 0 or a 1. SRAM is most
commonly used as the cache memory for most microprocessors, storing up to
several MBs of data. Device system registers will also often use SRAM as the
mode of memory.

3.1.2.2

Nonvolatile Memory

Memory that can retain data even when it is not receiving power is known as
nonvolatile memory. Nonvolatile memory is used as a secondary storage device.
This is where data that needs to be stored for long periods of time is located,
such as configuration files, OS software, and systems software. For the most
part, nonvolatile memory is slower in moving data than volatile memory. This
is the main reason that nonvolatile memory is used for storage.
3.1.2.2.1 Magnetic Storage Media

You might use magnetic storage a lot more than you are aware of. Not only are
computer hard disk drives and backup tape drives (and a few other storage

115

Page 115

Edwards c03.tex

116

Part I

■

V2 - 03/27/2009

Networking Nuts and Bolts

devices) magnetized data storage devices, magnetic storage is used in the
audio and video world as well. As a matter of fact, the strip on the back of
your debit and credit cards is magnetic storage for identification data that
communicates with the card reader used when you purchase something.7
Data stored on electronic media can be removed and the space that it was
occupying can be reused for other data. Data is written onto the medium with
electrical impulses that set a bit to either positive or negative polarity. When
data is accessed, the polarity of the bit is read, and the setting of the bit (1 or 0)
is determined.
3.1.2.2.2 Read-Only Memory

Memory used to store information that is not intended to be modified is known
as read-only memory (ROM). ROM is often referred to as firmware, which is
the software required for hardware-specific operations. ROM chips can retain
this data even without electricity applied to the device. There are arrays of
different ROM chip types; among these are:
Read-only memory (ROM) — Memory that is configured
and set by the manufacturer. It contains device systems software necessary for the proper operation of the device.
Programmable read-only memory (PROM) — A memory chip
that can be written to only once. This will allow someone other
than the manufacturer to write data onto the PROM. Just like
ROM, the data is there forever. A device known as a PROM
programmer (PROM burner) is used to write the data onto the
chip.
Erasable programmable read-only
memory (EPROM) — A memory
chip that can store data that may
need to be overwritten at some point.
The data on the EPROM is erased
by UV light and can then be reprogrammed with a PROM burner.

ACRONYM ALERT
PCMCIA — Personal Computer Memory Card
International Association

Electrically erasable programmable read-only memory (EEPROM)8 — A memory chip that can store data that may need to
be overwritten at some point. The data on the EEPROM is erased
by an electrical charge and can then be reprogrammed with a PROM
burner.
7 You
8 Say

can now ‘‘pay at the pump,’’ thanks to magnetic storage.
that five times real fast!

7:38pm

Page 116

Edwards c03.tex

Chapter 3

■

V2 - 03/27/2009

7:38pm

Network Hardware and Transmission Media

3.1.2.2.3 Flash Memory

Flash memory is a form of EEPROM that is used by a device for specific
storage purposes. Digital cameras, video gaming systems, laptops, many
network devices, and PCs all use flash memory. Examples of flash memory
are:
Memory cards for cell phones
Memory cards for digital cameras
Memory cards for video game systems
PCMCIA9 type 1 memory cards (3.3 mm thick)
PCMCIA type 2 memory cards (5.0 mm thick)
PCMCIA type 3 memory cards (10.5 mm thick)
Personal computer system BIOS chip
PC BIOS memory chips are
the most commonly used fixed
type of flash memory. The
POP QUIZ
other types of flash memory
What is the binary name for the binary
are removable and can hold a
value of 250 ?
lot of data. When feasible, flash
memory is preferred over hard
disk drive memory because it is
faster, smaller, lighter, and does not have any moving parts. On the downside,
flash memory is more expensive when comparing the cost of an equal amount
of storage space on a hard drive.

3.1.3

Encapsulation

Encapsulation is the act of including data from an upper-layer protocol within
a structure in order to transmit the data. As we discussed in Chapter 1, most
applications use either TCP or UDP. If data is transmitted from the Application
layer, the data that needs to be transmitted is passed to the Transport layer.
Let’s say that TCP is the protocol that is used. TCP adds a TCP header to the
datagram and then the datagram is passed to the Network layer where it is
encapsulated into an IP packet. The packet is then passed to the Data Link
layer where it is encapsulated into a frame (Ethernet, Token ring, etc.) and
then transmitted over the physical media to a destination. Figure 3-1 shows an
example of this.
9 Many people still refer to this type of memory card as a PCMCIA card. This is actually no
longer the appropriate term. PCMCIA memory cards are now simply called PC cards.

117

Page 117

Edwards c03.tex

118

Part I

■

V2 - 03/27/2009

Networking Nuts and Bolts

Application

Data

TCP
Header

Transport

IP
Header

Network

Data Link

Frame
Header

TCP Data

IP Data

Frame Data

Frame
Footer

Figure 3-1 Encapsulation

Information passed from layer to layer is called service data units (SDUs)
or protocol data units (PDUs). The difference between an SDU and a PDU is
that the PDU specifies the data that is to be transmitted to the peer layer at the
receiving end. The SDU can be considered the PDU payload. Recall from the
paragraph above, data is transmitted from Layer 7 to Layer 4, from Layer 4 to
Layer 3, and so on. The data that is put together to be passed from Layer 7 to
Layer 4 is the PDU. The SDU is what it becomes when it is encapsulated into
the PDU of the lower layer. Figure 3-2 shows an example of what PDU is used
at each layer in the OSI reference model.
Each layer within the OSI reference model creates a PDU for any data that
needs to be transmitted to the next lower level. In addition to the data in the
PDU, each layer assigns a header to the PDU as well. Refer now to Figure 3-3.
Data is being transmitted from Layer 7 to Layer 1, across a medium to the
Physical layer on the opposite end, and then up each layer until it reaches
Layer 7. Notice that each layer appears to communicate directly to the layer on
the opposite end. When each layer passes data to the layer below it, the data
(including the higher layer header) becomes an SDU. When the layer attaches
its header to the SDU, it becomes the PDU that is transmitted to the next lower
layer.

7:38pm

Page 118

Edwards c03.tex

Chapter 3

Layer

PDU

Application

Data

Presentation

Data

Session

Data

Transport

Segment

Network

Packet

Data Link

Frame

Physical

Bit

■

V2 - 03/27/2009

Network Hardware and Transmission Media

Figure 3-2 PDUs used at each layer in the OSI reference model

Application

Layer 7
Header

Data

Application

Presentation

Layer 6 Layer 7
Header Header

Data

Presentation

Layer 5 Layer 6 Layer 7
Header Header Header

Data

Session

Transport

Layer 4 Layer 5 Layer 6 Layer 7
Header Header Header Header

Data

Transport

Network

Layer 3 Layer 4 Layer 5 Layer 6 Layer 7
Header Header Header Header Header

Data

Network

Data Link

Layer 2 Layer 3 Layer 4 Layer 5 Layer 6 Layer 7
Header Header Header Header Header Header

Data

Physical

00101110010110001101111011011001110101

Session

7:38pm

Figure 3-3 Layer-by-layer encapsulation

Layer 2
Footer

Data Link

Physical

119

Page 119

Edwards c03.tex

120

Part I

■

V2 - 03/27/2009

Networking Nuts and Bolts

3.1.4 Data Communication Equipment
and Data Terminal Equipment
Data communication predominately takes place between
RANDOM BONUS DEFINITION
nodes that are known as either
data communication equipstraight-through cable — A twisted pair
ment (DCE)10 or data terminal
cable that is wired for normal DTE to DCE
communications.
equipment (DTE). In order for
crossover cable — A twisted pair cable that
communication to take place
is reverse-wired for DCE-to-DCE or
between nodes, one end of the
DTE-to-DTE communications.
connection must be a DCE and
the other a DTE. If you have to
connect a DCE to a DCE or a
DTE to a DTE, a null modem11 or a crossover cable12 must be used. The plug
connector of a hub (see Section 3.3.4) or a modem would be an example of a
DCE, whereas the plug connector on an NIC card (see Section 3.3.2.2) would
be an example of a DTE.
In data communications, synchronization between nodes is known as clocking. The DCE is responsible for providing the clock signal while the DTE is
responsible for synchronizing its clock based on the signal received. The DCE
uses what is called internal clocking, setting the clocking without any outside
influence. The DTE uses external clocking, which requires a signal in order to
set and synchronize its clocking.

3.1.5

All Your Base Are Belong to Us13

We don’t want to jump into Ethernet signaling at this point (Chapter 6,
‘‘Ethernet Concepts,’’ will cover this in depth). We do want to introduce some
terms that you will come across in this chapter (10BASE-T, 100BASE-TX, etc.),
so you will understand what they mean.
Baseband simply refers to the way data is transported on the wire. A baseband
signal is data that transported as digital data on an unmultiplexed channel
over the transmission medium. The BASE in the term 10BASE-T stands for
broadband. The number preceding BASE is the speed (for instance, 10BASE
means that the transmission medium can support Ethernet transmission at a
10

DCEs are also often called data carrier equipment.
cables that crosslink the transmit and receive wires. Also can be an adapter that is used
to cross the signals.
12 Normally a crossover cable is an Ethernet cable that is reverse-wired on each end. This will put
all output signals on one end of the cable to be the input signals on the other, and vice versa.
This is appropriate for other technologies, but is most common in Ethernet.
13 If you are an Internet gamer, you are probably familiar with this slogan. This broken English
translation appeared in a European release of the Japanese video game Zero Wing.
11 Serial

7:38pm

Page 120

Edwards c03.tex

Chapter 3

■

V2 - 03/27/2009

7:38pm

Network Hardware and Transmission Media

speed of 10 Mbps over baseband). All symbols following BASE identify either
a distance of transmission or a medium type (5 for 500 meters, T for twisted
pair, F for fiber optic).

3.1.6

Computer Buses14

Computers can be modified and any hardware that is added to the computer is known
as a peripheral. New peripherals come with
ACRONYM ALERT
software, known as a driver, that is loaded
PCI — Peripheral component interconnect
on your PC and provides the instructions
the computer will use to learn what it
needs to communicate and coexist with the
peripheral. Within the computer, there is a system that can logically connect
multiple peripherals within the same set of wires. This system is known as
the computer bus. Computer buses are also used to connect computer internal
components (more on this in a minute15 ).
A computer bus can operate as both a parallel bus and a serial bus. What’s the
difference? Glad you asked. Parallel buses transmit several bits of data at the
same time, in parallel on the bus, whereas serial buses transmit data one bit at
a time, sequentially to the destination. The main types of computer buses are
an internal bus and an external bus. The internal bus is the bus that is contained
within the computer and connects internal components to the shared bus; an
external bus is a bus that connects peripherals to the motherboard.

3.1.7

IP Addressing

Nodes in a TCP/IP network are assigned a numeric value, known as an IP
address. We will be discussing IP addressing throughout this book, so this is
a short overview. The IP address usually is unique and provides a network
identify for the node. Although there are new versions of IP that are growing
in popularity, currently16 IP version four (IPv4) is still what the majority of
networks are using.
An IPv4 address is a 32-bit number that is divided into four fields, called
octets, separated by dots. Each octet represents 8 bits of the total 32-bit number.
This is known as dotted decimal notation. An example of dotted decimal
14 Not

to be confused with a commuter bus.
This actually may take more or less than a minute. It depends on how fast you can
read and how many breaks you take.
16 IPv4 is popular at the time of this writing, although this may change in the near future, as a lot
of new vendor implementations are using IPv6.
15 Disclaimer:

121

Page 121

Edwards

122

Part I

■

c03.tex

V2 - 03/27/2009

Networking Nuts and Bolts

notation would be the IP address 192.168.1.1.17 The meaning of the octet that is
represented by each number depends upon what network class the IP address
belongs to. The entire IP address is separated into two parts: the network part
and the host part. Figure 3-4 shows an example of the difference in network
classes.
Class A
42

10

64

Network bits

114

Host bits
Class B
23

142

107

Network bits

14

Host bits
Class C

192

168

11

Network bits

122

Host bits

Figure 3-4 IP address network classes

The four18 network classes are as follows:
Class A — Class A addresses are identified by a number from 1
to 126 in the first octet. In Class A addresses, the first octet identifies the network and the remaining three octets identify the host.
These addresses are normally assigned to larger networks.
Class B — Class B addresses are identified by a number from 128
to 191 in the first octet. In Class B addresses, the first two octets
identify the network and the last two identify the host. These
addresses are normally assigned to medium-sized networks.
Class C — Class C addresses are identified by a number from 192
to 223 in the first octet. In Class C addresses, the first three octets
17 IP

addresses are identified in decimal (dotted decimal notation, to be specific). If converted to
binary, this number is 11000000.10101000.00000001.00000001 (note that there are 8 bits in each
field).
18 There is also a Class E network class, but it is not an approved standard and is experimental.

7:38pm

Page 122

Edwards c03.tex

Chapter 3

■

V2 - 03/27/2009

7:38pm

Network Hardware and Transmission Media

identify the network while the last octet identifies the host. These
addresses are normally assigned to small to medium-sized networks
Class D — Class D addresses are a little different than the other classes.
Class D addresses are used for multicasting. These addresses always
begin with the first 4 bits being 1110 and the remaining 28 bits identifying the network in which the multicast message is to be sent.
DID YOU JUST NOTICE THAT?
If you were paying attention during the previous discussion of IP network
classes, you may have noticed that the number 127 is skipped in the transition
from Class A (first octet containing 1–126) to Class B (first octet containing
128–191). This is because the number 127 in the first octet represents a special
type of IP address called a loopback address. Used mainly for troubleshooting,
the loopback IP simply loops datagrams back to the sender.
Some other special IP addresses include:
◆ 0.0.0.0 — Default network (where packets go when the router doesn’t know
where a host is)
◆ 1.1.1.1 — Broadcast to all on a specified network

3.2

Transmission Media

Transmission media refers to the
modes and materials by which
the data is transferred in a
POP QUIZ
network. Network cables, light
Define RAM.
waves, and so on are all considered transmission media. (If
you are referring to more than
one medium, it is called media.19 ) Transmission media provide a way for data
to be passed from one endpoint to another. The medium does not guarantee delivery nor is it concerned with what information is contained in the
datagram; it simply provides the path for the data.
In the United States, there are two forms of transmission media in data
communications. The first type, bounded or guided, is a communication line
(or any other type of solid medium) that transports waves from one endpoint
to another. The second type, unguided or wireless, is where data is passed
wirelessly from one access point (antenna) to another.
19 Another

one of those terms that is often misused but always understood.

123

Page 123

Edwards c03.tex

124

Part I

3.2.1

■

V2 - 03/27/2009

Networking Nuts and Bolts

Network Cabling

Wireless communication as a transmission medium is becoming more and
more popular, but network cabling is still the backbone of any network. There
are many different types of cabling, each serving a specific purpose to meet the
needs of the network. Often you will find different types of cabling running
side by side between nodes in the network. It’s important to understand
the cabling types that are in use on any network you configure and how to
maintain them. The major cable types are:
Twisted pair
Coaxial
Fiber optic
The type of cabling that is used depends on the network. Data traffic
requirements, the size of the network, the topology of the network, the
protocols in use, the nodes in place, cost considerations, and many other
things need to be taken into account when designing and/or maintaining a
network. In this section, we will discuss the more popular cable types and how
they work.
TIPS FOR INSTALLING AND REPLACING CABLES
Whenever you need to replace cables, or are tasked with designing and
implementing a cable run, there are a few hints you should be aware of that
will save you headaches in the future.
1. Use cable ties to keep cables grouped together. Do not use tape, staples,
glue, rubber bands, etc. The cable ties are easy to work with and easy to
remove when you need to.
2. Make sure to label the cables on each end of the link. It can be very time
consuming to try to track down a problem if the cables are not labeled.
Tape, glue, and even rubber bands work well for this task. Staples or tacks
do not.
3. Keep the cable off the floor. If you do not have a choice, then
make sure you cover the cable with a cable protector.
4. Stay away from anything that may cause electrical interference.
5. Cut your cables too long on purpose — leave some excess (on both ends) to
work with in the future.
6. Make a detailed drawing of the cables that are installed in
the building. The drawing needs to be easy to understand
when tracking cable routes and endpoint connections.
(continued)

7:38pm

Page 124

Edwards

Chapter 3

■

c03.tex

V2 - 03/27/2009

7:38pm

Network Hardware and Transmission Media

TIPS FOR INSTALLING AND REPLACING CABLES (continued)
7. Implement a ‘‘hands-off’’ policy for end users. Make sure you
know who is touching the cables and interfaces attaching
end-user nodes to the network. This is especially important
in coaxial runs. One glitch and all the users go down.

3.2.1.1

Twisted Pair Cable

Twisted pair cabling consists of two or more pairs of conductors that are
twisted together within the cable. The conductors are wrapped in plastic
and then all of the pairs are wrapped within the cable, making them less
susceptible to outside electrical interference. Twisted pair cables are used
primarily in areas with short to medium distances between nodes. Twisted
pair is less expensive than coaxial cable or fiber cable, and is often used as a
consideration in network design.
There are four pairs of twisted wires in a network Ethernet cable. These
are color coded in blue, brown, green, and orange. Each twisted pair has one
solid and one striped wire. Here is a list of the wires that are within a normal
twisted pair cable:
Blue
Blue/white
Brown
Brown/white
Green
Green/white

POP QUIZ
Define encapsulation.

Orange
Orange/white
There are two main types of twisted pair cabling in use in LANs. Unshielded
twisted pair (UTP) is the most popular copper cable type. Shielded twisted
pair (STP) is the other type. Ethernet and Token Ring both use twisted pair
cabling.
Unshielded twisted pair — UTP cabling is the type of copper
cabling that is used the most in networks today. UTP cables consist of two or more pairs of conductors that are grouped within
an outer sleeve. Figure 3-5 shows an example of a UTP cable.

125

Page 125

Edwards c03.tex

126

Part I

■

V2 - 03/27/2009

Networking Nuts and Bolts

Figure 3-5 UTP cable

UTP cable is often referred to as Ethernet cable, because Ethernet is the predominate technology that uses UTP cable.
UTP cabling is cheap, but does not offer protection from electrical interference. Additionally, bandwidth is limited with
UTP in comparison with some of the other cable types.
Shielded twisted pair — STP cabling is a type of copper cabling that
is used in networks where fast data rates are required. STP cables
consist of two or more pairs of conductors that are grouped together
and then an additional metal shield wraps around the twisted pairs,
forming an additional barrier to help protect the cabling. Finally, all
of the cables are grouped together and a final outer sleeve is placed
over the wiring. Figure 3-6 shows an example of an STP cable.

Figure 3-6 STP cable

STP cables are also referred to as Ethernet cables. STP cables provide
additional protection to the internal copper, thus data rates are
increased and more reliable. The conductors that are grouped together
can be shielded as individual pairs (in other words, each pair will
have its own shield), or all pairs can be shielded as a group.
The ANSI/TIA/EIA-568-B standard, Commercial Building Telecommunications Standard, is the standard that defines the requirements for installing and

7:38pm

Page 126

Edwards c03.tex

Chapter 3

■

V2 - 03/27/2009

7:38pm

Network Hardware and Transmission Media

maintaining cabling systems, component, and data transmissions in commercial buildings. In the standard, the categories (Cat) of twisted pair cabling are
outlined. As of the release of the ANSI/TIA/EIA-568-B standard, the only
categories that are recognized by the standards are Cat 5e and above.20 Table
3-5 lists all the categories, but you need only to know they exist. You should
focus on Cat 5e and above, as this is the direction the data world is heading.
Table 3-5 ANSI/TIA/EIA-568-B Standard Categories
CATEGORY

ANSI/TIA/EIA568-B STATUS

USED FOR

PERFORMANCE

Cat 1

Unrecognized

ISDN, ISDN basic rate
interface (BRI), doorbell
wiring, POTS voice
communication

Less than or equal to 1
Mbps

Cat 2

Unrecognized

Token Ring

4 Mbps

Cat 3

Unrecognized

10BASE-T Ethernet

16 MHz

Cat 4

Unrecognized

Token Ring

20 Mbps

Cat 5

Unrecognized

100BASE-T Ethernet

Less than or equal to
100 MHz

Cat 5e

Recognized

100BASE-T and 1000BASE-T Less than or equal to
Ethernet
100 MHz

Cat 6

Recognized

Backward compatible to Cat Less than or equal to
3, Cat 5, and Cat 5e cabling; 250 MHz
10BASE-T, 100BASE-TX, and
1000BASE-T Ethernet

Cat 6a

Recognized

10GBASE-T Ethernet

Less than or equal to
500 MHz

Cat 6e

Recognized

10GBASE-T Ethernet

Less than or equal to
625 MHz

Twisted pair cables can be hard-wired to endpoints or attached to a registered jack (RJ) connector. The most common connector is often referred to as
an RJ45 connector. The RJ45 connector resembles the connector for land-based
telephones, only larger. If you have plugged your PC into a network, then you
plugged in an RJ45 (see Figure 3-7).
20 This

does not mean that other categories are no longer in use. They probably are and will be
in networks that never change (which are rare). It simply means there are no plans to advance
the category (and you can bet there are not a lot of vendors out there that will continue to build
based on Cat 5 and below technology).

127

Page 127

Edwards c03.tex

128

Part I

■

V2 - 03/27/2009

Networking Nuts and Bolts

Figure 3-7 An 8P8C plug (RJ45)

STUFF YOU JUST HAVE TO KNOW
Let’s take a moment to talk a little about registered jacks. A registered jack (the
RJ in RJ45) is simply a standardized network interface. The pattern of the
wiring, as well as the construction of the jack itself, is based on the standard
for which the jack was developed. Although we have written mostly about the
RJ45 in this chapter, this does not imply that the RJ45 is the only type of
interface you will come across. So we have provided the following handy-dandy
reference list for your information.
◆ RJ11 — Used for telephone wires. If you pick up a phone (land line, of
course) and look at the wire that plugs into the phone, you are most likely
looking at an RJ11 connector.
◆ RJ14 — Same as above, but for two lines instead of one.
◆ RJ25 — For three lines.
◆ RJ61 — For four lines.
◆ RJ48 — Tor T1 and ISDN lines.
◆ RJ49 — Tor ISDN BRI lines.
◆ RJ61 — For twisted pair cables.

The term RJ45 refers to what is normally
attached to any 8 Position 8 Contact (8P8C)
jacks and plugs, but the true RJ45 standard
defines the mechanics of the interface as
well as a wiring scheme that does not match
the ANSI/TIA/EIA-568-B standard. There
are two parts to the 8P8C: the plug and the
jack. The plug is what was referred to in

ACRONYM ALERT
HDLC — High-Level Data Link Control

7:38pm

Page 128

Edwards c03.tex

Chapter 3

■

V2 - 03/27/2009

7:38pm

Network Hardware and Transmission Media

Figure 3-7 and is often called the male connector or male plug. The jack is the
interface that the plug goes into and is called the female connector or female jack.
There are eight pins, numbered 1 through 8 in an RJ45 connector. Sometimes
these are labeled on the plug. If they are not labeled, you can identify the pin
numbers by holding the connector in your hand with connector pins facing
upward and outward. The pin that is closest to you will be pin number 1 and
then they are sequentially numbered through pin number 8. (See Figure 3-8.)
Pin 1-

Pin 8-

Figure 3-8 RJ45 pin numbering

ANSI/TIA/EIA-568-B defines the pin to twisted pair definitions for pin
assignments when connecting the twisted pair to the 8P8C connector. The
definition of the pin/pair assignment21 is named T568A and T568B.22 The
standard to use depends on the 8-pin cabling system that is in use. T568A
and T568B define the order in which twisted pairs should be attached to the
8P8C adapter. Table 3-6 shows an example of the cable pin-outs for a T568A
straight-through cable.
The difference between the T568B pin-out definitions and the T568A pin-out
definitions is that the green pair and the orange pair are reversed. Table 3-7
shows the pin-outs for T568B.

3.2.1.2

Coaxial Cable

Coaxial cabling is not as popular as twisted pair cabling, but there still are some
networks that use it.23 Figure 3-9 shows an example of a coaxial cable. Within
the cable, there is either a single inner conductor or group of conductors that
are twisted together to form one. The conductor is then wrapped in a plastic
sleeve, which is wrapped in a metallic conducting shield. Finally, these are
all wrapped in an insulating sleeve. There may be a slight variation between
cable vendors, but the functions of the coaxial cable remain the same.
21 The

pin/pair assignment is often referred to as the cable pin-outs.
is not to be confused with the standard ANSI/TIA/EIA-568-B.
23
Most of these were networks that were built in the late 1980s and early 1990s. Most new
deployments use twisted pair.
22 T568B

129

Page 129

Edwards c03.tex

130

Part I

■

V2 - 03/27/2009

Networking Nuts and Bolts

Table 3-6 T568A Straight-Through Pin-Outs
8P8C PIN
NUMBER

WIRE COLOR

10BASE-T
100BASE-T SIGNALING

1000BASE-T
SIGNALING

1

Green/white

Transmit+

Bidirectional data
A+ (BI DA+)

2

Green

Transmit–

Bidirectional data
A– (BI DA–)

3

Orange/white

Receive+

Bidirectional data
B+ (BI DB+)

4

Blue

Not used

Bidirectional data
C+ (BI DC+)

5

Blue/white

Not used

Bidirectional data
C– (BI DC–)

6

Orange

Receive–

Bidirectional data
B– (BI DB–)

7

Brown/white

Not used

Bidirectional data
D+ (BI DD+)

8

Brown

Not used

Bidirectional data
D– (BI DD–)

Table 3-7 T568B Straight-Through Pin-Outs
8P8C PIN
NUMBER

WIRE COLOR

10BASE-T
10BASE-T SIGNALING

100BASE-T
SIGNALING

1

Orange/white

Transmit+

(BI DA+)

2

Orange

Transmit–

(BI DA–)

3

Green/white

Receive+

(BI DB+)

4

Blue

Not used

(BI DC+)

5

Blue/white

Not used

(BI DC–)

6

Green

Receive–

(BI DB–)

7

Brown/white

Not used

(BI DD+)

8

Brown

Not used

(BI DD–)

7:38pm

Page 130

Edwards c03.tex

Chapter 3

■

V2 - 03/27/2009

7:38pm

Network Hardware and Transmission Media

Figure 3-9 An example of coaxial cable

The inner conductor and the conducting shield work on the same axis
and work together to pass data — hence the name co (cooperative) and axial
(running on the same axis). Data is transmitted in the space between the inner
conductor and the outer conducting shield. Coaxial cables are best suited for
high-frequency or broadband signaling.
The connectors that are used to connect coaxial cable runs are known as
bayonet Neill-Concelman (BNC) connectors. There are two main types of coaxial
cabling, thin coaxial and thick coaxial, often referred to as thinnet and thicknet.
When used for Ethernet, they are called thin Ethernet (10BASE2) and thick
Ethernet (10BASE5).
Thin coaxial cabling, known as RG-58, is used for connections that use a low
power signal. In Ethernet, the maximum distance that data can be transmitted
is 185 meters. A node must be placed within that distance, or data corruption
and deletion may occur. Thick coaxial cabling, known as RG-8, is used for
connections that require a higher power signal. The maximum travel distance
between nodes using thick coaxial cables is 500 meters.

3.2.1.3

Fiber Optic Cable

When used in data networking, fiber optic cables are groups of thin strands
of glass or transparent plastic that is able to carry data for long distances. The
fibers are grouped together to form the core of the cable. The core is wrapped
in a cladding, which is denser glass material that reflects light back to the core.
Surrounding the cladding is a buffer. Finally, there is an outer wrap called a
jacket that helps protect the core from damage. Fiber optic cable has helped
make a lot of the advances in networking over the last few years. The use of
fiber cables provides for an increase in the distance data can travel between
nodes, as well as speeds that are, well, as fast as light.24 Optical signaling is not
hampered by electronic interference, so data loss is not seen as often as with
twisted pair or coaxial.
Fiber optic cabling works by sending reflections of light from one endpoint
to another. The light travels between the core and the cladding and back again.
The cladding reflects the light back to the core, much like a mirror does if you
shine a light into it. This is known as total internal reflection (see Figure 3-10).
24

Light signals can be transmitted at speeds of up to 40 Gbps.

131

Page 131

Edwards c03.tex

132

Part I

■

V2 - 03/27/2009

Networking Nuts and Bolts

Light
Cladding

Core

Figure 3-10 Total internal reflection in a fiber optic cable

Fiber optic cables are advantageous as a transmission medium for fast data
exchange over long distances. Fiber optic cabling can also save space in a
LAN as it requires less space than copper cables. There are two main types, or
modes, of fiber optic cabling used for data communications: single-mode fiber
(SMF) and multi-mode fiber (MMF).
Single-mode fiber optical cabling — SMF cables are thinner
than MMF cables. This is because SMF cables are designed to
carry a single beam of light. Because there are not multiple
beams involved, the SMF cable is more reliable and supports a
much higher bandwidth and longer distances than MMF cables.
The bulk cost of SMF cabling is much less expensive than MMF
cabling. Figure 3-11 shows an example of an SMF cable.
Cladding

Light

Core

Figure 3-11 Single-mode signaling

Multi-mode fiber optical
cabling — MMF cabling
POP QUIZ
is made for shorter disWhat is IEEE Standard 802.11?
tances. Unlike SMF, there
are multiple beams of
light, so the distance and
speed are less. Granted, supporting data rates of up to 10 Gbps for
distances as far as 300 meters is nothing to sneeze at. Because of the
additional modes, MMF cabling is able to carry much more data at
any given time. Figure 3-12 shows an example of MMF cabling.

7:38pm

Page 132

Edwards c03.tex

Chapter 3

■

V2 - 03/27/2009

7:38pm

Network Hardware and Transmission Media

Light
Cladding

Core
Multiple Modes

Figure 3-12 Multi-mode signaling

3.2.2

Wireless Communication

Wireless communication has really grown in the past few years. Many businesses, universities, and even some cities have now implemented wireless
access for anyone to use. There is nothing like being able to sit in a bookstore
or a coffee shop and being able to connect to the Internet and all that it offers.
Signals in wireless communication are sent via antennas, microwave stations,
satellite, or infrared light.
Wireless communication enables data to be transferred through the air via a
communication signal. Communication is normally handled by infrared light
or high-frequency radio waves. Infrared communication normally takes place
between nodes. The wireless signal between a PDA and a PC is an example
of nodes that use an infrared signal. Data communications, radio, and cellular
phones are all examples of nodes that use radio waves for data communication.
Section 3.3.3.9.3 covers the hardware that makes wireless communication as a
transmission medium a reality.

3.3

Network Hardware

A lot of different types of network hardware work together25 to issue, pass,
respond, receive, and otherwise transmit data in a network. Network hardware
performs the operations necessary to receive and forward data that it is
responsible for. Not all network hardware is created equal. Keep in mind,
however, the hardware is built to support the available standards that the
particular node should be able to support. Most of the hardware in networks
is nothing more than a big paperweight without the software loaded on the
device to teach it what to do and sometimes how to do it. To take this a bit
further, the hardware and software are useless without someone to configure
25

There are also times when the network hardware does not work well together, but we will save
that discussion until Chapter 16, ‘‘Troubleshooting.’’

133

Page 133

Edwards c03.tex

134

Part I

■

V2 - 03/27/2009

Networking Nuts and Bolts

it. Until computers are able to think for themselves, it is always going to take
human intervention to get a node to operate correctly in a LAN.
The following sections list network hardware common in networks today.
Not all the devices listed are in place in every network. They are available to
anyone who needs the device in order to support implemented or planned
standards within a network.

3.3.1

End-User Interface Hardware Types

A network exists to serve the needs of the end users. The network administrator
(head honcho, big daddy, C-3PO, or whatever else the person is called) plans
very carefully to ensure that the right equipment is purchased and brought
into the network. The hardware has to be able to support data traffic needs as
well as the necessary standards and protocols. Look at it this way: it wouldn’t
do you any good to buy a cell phone from one vendor and then order the
cell phone plan from another vendor. Most likely, the cell phone would never
work.26
The end users interface with some specific hardware devices that they need
to do their job. In Figure 3-13, you can see an example of some of the many
hardware devices that an end user may actually interface with. At the very
least, an Internet user will have a PC or laptop and an adapter of some sort
that will allow the PC to connect to a network. In many office environments,
multiple users will share the services of a printer, fax machine, or copy
machine. The network is what allows them to do this. For the purposes of this
chapter, we will not discuss the end-user direct access hardware. It would be
information that you are most likely familiar with.

3.3.2

Connecting End Users

Although there are many different user interface types out
there, we are going to focus on
the PC or laptop as the user
interface type for the remainder
of this book. If we enter into discussions of other user network
interfaces, we will define these
as they come up.
26 Jim

RANDOM BONUS DEFINITION
wireless fidelity (Wi-Fi) — A term that
describes certain types of 802.11 WLANs.

heard on the news the other day that a cell phone vendor out there claims its service will
work with any other vendor’s plan. Looks like maybe we can all get along.

7:38pm

Page 134

Edwards c03.tex

Chapter 3

■

V2 - 03/27/2009

7:38pm

Network Hardware and Transmission Media

Copy Machine

Laptop
Fax Machine

PC

Printer

Modem

Figure 3-13 End-user hardware types

The user interface is the device, software application, software program,
or other tool the user uses to complete a network transmission. The network
interface is the physical interface that allows the network node to connect to
the network.
It’s important to note the distinction between a network interface and a
user interface. Take a look at Figure 3-14. Really, you couldn’t tell a user to
go interface with a router and send an e-mail to 192.168.2.2. Now look at
Figure 3-15. The opposite holds true, as well: you can’t tell a router to send an
e-mail to your brother Joel in Abilene.
End users interface with cell phones, telephones, PDAs, PCs, e-mail programs, word processing programs, and a variety of other software and
hardware tools. They may go as far as installing a network adapter so
they can connect to the network, but the adapter really is not a user interface;
it’s a way for a PC (or other node) to pass and receive data to and from a
network.

135

Page 135

Edwards c03.tex

Part I

■

Networking Nuts and Bolts

“Hey Joel – tell the router to send
an email to 192.168.2.2”
Silly user, I only
speak in
datagrams!

OK

“Hey router – send
an email to
192.168.2.2

Figure 3-14 A user trying to interface with a router

I wonder why my
brother hasn’t
written??

irp

irp

Ch

irp

Ch

Ch

irp

irp

I told you that I
only speak in
datagrams!

Ch

irp

“Hey router – send
an email to my
brother, Joel”

Ch

136

V2 - 03/27/2009

Ch

irp

irp

rp

i
Ch

Ch

Joel

Ch

Figure 3-15 A router trying to send an email to a user

3.3.2.1

Network Interfaces and Adapters

Like many other things in networking, the terms interface and adapter
can have various meanings (and sometimes they mean the same thing).

7:38pm

Page 136

Edwards c03.tex

Chapter 3

■

V2 - 03/27/2009

7:38pm

Network Hardware and Transmission Media

We already discussed user interfaces and the types that are associated in that group. We are now
going to discuss network interfaces and network adapters.
Before we do that, take a look at
how Merriam-Webster defines
an interface and an adapter.

RANDOM BONUS DEFINITION
Worldwide Interoperability for Microwave
Access (WiMAX [IEEE 802.16]) — A task
force responsible for the IEEE 802.16
standards for broadband wireless access
(BWA) networks

in·ter·face27
noun
1: a surface forming a common boundary of two bodies, spaces, or phases
(an oil-water interface)
2 a: the place at which independent and often unrelated systems meet and
act on or communicate with each other (the man-machine interface)
b: the means by which interaction or communication is achieved at an
interface transitive verb
1: to connect by means of an interface (interface a machine with a
computer)
2: to serve as an interface for
adapt·or28
also adap·ter
noun
1: one that adapts
2 a: a device for connecting two parts (as of different diameters) of an
apparatus
b: an attachment for adapting apparatus for uses not originally intended

A network interface is any device or method that serves as an access point
to a data path among various network nodes within a network. A network
interface is also the point that connects users with a network that is outside
the boundaries of their LAN. Network interfaces provide a way for a node to
speak to other nodes, regardless of the standards that are in place along the
data path.
There is more to a network interface than simply installing it and then
plugging in a cable. The network interface is also able to convert data from
proprietary or noncommon standards to one that is shared, thus allowing
nodes to communicate with another one even if they don’t have the same
protocols implemented. A network interface connects end-user devices to a
network. The network interface controller (NIC) that is in a standard desktop
computer is a type of network interface. The point at the boundary of a LAN,
27 Merriam-Webster

Online Dictionary. Retrieved May 9, 2008, from www.merriam-webster.

com/dictionary/interface.
28 Merriam-Webster

Online Dictionary. Retrieved May 9, 2008, from www.merriam-webster.

com/dictionary/interface.

137

Page 137

Edwards c03.tex

138

Part I

■

V2 - 03/27/2009

Networking Nuts and Bolts

which connects the LAN to an outside network, is another type of network
interface. In Layer 3 environments, interface is often the term used to describe
a network connection and really isn’t considered hardware.
Network adapter is usually the term given to the hardware interface to the
network. Previously we said that an NIC card is a network interface that a
computer uses. An NIC card is also referred to as a network adapter.29 The NIC
card adapts to the computer, allowing it to have an interface to the network.
Confused yet? Wait — there’s more. There is also what is known as a virtual
network adapter, which is an application that assists a computer to connect to
the Internet without a physical adapter. This is usually done over WiFi or
WiMAX.
We really shouldn’t dwell on this much longer. With practice, you will
learn how to adapt to your fellow networking gurus and can interface with one
another while talking about how great this book is and how much you enjoyed
reading it.30 You will get a better feel for adapters and interfaces throughout
the remainder of this book. It’s not as difficult as it may seem, we promise.

3.3.2.2

Network Interface Controllers

The network interface controller (NIC)31 is a hardware card that allows a PC to
participate in passing and receiving data on a network. An NIC is commonly
referred to as an NIC card, LAN card, LAN adapter, network card, network adapter,
Ethernet adapter, and a few other names. Often the name may be a reference
to technology the NIC is supporting (i.e., an Ethernet card). All are entirely
acceptable and, regardless of what term you use, generally understood by
whoever is participating in the discussion.32 Figure 3-16 shows an example of
an NIC card.
NIC cards operate at Layers 1 and 2 of the OSI reference model. Because
NIC is a physical connecting device, providing a user with network access, it is
a Layer 1 device. However, because it uses a system for addressing nodes, it is
also a Layer 2 device. NIC cards33 have a 48-bit serial number assigned to them,
which is the MAC address. NIC cards normally take one of two forms; they
can be an expansion card that has to be physically inserted into the bus on the
PC motherboard or they can be integrated into the motherboard. You may also
have interfaces that have a difference connector type, such as a USB interface.
29

A good portion of the time if someone says ‘‘network adapter,’’ they are talking about an NIC
card. Or the adapter at the end of a cable (serial adapter, Ethernet adapter, etc.).
30
It seemed like a good time for a shameless plug.
31 Some people assume that NIC stands for network interface card. This is not correct, although
the term NIC card is accepted by most. If NIC were network interface card, then an NIC card
would be a network interface card card.
32 If you are ever unsure, just ask someone.
33 Okay. We said that it was a funny term, but it’s one we are comfortable with. It is less awkward
to ask someone, ‘‘Who do you buy the NIC card from?’’ than ‘‘Where did you get that NIC?’’

7:38pm

Page 138

Edwards c03.tex

Chapter 3

■

V2 - 03/27/2009

7:38pm

Network Hardware and Transmission Media

Figure 3-16 An NIC card

3.3.3 To Boldly Go Where Data Needs to Flow
(or, How Does that E-mail Get to Brother Joel?)
We have our cables, computers, NIC cards, buses, and all the things we need
to get our bits to hit the NIC card and travel across our UTP to a destination
on the other side of the LAN. As you can see in Figure 3-17, our bits just
are not going to go very far. The application sends the data to our NIC card,
who forwards it on to the medium, who just cannot figure out where the bits
should go.
We all know that the example in the preceding paragraph is simplistic, but
if you think about it, that is about all we have covered so far. Well, folks,
it’s time now for us to talk about the nodes in the network. Some of these
nodes you may not ever come across in real life, and others you will become
very familiar with. There are a lot of different nodes in a network, and often
equipment from many different vendors of node types is implemented within
the same network.34 When designing a network, it is important to put the
right node in place to perform the right job. You really don’t need a router
34 Don’t

put all of your eggs in one basket.

139

Page 139

Edwards c03.tex

Part I

■

Networking Nuts and Bolts

in a bridged network, nor would you try to use a repeater to connect to your
Internet service provider (ISP).

101010
001101
011100
110011
001100
111110
001010
100110
011001
010110
011110
0 0??1 1??010??1011??01?1000??1101??011001??110101?011??0
0

1

1
0

0

0

0

1

0

1

1
1

0
0

1

0

1

0

1

0

1

140

V2 - 03/27/2009

1

Figure 3-17 Sending data to the pseudo-net

This section does not provide an in-depth discussion of the standards
involved with and the modus operandi of any individual node. Most of these
will be covered in upcoming chapters. This section is more of an introduction to
networking hardware. Where does the data go when it leaves your computer?
What other nodes might you be using and not even realize it? These are the
types of questions you will be able to answer when you are done with this
section. The next time you hear someone say, ‘‘Hey, what’s all the hubbub?’’
you may be able to come up with a witty quip in response.

3.3.3.1

Concentrators

A network concentrator is a node that is able to multiplex signals and then
transmit them over a single transmission medium. Most concentrators support
multiple asynchronous35 channels and one high-speed synchronous channel.
The term concentrator is often used generically when referring to some nodes
35 In data communication, an asynchronous process is one that does not require a clocking
mechanism in order to work. A synchronous process does require clocking — in other words, it
has to be synchronized in order to work.

7:38pm

Page 140

Edwards c03.tex

Chapter 3

■

V2 - 03/27/2009

7:38pm

Network Hardware and Transmission Media

known as hubs (see next section). A concentrator usually provides point of
presence (POP) access for remote users, as well as performing other functions.36

3.3.3.2

Hubs

Hubs are commonly used to connect devices within network segments37 to
one another. Figure 3-18 shows an example of a typical hub deployment in a
network segment. Notice in the figure, the hub actually supports data rates of
both 10 Mbps and 100 Mbps. There are a lot of different types of hubs, with
varying numbers of hosts supported. Some support multiple data rates while
some only support a single data rate. The hub that is appropriate for your
environment should be chosen based on the needs of the network and the end
users.

10

0

ps

M

bp

Mb

s

10

ps

100 Mb
10/100 Mbps Hub

10

M

bp

s

100 Mbps

Figure 3-18 Hub deployment

When data is received by a hub, the hub
forwards the received data to all the nodes
that connect to it. All ports see datagrams
ACRONYM ALERT
received on any other ports within the hub.
FPGA — Field-programmable gate array
Hubs are considered shared media, as there
are multiple hosts sharing a common transmission medium. If a hub is made aware of a collision (data that collides
when two or more hosts try to pass data at the same time), it will signal
the other ports to stop transmitting until the collision is resolved. Hubs also
36 Some

concentrators are also able to perform high-layer functions, such as routing.
are areas of a LAN that are contained within a boundary with the boundary
termination node being a router, switch, or a bridge.

37 Segments

141

Page 141

Edwards c03.tex

142

Part I

■

V2 - 03/27/2009

Networking Nuts and Bolts

typically determine if one of the ports is having problems (excessive collisions,
corrupted data, etc.). If so, the hub can react and shut the port off from the rest
of the shared media. Hubs are considered Layer 1 nodes.
Hubs have largely been replaced in recent years, due to the popularity and
cost reduction of network switches, though they are still in use for many
home and small business networks. Additionally, hubs can be used to copy
datagrams that are sent to or received by a specific node and have that
information forwarded to one or more network monitoring connections.

3.3.3.3

Media Access Units

Media access units (MAUs), also referred to as multi-station access units,38
function similarly to hubs, but for Token Ring networks. Data flows through
the MAU in a logical ring topology, although the physical topology is a star
topology configuration. The MAU can recognize any hosts that are inactive
and disable the port the host is on so as not to disrupt the operation of the
logical ring. MAUs are considered Layer 1 nodes.
Take a look at Figure 3-19. You see that all hosts are physically connected
to the MAU in a star topology, while communication between the hosts is still
performed as if the hosts were physically connected in a ring.

MAU

Figure 3-19 An MAU — physical star, logical ring

38 There are two acronyms that are common when referring to the multi-service access unit, MAU

and MSAU.

7:38pm

Page 142

Edwards c03.tex

Chapter 3

3.3.3.4

■

V2 - 03/27/2009

7:38pm

Network Hardware and Transmission Media

Repeaters

Repeaters are used to give data
the extra push it needs to reach
POP QUIZ
an endpoint. Transmission media
has distance limitations before
What does MAU stand for?
the signal experiences degradation, known as attenuation or signal loss. When the distance limit has been reached, instead of placing another
switch, hub, or router in the path, a repeater is used.
The role of the repeater is simple: it accepts data and then retransmits it to
the other side. Copper and fiber optic cabling are both supported by repeaters
geared for the cabling type. Additionally, there are repeaters available for
networks that use wireless as a transmission medium.

3.3.3.5

Bridges and Switches

Functionally, bridges and switches are pretty much interchangeable. Both are
Layer 2 devices that support and perform the same basic function of joining
network segments within the LAN (see Figure 3-20). Bridges traditionally were
very small (some had only two port interfaces). When sold on the market, some
bridges fetched a very expensive price, especially if they could support data
rates that matched the rates supported by the transmission media in place.

Server Farm

Server Farm

Switch

Users

Segment A

Users

Segment B

Figure 3-20 An example of a switch bridging two LAN segments to one another

143

Page 143

Edwards c03.tex

144

Part I

■

V2 - 03/27/2009

Networking Nuts and Bolts

In the late 1980s and the early 1990s, the demand started growing for faster
systems and faster networks. LANs were expanding to the point where a shared
media network was no longer able to handle the demand. Advancements in
technology paved the way for system resource (processor and memory)
advancements, which allowed vendors to build nodes with more flexibility in
the number of ports than traditional bridges could support, all at the speed
supported by the connected transmission medium. These nodes were termed
switches, but their functions remained the same as what a bridge did — the
switch just was able to do more of it. The term switch is more of a marketing
term, used to separate the legacy nodes from the new and improved version.39
For the most part, a bridge is a switch and a switch is a bridge and both do
more than a hub.
AN UNRELATED MOMENT OF PAUSE
Too bad they didn’t think of these:
◆ AMIGA — A Merely Insignificant Game Addiction
◆ BASIC — Bill’s Attempt to Seize Industry Control
◆ CD-ROM — Consumer Device, Rendered Obsolete in Months
◆ COBOL — Completely Obsolete Business-Oriented Language
◆ DOS — Defective Operating System
◆ ISDN — It Still Does Nothing
◆ LISP — Lots of Infuriating and Silly Parentheses
◆ MIPS — Meaningless Indication of Processor Speed
◆ PCMCIA — People Can’t Memorize Computer Industry Acronyms
◆ PENTIUM — Produces Erroneous Numbers Through Incorrect Understanding
of Mathematics
◆ SCSI — System Can’t See It
◆ WWW — World Wide Wait

Switches have almost completely replaced hubs in today’s networks. The
prices of switches and hubs are fairly close when taking into account the
number of supported hosts. Some reasons why switches are preferred over
hubs are that switches are configurable, support more hosts within a single
node, and perform faster and more reliably than a hub.
39 The

sales and marketing folks continue to do this today. In Sections 3.3.3.7 and 3.3.3.8, we will
discuss upper-layer switching (Layer 3 switching, web switching, application switching, etc.),
which is nothing like traditional switching, but it sounds good and it sells.

7:38pm

Page 144

Edwards c03.tex

Chapter 3

■

V2 - 03/27/2009

7:38pm

Network Hardware and Transmission Media

Switches are deployed in varRANDOM BONUS DEFINITION
ious locations in a network.
Switches are able to determine
buffer — A block of memory used to store
the best path to a network segdata temporarily.
ment through the use of the
Spanning Tree Protocol (STP).
STP allows a network to be
designed to include redundant links, which ensures that data gets to its
destination if the primary link fails. STP also ensures that there are no loops in
the network, which might be introduced with the addition of the redundant
links. Spanning Tree has had many improvements made in the past few years.
We will discuss the Spanning Tree Protocol further in Chapter 11, ‘‘The Data
Link Layer.’’
Switches are also capable of being configured with multiple virtual LANs
(VLANs), which allow nodes to communicate as if they were all connected
within the same LAN segment, regardless of where the nodes physically
reside. In a VLAN environment, broadcast messages are only sent to the
interfaces that are members of the VLAN, leaving the remainder of the switch
the opportunity to serve other areas. Figure 3-21 shows an example of the
logical topology of a fully meshed switched network.

VLAN 108

VLAN 101

VLAN 1

VLAN 1

VLAN 1

VLAN 1

VLAN 109

VLAN 105

Figure 3-21 LAN switch deployment

Take note of all the available links and let’s take a moment to discuss what
problems may occur if there were no way to control the flow of data. Keep in
mind that switches forward data in the direction of the node that knows where
the MAC address of the destination is. In the example, if a host in VLAN 108
needs to get data to a host in VLAN 105, and there is nothing configured on
the switch to assist in forwarding decisions, which path would the data take?

145

Page 145

Edwards c03.tex

146

Part I

■

V2 - 03/27/2009

Networking Nuts and Bolts

Each switch would flood the data out all other switches and would continue
to do so at an alarming rate. Keep in mind that there are other nodes in other
VLANs doing the same thing. A basic example, but enough for you to see
that there are problems. That is what makes switches special — all the tools
available today to address these issues and many more that may arise. We will
discuss switching in more detail in Chapter 11.

3.3.3.6

Routers

Routers make it possible for our e-mails to make it to their destination. They
make the decisions that are necessary to get data from one user to another. It
would be virtually impossible to meet the demands of users today without a
router in the mix, helping make decisions on how to get data from point A to
point B.
Routers are advanced network nodes that connect networks of different
types. Routers are intelligent enough to know how to get data from a Token
Ring subnet to an Ethernet subnet, without data corruption of any kind. Routers
support many protocols and standards that allow much more flexibility in
their deployment. A router can be placed in the network to join two or more
LANs together, two or more WANs, a LAN to an ISP, and so on. Figure 3-22
shows a router joining two networks to one another and joining both of them
to the Internet.

Token
Ring
Catenet

Switch

The Internet

Router

Switch

Ethernet
Catenet

Figure 3-22 An example of a router deployment

7:38pm

Page 146

Edwards c03.tex

Chapter 3

■

V2 - 03/27/2009

7:38pm

Network Hardware and Transmission Media

Routers operate at Layer 3 of
POP QUIZ
the OSI reference model and use
IP addresses for data delivery.
At which layer of the OSI model does a
Routers also are able to comswitch operate?
municate with other routers and
share path information, so when
a packet is received, it can be
sent toward its destination over the best path possible. Routers run algorithms
to assist in determining the best path, and they share information with one
another, so every router can be on the same page. Routers ensure that data
gets to where it is supposed to go.
Routers maintain routing tables that help determine where the best path
is to a destination. The routing table includes information that shows what
subnets the router has learned and the path to the next node (next hop) that
leads to the destination IP address. The routing table is able to place a metric
or cost to a destination to assist in routing decisions. The entries in the routing
table can be configured (static) or learned via a routing protocol such as RIP
or OSPF. Following is an example of a routing table:
Active Routes:
Network Destination Netmask Gateway Interface Metric
0.0.0.0 0.0.0.0 192.168.1.1 192.168.1.104 1
127.0.0.0 255.0.0.0 127.0.0.1 127.0.0.1 1
192.168.1.0 255.255.255.0 192.168.1.104 192.168.1.104 1
192.168.1.104 255.255.255.255 127.0.0.1 127.0.0.1 1
192.168.1.255 255.255.255.255 192.168.1.104 192.168.1.104 1
224.0.0.0 224.0.0.0 192.168.1.104 192.168.1.104 1
255.255.255.255 255.255.255.255 192.168.1.104 192.168.1.104 1
Default Gateway: 192.168.1.1

In the example, you can see that the routing table has information on the
destination addresses that it is aware of, the subnet mask that is assigned
to the destination IP address, the gateway (next hop to destination), the
interface through which the data needs to go in order to reach the gateway,
and the metric assigned to the destination. The metric is the number of
hops to a destination. If there is only one route, the metric is ignored. If
there are multiple routes to a destination, the one with the lowest metric
is used.
Routers can be as simple as a router in a home office to as complex as an
Internet backbone router. Routers support multiple protocols and interfaces,
which allows them to be operated and translate data coming from multiple
network types. Routers are discussed in greater detail in Chapter 10, ‘‘The
Network Layer.’’

147

Page 147

Edwards

148

Part I

■

3.3.3.7

c03.tex

V2 - 03/27/2009

Networking Nuts and Bolts

Layer 3 Switches

Section 3.3.3.5 discussed traditional Layer 2 switches and the
RANDOM BONUS DEFINITION
functions they perform. Layer 3
bit — A unit of data that is either a 0 or a 1.
switches can operate at Layer 2,
as well as function like a router.
Layer 3 switches can be configured to make routing decisions to send data to a destination. Routers use
software to perform logic decisions for operation and use a microprocessor
to perform packet switching. Layer 3 switches have replaced the need for
software logic decisions and some hardware that routers rely on with integrated circuitry to perform these tasks. The circuitry that is used is known as
application-specific integrated circuits (ASICs).
Layer 3 switches combine the wire speed technologies used by Layer 2
switches and the tools necessary to route packets as a router. Layer 3 switches
make routing decisions based on the same routing table information as a
traditional router does. As far as the hardware design, a Layer 3 switch and a
router look a lot alike in many cases. Both are configurable and the higher end
ones have slots where different types of modules can be inserted, increasing
the protocols that are supported by the node.
Layer 3 switches are predominately developed for larger corporate LANs.
The Internet still utilizes routers in the core to get data to a destination. Most
Layer 3 switches are not able to support the WAN interfaces required for
routing Internet data. Layer 3 switches are often referred to as routing switches
or Ethernet routing switches.
Layer 3 switches also have the ability to control the flow of data by
implementing what is known as class of service (CoS), which provides for
packet queuing into classes of service to ensure that data with a higher priority
is attended to before data with a lower priority.

3.3.3.8

Upper-Layer Switch Types

There are nodes that perform functions at Layer 4 and above of the OSI
reference model. The term switch is more of a marketing term, as these nodes
are nothing like traditional Layer 2 switches. Some of the terms that are
assigned to switches that fall in the upper-layer category include:
Multilayer switches
Server load balancer switches
Web switches
Layer 7 switches
Application switches
Layer 3 switches
Layer 4 switches

7:38pm

Page 148

Edwards c03.tex

Chapter 3

■

V2 - 03/27/2009

7:38pm

Network Hardware and Transmission Media

Layer 4–7 switches
Content switches
The previous section discussed the Layer 3 switch. The
POP QUIZ
Layer 3 switch is able to route
At which layer of the OSI model does a
data much like a router at wire
router operate?
speed, as well as function as traditional Layer 2 switches. Layer
3 switches are also sometimes
referred to as multilayer switches.
A Layer 4 switch operates at the Transport layer and expands the functions
that are performed by Layer 2 and Layer 3 switches. Layer 4 switches prioritize
data based on applications that are in use. A Layer 4 switch provides for CoS
to be deployed throughout the LAN (not just within the switch). An example
of providing priority for applications would be in a LAN where e-mail traffic
takes precedence over Telnet traffic. These parameters can be configured so if
there are some users who need Telnet more than e-mail, it can be configured
to allow for this. Layer 4 switches are also referred to as multilayer switches.
Server load balancers (SLBs) distribute traffic destined for a server. They
share the load for requests between multiple servers, without the end user even
being aware that there is any node between them and the server. Figure 3-23
shows an example of a switch performing load balancing for HTTP requested
to a website.

Web Server A

Web Server B

Internet
Load
Balancer

Web Server C

Figure 3-23 Deployment of a server load balancer

149

Page 149

Edwards c03.tex

150

Part I

■

V2 - 03/27/2009

Networking Nuts and Bolts

Load balancers also spoof the IP address of the server, which helps secure
the servers from attack. Load balancers divide requests destined for the server
among all the servers that are attached to the load balancer. If a load balancing
solution is not in place, all traffic hits the same server, which could potentially
cause latency and rejecting of requests to the server.
Some of the upper-layer switches are also able to cache data for speedy
access. These functions are known as data acceleration. Some also support
cryptographic protocols — for instance, Secure Sockets Layer (SSL) and Transport Layer Security (TLS). Load balancing, data acceleration, cryptographic
protocols, and many more things.40 Who could ask for anything more?41

3.3.3.9

Remote Access

Network nodes that are used to provide remote users the capability of accessing
a computer or network from a remote location are known as remote access nodes.
Many corporate LANs utilize VPN technology to allow users into the LAN
from any location, as long as they have access to the Internet. Some users may
not have access to the Internet, and in those cases, they can use a modem to
connect to the remote location.
Home users also have
modems that allow them to conRANDOM BONUS DEFINITION
nect to the service provider.
modulation — The process of manipulating
Once connected, the users can
a waveform to create a signal that sends a
digitally travel to almost anymessage. In data communications,
where in the world. They can
modulation is performed by a node that
also use VPN client software
converts a digital signal to an analog signal,
to connect to the VPN server
in order to be communicated over a phone
line.
(or rather, to the node that is
running the server software).
Remote access technology, like
many other networking technologies, has grown by leaps and bounds in the
last decade. Remote access (with the necessary applications) allows people to
telecommute and work from remote locations as often as necessary.42 Additionally, remote access gives small offices the capability to connect to the
corporate LAN to conduct business. This is a much cheaper option than what
was provided in the 1980s to early 1990s.

40

That’s what Layer 4–7 switches are made of.
assure you: someone is always asking for more.
42 Or as long as the boss will allow them to do so.
41 We

7:38pm

Page 150

Edwards c03.tex

Chapter 3

■

V2 - 03/27/2009

7:38pm

Network Hardware and Transmission Media

Remote access gives clients, vendors, and partners the capability to connect
to the corporate LAN. The system administrator controls who gets to go where
once they are on the LAN. In this section, we discuss the hardware nodes that
provide an avenue for these technologies to exist.
3.3.3.9.1 Modems

The term modem is derived from its two main functions. A modem modulates
and demodulates. This means that a modem converts digital data to an analog
signal and then converts it back again when the data reaches the modem that
is connected to the destination node. Figure 3-24 is an example of remote users
accessing a corporate network segment via a modem.

Home user

Modems

Switch

Figure 3-24 Modem remote access

Data that is sent and received by a modem is measured in bits per second
(bps) or by its baud rate. Bps is a measure of the amount of data (number of
bits) that can be sent in one second. Baud rate is determined by the type of
modulation used and represents the number of times that a signal is changed
in one second. The baud rate and the bps rate are not the same number.
Modems that connect a user’s PC to a phone line are called dialup modems.
Dialup modems are not the only modem type that is available. Internet access
is now available to most people in the United States and other parts of the
world at very high data rate speeds. There are different types of modems

151

Page 151

Edwards c03.tex

152

Part I

■

V2 - 03/27/2009

Networking Nuts and Bolts

available to the average user as well as businesses and other organizational
types. Here is a list of a few of these:
Cable modem
Asymmetric digital subscriber line (ADSL) modem
Digital subscriber line (DSL)
Microwave modem
Optical modem
Wi-Fi modem
The type of modem to use really depends on the needs of the user(s). A
person who plays video games online would be much happier with a cable
or DSL modem over the traditional dialup modem. Someone who goes online
to send and receive e-mail once a week can probably survive with a dialup
modem.43
3.3.3.9.2 VPNs

VPN technology provides a way for a remote user or branch office to connect
virtually to a remote LAN over the Internet. A VPN supporting node has three
main functions:
Provide remote access for individual users
Provide remote access for a branch office or other LAN
Ensure that only authorized individuals are able to access the LAN
There are many different
types of nodes that support VPN
POP QUIZ
technology. Some are called
What is the common name for a
VPN routers, VPN switches,
modulator/demodulator?
extranet routers, and extranet
switches. As long as the node in
question’s predominate jobs are
remote access, authentication, and encryption, the node is VPN-compatible.
VPN hardware supports enhanced security, load-balancing methodologies,
and the capability to support an increased number of clients that can be
connected at the same time, based on the processing power of the node.
43 But

good luck with opening some of those attachments.

7:38pm

Page 152

Edwards c03.tex

Chapter 3

■

V2 - 03/27/2009

7:38pm

Network Hardware and Transmission Media

3.3.3.9.3 Wireless

Wireless remote access is a growing technology. Many business and companies
are providing access to the Internet and/or the LAN for their customers and
employees. There are two main nodes that are needed for wireless remote
access. You need to have an end user with a wireless NIC (WNIC) and an
access point for them to connect to. The end user is known as the wireless
client. Access points are the boundary nodes for the network. A wireless
client would be any node that is used to connect to the network without a
solid communication path. Figure 3-25 shows an example of wireless remote
access.

Corporate LAN

Figure 3-25 Wireless remote access

Some examples of these client node types would be:
Cellular phones
IP phones
Laptops
Workstations
Computers
Notice that a wireless client does not have to be a portable device. It can be
a stationary device as well, as long as it has an interface that supports wireless
technology. There are many access point nodes; some are integrated into other
network node types. Within networks that are completely wireless there are
wireless bridges, switches, routers, and so on, just as there would be in any
wired LAN.

153

Page 153

Edwards

154

Part I

■

c03.tex

V2 - 03/27/2009

Networking Nuts and Bolts

3.3.3.10

Servers

Network servers are nodes that manage the resources available to the users of
the network. There are many different types of servers, normally named for
the function they perform. A few examples include:
Print servers — Manage traffic destined to a network printer.
File servers — Store files for network users.
Network servers — Manage the traffic on the network.
FTP servers — Manage file transfer.
Mail servers — Manage e-mail traffic.
Fax servers — Manage incoming and outgoing fax messages.
List servers — Manage mailing lists.
Proxy servers — A node that resides between a client and a server,
whose purpose is to manage requests destined to the server. Proxy
servers allow for shared connections and free the server up so the performance of the server from a end-user perspective is greatly improved.
Network servers are nodes
that are dedicated to the technology they are configured to
RANDOM BONUS DEFINITION
support. These nodes have nothAppleTalk — A protocol suite developed by
ing else to worry about but that
Apple Computer.
specific function. Some servers
can have multiple applications
running and therefore have the
resources necessary to support each of those. Even if the node is running
multiple applications, the application itself is the server and is still referenced
by the function it is set to do.

3.4

Chapter Exercises

1. Explain what ‘‘10 half or 100 full?’’44 means to you, what the difference is between 10 half and 100 full, and list pros and cons of each.
2. List three types of interfaces and three types of adapters.
3. Why is an NIC card considered both an interface and an adapter?
44 We

told you that someone would ask this someday.

7:38pm

Page 154

Edwards

Chapter 3

■

c03.tex

V2 - 03/27/2009

7:38pm

Network Hardware and Transmission Media

4. List three examples of flash memory.
5. List the PDU for each of the OSI layers:
Layer

PDU

Application
Presentation
Session
Transport
Network
Data Link
Physical
6. What is the difference between volatile and nonvolatile memory?
7. What is the difference between STP and UTP cabling?
8. Explain when you would want to use MMF cables instead of SMF cables.
Next, explain in what instances SMF cabling would be preferred over
MMF cabling.
9. Define modulation.
10. What is the main difference between a Layer 3 switch and a router?

3.5

Pop Quiz Answers

1. The decimal number 211 is equal to what binary number?
11010011
2. The binary number 01011100 is equal to what decimal number?
92
3. What is the binary name for the binary value of 250 ?
Pebibit (Pibit)
4. Define RAM.
Volatile memory that is available for data storage and access,
regardless of the order in which it was received.
5. Define encapsulation.
Encapsulation is the act of including data from an upper-layer
protocol within a structure in order to transmit the data.

155

Page 155

Edwards c03.tex

156

Part I

■

V2 - 03/27/2009

Networking Nuts and Bolts

6. What is IEEE Standard 802.11?
IEEE 802.11 is the standard that is maintained by the IEEE outlining
WLAN communications. Sometimes, IEEE802.11 is also referred to as
Wi-Fi, although traditional Wi-Fi standards are not included in IEEE
802.11.
7. What does MAU stand for?
Media access unit
8. At which layer of the OSI model does a switch operate?
Layer 2
9. At which layer of the OSI model does a router operate?
Layer 3
10. What is the common name for a modulator/demodulator?
Modem

7:38pm

Page 156

Edwards

c04.tex

V3 - 03/27/2009

11:04am

CHAPTER

4

Operating Systems and
Networking Software
Part of the inhumanity of the computer is that, once it is competently programmed
and working smoothly, it is completely honest.
— Isaac Asimov

This quote by Isaac Asimov points out the basic difference between human
intelligence and that which is attributed to computers. True computers can
be designed and built to calculate, retain, and retrieve vast amounts of data
in microseconds and display it in graphics and color beyond what human
language is able to relate.1 However, computers are programmed devices that
are only able to operate on a set of rules designed by humans.
True, there are programs that attempt to give computers a form of artificial
intelligence, but being only machines that work within a defined rule set,
they can only respond in a completely honest manner. On the other hand,
humans are capable of lying at any time and often do. We will not get into
the philosophical or psychological reasons for why humans have a tendency
toward lying. Whatever their reason may be, humans can be whimsical,
whereas when a computer acts in that manner, it usually gets its guts torn out.
So, now aren’t you happy you are not a computer?
The essential piece of software each computer requires is an operating
system. Without it, a computer would just sit and not do a meaningful piece
of work, just like some humans we know. It is the basic process that operates
on human requests and responds accordingly, if programmed to act in that
manner. The network drivers embedded in the operating system communicate
with the portions of a computer that interact with the network. The operating
system assists other application programs to communicate with a server that
1 Try

to tell the average human to produce a fancy graph on the fly!

157

Page 157

Edwards

158

Part I

■

c04.tex

V3 - 03/27/2009

Networking Nuts and Bolts

is located remotely and can only be reached over the network. There are other
programs involved in the network arena, but the purpose of this chapter is to
cover the basic computer operating system and how it interacts with network
components. There will also be discussion on network operating systems
(NOS) and their place in the network.

4.1

Computer Operating System Basics

To understand computer operating systems and their place in the universe, it
is essential to first discuss some computer
design basics. Everyone by now has heard
ACRONYM ALERT
the acronym CPU (central processing unit).
ARE — All routes explorer
Some may say it means the computer itself,
such as a personal computer, without any
peripherals attached to it. In days gone by,
a CPU could have taken up some serious floor space, filling a large room
or many rooms with racks of equipment. Today, a desktop computer has
roughly a footprint of one square foot. This represents a significant difference
in floor space, but today’s CPU also has major advantages in speed, storage,
processing power, and energy consumption. Even though modern computers
are far more capable than their early predecessors, they still operate pretty
similarly when it comes to handling data.

4.1.1

CPU Basics

The CPU is the heart of any computer. Data and instructions flow into it so
the data can be manipulated and acted upon in a controlled manner. Data and
instructions are stored within the memory system of the computer. Figure 4-1
shows a block diagram of a basic CPU.
The memory storage area can be constructed of various storage devices
ranging from semiconductor to magnetic media. For this section, all you need
to know is this is where the instructions of a program and the data that
program is to operate on reside. The memory interface contains circuitry that
provides addressing information to the memory storage devices so that data
may be retrieved. Once the data is received, it is passed to circuits that decode
the retrieved data to determine if it is an instruction or data that needs to be
operated on. If the latter, the appropriate input registers are loaded with the
data. If it is determined that the retrieved data is an instruction, the arithmetic
logic unit (ALU) is given the instruction. Depending on the instruction the
ALU receives, it performs an operation on the data contained in the input

11:04am

Page 158

Edwards

Chapter 4

■

c04.tex

V3 - 03/27/2009

11:04am

Operating Systems and Networking Software

registers and places the result of that operation in the output registers so that
data can be moved back to the memory system for storage.

Arithmetic
Logic Unit

Output
Registers

Input
Registers

Instruction
Fetch/Decode

Data Path

Memory
Interface

Memory Storage

Figure 4-1 A block diagram of a basic CPU

The ALU is the device that performs mathematical operations on the data it is
presented with. These are not only the basic functions of addition, subtraction,
multiplication, and division, but also Boolean logic2 such as or, and, and
their negated logical functions. The ALU is solely responsible for actual
mathematical manipulation of the data it is presented with. The remainder
of the CPU functional blocks is solely for the purpose of retrieving data and
seeing that it is returned to the memory system properly so it can be easily
accessed if needed.
2 A system of logical operations. The term Boolean comes from the name of the inventor of Boolean

algebra, George Boole.

159

Page 159

Edwards

160

Part I

■

c04.tex

V3 - 03/27/2009

Networking Nuts and Bolts

QUICK REVIEW
The Boolean algebra or function is usually indicated by a + sign between
variables, such as A+B=C. A variable is usually true when its value is equal to 1
and false when its value is equal to 0. An or function result is true if any of the
variables making up the function is true. A negated or function is usually
referred to as a nor function and its value is false if any of the variables making
up the function is true.
The Boolean algebra and function is indicated with a ‘‘·’’ sign between
variables, such as A·B=C. An and function result is only true if all of the
variables making up the function are true. A negated and function is usually
referred to as a nand function and its value is only false if all the variables
making up the function are true.
The following table shows two variables and the resultants of the or, nor,
and, and nand functions.

A

B

OR

NOR

AND

NAND

0

0

0

1

0

1

0

1

1

0

0

1

1

0

1

0

0

1

1

1

1

0

1

0

This discussion is a simplification of what a CPU is. However,
what once took racks of equipPOP QUIZ
ment is now contained on a
What function does an arithmetic logic unit
single microprocessor chip. Curprovide?
rent microprocessors are magnitudes more powerful than those
early computers and use much
more sophisticated designs that take advantage of bigger data paths, larger
addressing capabilities, caching, look-ahead memory fetch,3 parallel and multiple processor technologies — to name a few.
The next section discusses the overall computer architecture and how the
CPU interacts with those other computer subsystems.

3A

memory fetch grabs the immediate contents of a memory location. Look ahead memory
fetch is intuitively retrieving data from memory using the idea that memory fetching is mostly
sequential and to save time memory contents would be retrieved in blocks of sequential memory
addresses.

11:04am

Page 160

Edwards

Chapter 4

4.1.2

■

c04.tex

V3 - 03/27/2009

11:04am

Operating Systems and Networking Software

Computer Basics

A computer is a collection of subsystems under the control of the operating
system, which is the driving intelligence behind the electrical circuits it runs
over. Without an operating system, a computer is just a pile of chips, boards,
wires, and circuits that would not do any useful thing. But, then again, an
operating system is just a collection of ones and zeroes, which is just a bunch
of useless information without a computer to execute those commands and
instructions. So computers and their operating systems need each other to
make a complete package.
In this section, we will be discussing a generic computer system. Most
computers have the subsystems being discussed or at least some compatible
variation of those subsystems. Figure 4-2 illustrates a block diagram of a basic
computer system.
Central
Processing Unit
CPU

Address Bus

Data Bus

Read Only
Memory
ROM

Read Access
Memory
RAM

Mass Storage
System

Input/Output
System

User Interface
&
Peripheral
Devices

Figure 4-2 A block diagram of a basic computer

We already discussed the CPU portion of a computer. You know that it
executes instructions and operates on data, but where is that data obtained?
In Figure 4-2, the memory system is distributed across the ROM (read-only
memory), RAM (random-access memory), and mass storage System. Why
the need for different memory systems? Each has its own purpose within a
computer system.

161

Page 161

Edwards

162

Part I

■

4.1.2.1

c04.tex

V3 - 03/27/2009

Networking Nuts and Bolts

Read-Only Memory

When power is first applied4 to a computer,
commands must be inputted into the CPU
to initialize the computer system. A CPU is
designed to output an initial address to the
ACRONYM ALERT
address bus to retrieve the first instructions
CRC — Cyclic redundancy check
from the ROM. The ROM is a fixed storage subsystem that has the initial boot-up
instructions to initialize the system. Most boot-up programs perform both an
initialization of the computer and a check of the subsystems to ensure they are
functional. The ROM may consist of semiconductor devices that contain bits
of the data making up the instructions to be executed that are not alterable by
the user. However, current personal computer systems do allow for updates
to the ROM software program for bug fixes or feature enhancements. ROM
devices in this category are usually called electrically alterable read-only memory
devices.5
This means the device can
be written to if necessary using
special sequences under control
POP QUIZ
of the operating system. The
Would it be advisable to cycle power to the
boot-up code is critical for comcomputer while a ROM upgrade is in
puter initialization. If this code
process?
becomes corrupted for any reason, the computer may not be
usable and may require professional maintenance to restore it back to operation. For this reason, many
computers flash warning messages and precautions when the ROM is being
accessed under user control. ROMs can be upgraded safely, but do not attempt
an upgrade without fully understanding the upgrade process. Typically, once
the process has been initiated, it cannot be interrupted until it has completed
and the computer has rebooted. If you ever have any questions about upgrading ROM, consult your computer documentation and, if necessary, contact the
support staff of the computer’s manufacturer.

4.1.2.2

Random-Access Memory

Random-access memory (RAM) consists of semiconductor devices that are used
for temporary storage of program instructions and data. The usual design is
4 Technically,

you have power within the PC as soon as the battery is plugged in — in other
words, when you press the ‘‘on’’ button on the node.
5 The actual devices used in today’s computers are called EEPROM (electrically erasable programmable read-only memory).

11:04am

Page 162

Edwards

Chapter 4

■

c04.tex

V3 - 03/27/2009

11:04am

Operating Systems and Networking Software

an array of these devices residing in the address space of the CPU. As their
name implies, they can be accessed randomly no matter what address the
information to be retrieved is residing at. This also means the CPU under
program control may write data to locations within its address space and
store the information for later retrieval. RAM space is usually controlled by
the operating system, which designates locations for fixed buffer space for
functions under its control and for use by the application programs that
may be running at the time. Modern operating systems are capable of running
multiple processes at the same time. Each of these processes require operational
memory space, so it is critical that memory management be handled properly
and as efficiently as possible.
All programs running under the control of the operating system must be
well behaved and adhere to the memory space allocation given. When a
program violates its memory space allotment, it may overwrite locations being
used by other applications or the operating system. If a rogue application
overwrites memory used by the operating system for control of the computer,
there is a strong likelihood that machine control will be lost and the user will
no longer be able to operate the computer under normal conditions. It is in
these times that a computer may need to be rebooted to restore operation.
The amount of memory space a computer may contain is determined by how
large an address a CPU is able to generate. In the early microprocessor-based
PCs, the number of bits of address was only 16, which would allow for
a maximum of 65,536 discrete memory addresses. You can determine the
address space of a device by taking the number 2 and raising it to the power
of the number of address bits that are generated by the CPU. For example:
216 = 65,536 for 16
address bits
220 = 1,048,576 for 20
address bits
224 = 16, 777,216 for 24
address bits
232 = 4,294,967,296 for
32 address bits

RANDOM BONUS DEFINITION
active monitor — A node in a Token Ring
LAN that is responsible for handling many
boundary conditions and housekeeping
functions, including generation of a
common clock, elastic buffering, and
removal of circulating high-priority tokens.

Earlier PCs were mostly character-based computers. Programs were smaller
and not as memory-intensive as the visually oriented operating systems of
today. As processor capabilities expanded with increased processing speeds
and greater addressing ability, software became more sophisticated by taking
advantage of these increased capabilities. In the early days, there was a
constant battle between hardware designers and their software counterparts.
The standing joke used to be that software is like a gas; it will occupy the space

163

Page 163

Edwards

164

Part I

■

c04.tex

V3 - 03/27/2009

Networking Nuts and Bolts

that is provided. This is still pretty much true, but to the software developers’
credit, they have done some totally marvelous things with the space they filled.
The real battle lines were
drawn on the lines of cost.
Hardware had fixed costs and
POP QUIZ
increased rapidly as memory
True or false: The information contained
needed to be expanded. Those
within RAM is saved when the computer is
lines have been obliterated
powered off.
somewhat by the advances in
chip design, with increased densities and lower power consumption of newer processor and memory chips. Costs dropped dramatically
and the capabilities of PCs expanded exponentially. This leads to the conclusion that there is a direct correlation between memory size and computer
performance. A general rule of thumb is to buy as much memory as you
can afford. However, it is really application-dependant. Applications such as
gaming software require much more memory and processor speed, whereas
someone who just wants to type a few reports can get by on a relatively smaller
amount of memory and decreased processor speed. The marketplace puts PCs
on the cutting edge of technology as consumers become more sophisticated.
It can only keep pushing the demands on memory and processors to increase
their abilities, and this is the driving force for today’s technology.

4.1.2.3

Mass Storage System

The mass storage system is comprised of a collection of multiple devices
storing programs and information either in magnetic or optical media formats.
The very earliest PCs used floppy disks to write and retrieve information in
a somewhat nonvolatile manner when the computer was powered off. The
‘‘somewhat nonvolatile’’ comment is for anyone who had to suffer through
the loss of information due to a flaw in the magnetic media or the electronics
of the device controlling this media. If it can be easily written, it can be easily
removed or erased.
Just as memory chips underwent improvement, so did magnetic media devices.
Floppy disks went from single-sided to
double-sided and higher densities. The
ACRONYM ALERT
last floppy disks were high-density 3.5-inch
BOOTP — Bootstrap Protocol
plastic-encased disks that were more reliable than their predecessors but still could
suffer similar data losses. The highest
density obtained with floppy disks was 1.44 MB, which is a lot for a
typewritten document but far from having the capacity to store some of

11:04am

Page 164

Edwards

Chapter 4

■

c04.tex

V3 - 03/27/2009

11:04am

Operating Systems and Networking Software

today’s programs. Programmers had to develop schemes to distribute their
software using multiple floppy disks. A user had to sit by the computer
during the installation of such a program and wait for the message to load the
next disk. The process was tedious and time-consuming.
The development of optical storage devices, such as CD-ROM, increased
storage capacities in a movable media format from just over 1 MB to the
vicinity of 700 MB. This was a boon to both software developers and computer
users. DVD devices, with their higher capacity for data storage, increased what
CD-ROM could store by a factor of 10 — or roughly the ability to store 7 GB of
information. Current day computers are shipped with optical drives that can
read and write both CD-ROM and DVD media formats. Optical media now has
read-write capability, but the process is slower than that of magnetic media.
However, as a removable media storage system, it has many advantages over
its magnetic predecessors. Even though optical disks are more robust as far
as data retention, they still can be rendered unusable by physical damage. A
severe scratch can make an optical disk unreadable.
Nonremovable disk storage
systems are referred to as hard
disks. They are ‘‘hard’’ because
POP QUIZ
the magnetic media was origWhen a computer is first powered on, the
inally sprayed on the surfirst device it is most likely to read its initial
face of rigid aluminum disks,
set of instructions from is the
which were mounted within an
.
enclosed airtight container to
eliminate data corruption due
to dust and other contaminants.
Magnetic media was bonded to a soft pliable Mylar surface, thus the name
‘‘floppy disk.’’ The advantages of hard disks are their ability to store vast
amounts of information and its fast retrieval times. Initially, hard drives
were commercially available only to users of large mainframe computers,
but as development progressed on these devices, the pricing was such that
it was commercially feasible to sell them to the PC market. The first PCs
shipped with a whopping hard disk storage capacity of 5 MB. Many of today’s
graphics-intensive programs would not be able to load onto the drive, let alone
the operating system or any other user data. It is not uncommon today to see
laptops with 200 GB hard drives and desktops with 500 GB6 storage capacities.
Hard drives are usually mounted within a computer’s case, but many drives
are sold as external drives communicating between the drive and computer
over the USB port.
6 This

really is an amazing amount of data storage. Can you imagine what increases will be made
within the next decade?

165

Page 165

Edwards

166

Part I

■

4.1.2.4

c04.tex

V3 - 03/27/2009

Networking Nuts and Bolts

Input/Output System

A computer is not very useful if information cannot be entered into it or
retrieved from it. The input/output system is a collection of circuits that allow
for information to be entered by the user via a keyboard, pointing device,
scanner, etc. It also provides a method for information to be displayed to the
user. This can be in the form of video screens, teletype, printers, plotters, etc.
These are the most common methods of input and output from a computer
system. There are many specialized input/output devices for data entry and
retrieval not mentioned in this section, but the idea is always the same: move
information into the computer and retrieve it from the computer after it has
operated on it.
Because input/output devices interacting with other
physical devices and humans
RANDOM BONUS DEFINITION
may experience timing differbit stuffing — A technique that provides a
ences with the CPU, there needs
unique frame delimiter pattern yet
to be a way of storing the informaintains payload data transparency by
mation and notifying the CPU
inserting an extra 0 bit after every
when the data is present. Generoccurrence of five 1 bits in the payload data
stream.
ally two schemes were devised
to accomplish this. One is where
the input/output devices are
mapped to dedicated memory addresses and the CPU polls these locations
to see if there is information that needs to be acted on. This is referred to as
memory-mapped I/O. The other scheme is interrupt-driven I/O, where a device
writes information into a dedicated register at a fixed port location and sets an
interrupt requesting service from the CPU.
In a memory-mapped I/O system, the CPU determines which location it
should poll under operating system control. In an interrupt-driven I/O system,
the CPU responds to interrupts (and there may be many, depending on the
number of I/O ports to be serviced). Interrupts adhere to a fixed interrupt
priority scheme, which is hierarchal. The CPU can be processing an interrupt
request and be preempted by a higher priority interrupt request.
Regardless of which I/O scheme is used in a computer, the operating
system must be able to deal with input/output data requests. It must be able
to determine when a device is acting unresponsive and either notify the user
or take other action as determined by the program. Generally the operating
system is responsible for data movement between the various systems within
the computer. However, a user may be running an application, such as a word
processor, which is running over the operating system. When a user depresses
a key on the keyboard, the operating system reads the key and presents that

11:04am

Page 166

Edwards

Chapter 4

■

c04.tex

V3 - 03/27/2009

11:04am

Operating Systems and Networking Software

information to the word processor program, which may request that it also be
displayed on the video screen.
On PCs, input/output connections are in the form of ports
dedicated to either serial or
POP QUIZ
parallel data communications.
Name a device that you might find
Serial communications refers to
connected to a serial port.
the information being passed
one bit for each time interval, which is determined by the
speed of the port. Generally serial devices are slow data rate devices such
as keyboards, modems, pointing devices, scanners, etc. However, with the
development of Universal Serial Bus (USB), high-speed serial ports, devices
such as hard disk drives and printers can be used due to the increased data
rates on these ports. Parallel ports on older PCs were mostly relegated as
printer ports. Parallel data communications means that data is sent a whole byte
at a time for each cycle of the port. USB has become today’s de facto standard
for peripheral ports.

4.1.3

Operating System Basics

Operating systems in one form or another have been around since the inception
of the first computer. Of course, the first computers were of the mainframe
variety with character-oriented terminals.7 Users entered commands and data
in the form of alphanumeric characters that could be found on any typewriter.
Data retrieved from the computer could be displayed on the terminal screen
for small queries, or, for larger reports, outputted to a printer.
The most basic form of an operating system is a file manager. It is able to create new
files on the storage medium being used. It is
also able to catalog the files for easy retrieval
ACRONYM ALERT
and has some sort of indexing ability simiDMA — Direct memory access
lar to that of a filing cabinet. Computers and
their operating systems were first designed
to adopt systems that were similar to the
business practices of those days. The earlier computers were a high-speed
filing system able to store, index, and retrieve data faster than a filing clerk.
Operating systems underwent some dramatic revisions with the introduction of the PC. Initially, these operating systems were similar to those found
7 The first terminals were alpha-character-oriented. They were merely an electronic form of a
typewriter. Graphic terminals that could display some sort of graphic (usually at low resolution
by today’s standards) were a later innovation in terminal design. Terminals connected to the
computer via serial cable.

167

Page 167

Edwards

168

Part I

■

c04.tex

V3 - 03/27/2009

Networking Nuts and Bolts

on the larger computers. They too were character-oriented. The major early
PCs initially ran on proprietary operating systems such as Apple’s DOS (Disk
Operating System) and Tandy Radio Shack’s TRS-DOS (usually phonetically
pronounced tris-dos). The first cross-platform PC operating system to gain
popularity was Digital Research’s CP/M (Control Program for Microcomputers), originally designed to run on Intel 8080/8085 microprocessor-based
computers. It migrated to the Zilog Z80 which was capable of executing the
Intel 8080-based instruction set and was a mainstay of the Z80-based PCs for
a number of years.
The major limitation of CP/M was that it was designed for 8-bit microprocessors and was only capable of addressing 64 KB of memory. As microprocessors
moved up in capability, CP/M began to lose ground to other operating systems, mainly Microsoft’s MS-DOS. Digital Research did finally release a 16-bit
version as CP/M86, but it was not able to compete against the IBM/Microsoft
juggernaut.
Initially, MS-DOS was locked up by IBM and was sold with the IBM PC
as IBM DOS. Other PC manufacturers were on the outside looking in and
attempted to adopt CP/M86, but the popularity of the IBM PC running
MS-DOS left them far behind on the number of PCs being sold. The off-brand
manufacturers eventually developed clone PCs that were able to run MS-DOS,
thus boosting their PC sales. The developer of CP/M and CP/M86, Digital
Research, also developed a clone to MS-DOS called DR-DOS to compete with
Microsoft. The number of PCs now running MS-DOS caused IBM to lose their
competitive edge and to eventually give up on the PC market.
Although CP/M was a cross-platform operating system, the hardware it was
running over could have major differences. As a result, a CP/M program on one
computer could not run on another computer from a different manufacturer.
The portability of CP/M was the core operating system (sometimes referred
to as the kernel). The CP/M kernel provided a common interface for user input
and application programs that would run over different computer platforms.
The computer manufacturers had their own software designer teams that
would write the software code needed to allow the kernel to communicate
with other hardware systems of the computer system. These pieces of code
were referred to as hardware drivers.8 Each subsystem in a computer system
could have its own driver if needed. An example of this is the mass storage
subsystem. The kernel would call for a file and the driver would cause the
floppy drive to seek the track and sector where the beginning of the file
was located. The point is, although there was commonality as far as user
interfaces and the applications able to run on CP/M, they could have been
8 Hardware

drivers are synonymous with device drivers. It is the code that is designed to allow
the kernel of the operating system to properly communicate with the device/hardware no matter
how different in design they may be. The device driver acts as a translator to allow for the correct
operation of the device/hardware.

11:04am

Page 168

Edwards

Chapter 4

■

c04.tex

V3 - 03/27/2009

11:04am

Operating Systems and Networking Software

operating on computers whose hardware had substantial differences from one
manufacturer to the next.
Soon after the IBM PC was
introduced and its hardware
specifications were published,
RANDOM BONUS DEFINITION
clone PCs began to enter the
byte — A unit of data that is equal to 8 bits.
marketplace. Since IBM opened
its architecture, it was not able
to legally protect its design, and
the PC marketplace ballooned overnight with clones from a number of hardware manufacturers. This phenomenon led to a PC base that not only was
able to have the same operating system but also had hardware commonality,
which was a boon to the peripheral manufacturers.
With the consolidation of
today’s PC marketplace, there
are really only two variations
POP QUIZ
of PCs. Today’s PC users are
What is the acronym for a user interface that
either in the Apple Mac domain
uses a point-and-click method of executing
or the PC domain (PCs from
computer commands?
various manufacturers able to
run the various iterations of
Microsoft DOS). Today, Apple
manufactures and markets laptops and desktop PCs based on its Macintosh family of computers. Macs were the first PCs that took advantage of a
point-and-click–based operating system.9
Today’s PC world is divided between the Mac operating system and
Microsoft Windows operating system. Both are GUI (graphical user interface)
based and use a graphical display screen and some sort of pointing device.
However, even with the whiz-bang colorful interfaces, the operating system
is basically performing the same functions as its predecessors. The only
difference is that instead of parsing text instructions, the user input interpreter
uses positional information, and if a mouse is used, a right, left, or double-click
will cause the operating system to act on the object that is being pointed at on
the video graphical display screen.

4.2

Network Operating System Basics

As the need grew for PCs to interconnect and share data and common
resources, the opportunity arose for the design and marketing of network
9 If

this had caught on before Windows came out, it might have been a much different world
today.

169

Page 169

Edwards

170

Part I

■

c04.tex

V3 - 03/27/2009

Networking Nuts and Bolts

operating systems. The most common design of network operating systems
was the client/server implementation. PCs were clustered for individual users
(clients) to share files on the file server or print data files on printers under the
control of a print server. Figure 4-3 illustrates an example of network running
a network operating system (NOS).
File Server

Print Server

Database

Database

User Client PC stations

Figure 4-3 A computer network under the control of a network operating system

Actually, ‘‘network operating system’’ is a bit of a misnomer in that the NOS
really runs on computers that are servers placed in the network. Figure 4-3
shows a single10 file server and a single11 print server. In reality, on large
networks there could be multiple servers in use. Also, for a small office,
the functionalities of both the file server and print server can be combined
in a single server. Being a client/server application, the responsibility for
authentication of clients with the authority to connect to the server depends on
the server to verify that clients have the necessary valid security credentials.
In larger networks with many clients, that function can be placed in entirely
separate servers solely responsible for granting network access as well as the
permission levels a user will have while logged into the network.
There are networks where the software that is being run on a local PC is
actually an application located on the server. An example of this is a word
processor program that has a fixed number of network licenses. The theory is
that not all users would use the program simultaneously, so a company could
save some costs by sharing applications over the network. Once all the licenses
10 Just
11 See

because they are single does not mean they are available.
footnote 7.

11:04am

Page 170

Edwards

Chapter 4

■

c04.tex

V3 - 03/27/2009

11:04am

Operating Systems and Networking Software

are occupied, subsequent users would need to wait until another user logged
out of the program, thus releasing the license. Users could be prevented from
loading a program from a server if the network or the server is being heavily
worked. Once the program is downloaded to the local PC, there is no further
network interaction required until the application is released by the user. This
interaction was called the file services portion of the NOS.
Print services were also an important
piece of NOS. Printer requests would be
queued to the print server servicing that
portion of the network. A print server could
ACRONYM ALERT
have one or many printers under its conFIFO — First in, first out.
trol. As print job requests arrived at the
print server, it would determine the printer
the print job was to be outputted to. The
print server queued the print jobs on a first-come, first-served basis. Print jobs
were stored on the print server and parceled out to the printer as fast as the
printer was able to take the data. Today’s network-ready printers are basically
their own print server with the intelligence and storage capacity required to
queue print jobs from a large user base.
There were many networking operating systems, but the most popular
were Novell NetWare and Microsoft Networking. Novell utilized an IPX/SPX
protocol stack to provide communications over its network. Both Novell
and Microsoft have since migrated to supporting the TCP/IP protocol suite
over their networks. TCP/IP is not a NOS; it is a protocol that controls
communications between peers. A client/server application can be run over a
network that uses TCP/IP protocol for communicating over the network, but
the actual client/server application is independent on the protocol itself.
The majority of today’s networks are TCP/IP-based networks that have
a wide range of applications running over them. A workstation may have
multiple sessions to various servers on the network simultaneously. Most
people use e-mail and may be logged into a corporate mail server while
running other applications to other servers over the same network. The need
for a network server running a NOS is not required when running the TCP/IP
protocol over a network.

4.2.1

Peer-to-Peer Networking

When discussing network operating systems, the context of the discussion
is usually based around client/server networks. To perform peer-to-peer
networking, where one computer can share data and resources with another
computer, requires some sort of application program. The earlier versions of
peer-to-peer networking were crude and cumbersome to configure and use.
However, as Microsoft evolved its Windows operating system, they added

171

Page 171

Edwards

172

Part I

■

c04.tex

V3 - 03/27/2009

Networking Nuts and Bolts

peer-to-peer as well as workgroup network capabilities. Windows was the first
GUI-based operating system that was able to support this type of networking.
Windows users are able to share drive space and locally attached printers
with other users on the same network using what is commonly referred to as
Windows networking. Windows networking depends on the host names of each
computer to be different if they reside within the same network. This was first
accomplished with NetBIOS API (application programming interface) running
on each Windows computer on the network. In today’s networks, NetBIOS is
usually run over the TCP/IP protocol. In this scenario, each computer has both
a unique computer name and an IP address. The services NetBIOS provides
are related to the Session layer of the OSI model.
On smaller networks, the computer broadcasts the name of the computer that
it wants to establish a session with. On large networks, broadcasts can become
intrusive and affect network throughput speeds. Large Windows networks
will utilize a WINS (Windows Internet Name Service) server for computer
name resolution. It maps computer host names to network addresses, thus
eliminating multiple broadcasts on the network. WINS can be thought of as
the name service for NetBIOS networks and is similar to a DNS (Domain Name
Service) server in operation on a TCP/IP network.
Figure 4-4 shows a small peer-to-peer Windows-based network.

Computer A
Computer B

Hub/Switch

Shared Printer

Computer C

Computer D

Figure 4-4 A small, Windows-based peer-to-peer network

In this figure, the PCs are labeled Computer A, B, C, and D. However, they
may be named in any manner a user or network administrator chooses. It is

11:04am

Page 172

Edwards

Chapter 4

■

c04.tex

V3 - 03/27/2009

11:04am

Operating Systems and Networking Software

a good idea to select meaningful names such as joes pc, jims pc, and so on,
to give a frame of reference for the PC. In larger companies, the computers
may be named by department and function. Naming is purely arbitrary, but
knowing what each PC is named can be helpful, especially when trying to
troubleshoot network issues.
Within this network, NetBIOS provides computer name
registration and resolution, a
RANDOM BONUS DEFINITION
connection-oriented communicheapernet — Another name for 10BASE2.
cation session service, and a connectionless communication for
datagram distribution service.
Before a computer can either start a session or distribute datagrams on
the network, it must use the NetBIOS name service to register its name. NetBIOS utilizes UDP port 137 for the name service. The NetBIOS name service
functions are to add a name or group name, delete a name or group name, or
find a name on the network.
Since in today’s networks NetBIOS is run over TCP/IP, NBT (NetBIOS over
TCP/IP) utilizes TCP port 139 for the session service. The session mode of
NBT allows two computers to establish a connection to pass communications
between them. The NetBIOS primitives12 associated with the session service
are as follows:
Call — Opens a session to a remote computer using its NetBIOS name.
Listen — Listens for session requests using NetBIOS name.
Hang Up — Ends a session that had been previously established.
Send — Sends a packet to the computer that a session has been established with.
Send No ACK — Similar to Send but does not require a
returned acknowledgement that the packet was received.
Receive — Waits for the arrival of a packet from a computer a session
has been established with.
The datagram distribution service is a connectionless service where messages
are sent without regard to error detection or remediation. It is incumbent upon
the application using this service to provide the necessary data error detection
and recovery when needed. UDP port 138 is used by NBT for this datagram
distribution service.
12 This

list is almost the same responses that one can expect from the family teenager. However,
for a NetBIOS session these are the root terms used to describe a particular sequence within the
session.

173

Page 173

Edwards

174

Part I

■

c04.tex

V3 - 03/27/2009

Networking Nuts and Bolts

The primitives used for datagram distribution by NetBIOS are as follows:
Send Datagram — Sends a datagram to a remote computer using its
NetBIOS name.
Send Broadcast Datagram — Sends a datagram to all the
NetBIOS names that are registered on the network.
Receive Datagram — Waits for the arrival of a packet from a Send Datagram process.
Receive Broadcast Datagram — Waits for the arrival of
a packet from a Send Broadcast Datagram process.
Fortunately, setting up a small Windows-based local network is easy to do.
The previous discussion in this section gives you an appreciation of what is
going on under that colorful GUI screen. The unfortunate part is that Windows,
with all its various generations, had added twists and bends to the methods
used to configure networking on a PC using the Windows operating system
for its OS. It is the author’s recommendation to review the documentation for
your particular version of Windows before attempting to configure your PC
for networking. The configuration overview as well as the screenshots in the
remainder of this section are based on Windows XP.
Most of the PCs purchased within the last couple of years come pretty much
network-ready. Many desktops come with an Ethernet NIC card13 installed,
and many laptops not only have a hard-wired NIC for Ethernet connectivity
but also have some sort of wireless connection interface. However, if you
have an older PC that you would like to add to your network and it does
not have a NIC installed, you have choices available to you to make your PC
network-ready. Desktop computer models may either use an internal card, if
there is an interface card slot available, or some sort of external solution. There
are network interfaces available that will plug into the USB port. If you are
not all that computer savvy, I recommend taking down as much information
you have about your PC and visiting your local computer store. The sales
clerk or computer support staff should be able to assist you in purchasing the
appropriate solution to make your computer network-capable.
Older laptops can be easily made network-ready with the addition of a
network PCMCIA card. The usual choice is either a card that supports a
hard-wired Ethernet solution or a WLAN PCMCIA card, which enables you
to connect to your local network wirelessly. The choice is solely dependent
upon the current installed network. If this is an initial setup, I strongly suggest
investigating a wireless solution. The beauty of a laptop is its mobility, and to
have it tethered by an Ethernet cable may not be the ultimate network solution.
13 Keep

in mind, NIC = network interface controller.

11:04am

Page 174

Edwards

Chapter 4

■

c04.tex

V3 - 03/27/2009

11:04am

Operating Systems and Networking Software

NICs require drivers to be able to interoperate with the operating system. Windows
ACRONYM ALERT
has moved to the plug-and-play philosophy where the Windows operating sysIP — Internet Protocol
tem detects when new hardware has been
installed. In most cases, with interface cards
from larger manufacturers there is a high probability that Windows will have
and load the appropriate driver. If your card is one that Windows is unable
to auto-detect, the Windows wizard may request that you load a driver disk
to complete the installation of the card. In most cases, there is usually a disk
in the box with the card or documentation that will point you to a website or
FTP server where the appropriate driver14 can be downloaded.15 You can use
that downloaded file to complete the installation of the card.
With your wired Ethernet Interface installed, you can navigate to your local
area connections properties. On Windows XP, click Start  Control Panel. On
the Control Panel screen, select Network Connections for the classic view, or
if using category view, select Network and Internet Connections. Select the
Local Area Connection that is associated with the NIC card you have installed.
With the icon for the interface selected, right-click and scroll to Properties. A
window should appear labeled Local Area Connection Properties, similar to
Figure 4-5.

Figure 4-5 Windows XP Local Area Connection Properties
14 Not

to be confused with diver, one who deliberately jumps headfirst into water.
that you have another computer that has network capability and is able to reach the
Internet to get the file to download.

15 Assuming

175

Page 175

Edwards

176

Part I

■

c04.tex

V3 - 03/27/2009

Networking Nuts and Bolts

On this PC, Client for
RANDOM BONUS DEFINITION
Microsoft Networks is already
installed and enabled. If it
flooding — The process of sending a frame
is not yet installed on your
to all of a switch’s ports, with the exception
PC, select the Install button
of the port the frame came in on.
and a new window will open
labeled Select Network Component Type. Select the Client component and click on the Add button. The Select
Network Client window will open. Select Client for Microsoft Networks and
click OK. If you want to share parts of your file system or locally attached
printers, you must enable File and Print Sharing. In the Local Area Connection
Properties window, click the Install button. When the Select Network Component type window appears, select Service and click on the Add button. The
Select Network Service window will appear. Select File and Printer Sharing for
Microsoft Networks, and then click OK. You now have Microsoft Networking
enabled with file and printer services enabled. We will revisit both file and
printer sharing in a bit. For now, it’s on to how we get TCP/IP on this puppy.
If you do not see Internet Protocol (TCP/IP) in the Local Area Connection
Properties window, the protocol must be added. Click on the Install button
in the Local Area Connection Properties window. When the Select Network
Component window appears, select Protocol and click on the Add button. On
the Select Network Protocol window, select Internet Protocol (TCP/IP) and
click OK. The protocol has now been installed but must be configured.
Before getting into the configuration of
TCP/IP on this Windows PC, a brief
description is in order of the difference
between a statically assigned IP address
ACRONYM ALERT
and an IP address that has been assigned
MIB — Management information base
by a server acting as a DHCP server. This
topic will be covered and mentioned in
other chapters, and by the time this book
is finished there will be no question that you will know the differences and
how they come to be assigned. First, a statically assigned IP address is pretty
obvious. It is an IP address that is assigned to the PC by a user or administrator
and is the same IP address the computer will have assigned to it each time the
PC is booted up.16 The only things that have to be known prior to assigning the
static IP address is that the IP address is unique and not assigned to another
computer on the same network segment, that the address to be assigned fits
into the addressing scheme being used on that network segment, and, lastly,
that the subnet mask assigned with the IP address is compatible with the IP
16 What

it is not is an address that is applied via a static charge.

11:04am

Page 176

Edwards

Chapter 4

■

c04.tex

V3 - 03/27/2009

11:04am

Operating Systems and Networking Software

address and is the subnet mask assigned to that network segment. Static IP
address assignment is not difficult in a small network, but it can become rather
unwieldy in a large network. And if a network redesign is required with a
change in IP address assignment for that network, it can become a support
nightmare in very short order. If it can be avoided on the network you are
setting up, it is recommended to do so and use a DHCP server for IP address
assignment.
So, how does one come up with a DHCP server for their network? Of
course, you could have an actual server running a DHCP service, but for
a small network, such as that shown in Figure 4-5, it would be a waste of
resources. There are many newer network devices that do run a DHCP service
if configured to do so. Most routers, both wired and wireless, are capable of
running a DHCP service. If the hub/switch shown in Figure 4-5 were replaced
by a mini-router like those used for cable/DSL Internet access, you could have
a DHCP service running on that network. The beauty of having a local DHCP
server is that if there is ever a need to change a network’s addressing scheme,
default gateway, or the DNS servers being used, there is just a single point
that requires configuration change. So there is a major support advantage of
running a DHCP service on your network. It is easy to see the advantages
of having such a service on large networks with many PCs. One reason to
consider DHCP even for a small network is if there are laptops being used. The
advantage of using a laptop for a PC is its portability and its mobility of moving
from one network to another. Although it is doable, having to configure your
TCP/IP setting each time you move from one network to another can grow
old very quickly.
To set the IP properties of the installed NIC, click on Start  Settings 
Control Panel. On the Control Panel, select Network Connections. Right-click
on the Local Area Connection you are going to configure IP addressing on,
and then select Properties. Select Internet Protocol (TCP/IP) and click on
the Properties button. The window where properties can be configured will
appear and look similar to that shown in Figure 4-6.
Notice that this interface is configured for obtaining an address dynamically from a DHCP server somewhere on the existing network. To do this,
only the two radio buttons to automatically select these addresses need be
selected. However, if you select to statically assign the IP address, each
of the grayed fields needs to filled in with the appropriate information.
IP address — A unique
IP address that is not currently used on the network
segment where the computer is to be connected

RANDOM BONUS DEFINITION
host — Any node in an IP network.

177

Page 177

Edwards

178

Part I

■

c04.tex

V3 - 03/27/2009

Networking Nuts and Bolts

Subnet mask — The subnet address assigned to the network segment
that the computer is to connected to.
Default gateway17 — The IP address of the node that acts as the default
gateway for the network segment the computer is connected to.

Figure 4-6 Windows XP Internet Protocol (TCP/IP) Properties screen

The DNS (Domain Name Service) server
is required if the computer is going to
attempt to connect to remote computers
ACRONYM ALERT
by using a domain name.18 In Figures 4-3
and 4-4 the networks are self-contained and
ns — Nanoseconds.
it is assumed that someone is keeping track
of IP addresses that have been assigned. In
those situations, there is no need for a DNS server to reach the other PCs
on the network. Each user will need a list of what those IP addresses are
for all computers and other network resources, such as printers. However, in
17

A quick definition of a default gateway is that it is the IP address of a node that is used when a
computer needs to start a session with a computer that is not resident on the same network.
18 A domain name server is a computer residing on the Internet providing requested services.
For example, a web server may have a name like www.mywebsite.com. Since the IP protocol is
dependent upon finding an address using numerical addresses, someone needs to resolve the
name to a numeric address. This is the role of a DNS server and it gets its information from the
authoritative service on the Internet where the name has been registered.

11:04am

Page 178

Edwards

Chapter 4

■

c04.tex

V3 - 03/27/2009

11:04am

Operating Systems and Networking Software

this current interconnected world the need for DNS is paramount. Figure 4-7
shows a small local network connected to the Internet using a router with a
high-speed connection.
www.mywebsite.com

Wireless
Enabled Laptop

Wireless
Access Point

Internet

Router
Ethernet Network Segment

User Computer Workstations

Figure 4-7 A small local network connected to the Internet

Usually when a user or company signs up with an Internet service provider
(ISP), they are provided information such as the public IP address that is to be
used on the router and its default gateway’s IP address. The ISP also provides
local DNS service located within the ISP’s network, which can be pointed to
for DNS name resolution. In a statically assigned IP scheme, these addresses
would need to be entered in the appropriate fields of the Internet Protocols
(TCP/IP) Properties window to enable the computer to query the provided
DNS servers for name resolution when needed. This will need to be done for
every computer on the network if they are to be able to connect to computers
by IP host name. Most ISPs provide two DNS server addresses. Normally
these would be called a primary DNS address and a secondary DNS address. The
primary DNS address is entered in the Preferred DNS server box, whereas the
secondary DNS address is entered in the Alternate DNS server box. The PC
is now configured to communicate with other PCs on the local network and
other computers that may be found on the Internet.

179

Page 179

Edwards

180

Part I

■

c04.tex

V3 - 03/27/2009

Networking Nuts and Bolts

QUICK TIP
There are a couple of quick tests you may want to perform to verify the
operation of the NIC card and the connectivity to the local network and the
Internet.
1. Click on the Start button in the lower-left portion of your Windows screen.
2. Select Run.
3. In the Run window, enter cmd and click OK. A DOS window will open where
DOS commands can be entered.
4. Type the command ping 127.0.0.1. You should receive back four messages
stating ‘‘reply from 127.0.0.1.’’ This indicates that your NIC card is
working properly with Ethernet and TCP/IP. If you receive ‘‘Request
timed out’’ messages, your card has not been properly configured.
5. To verify your network connectivity, attempt to ping the local
default gateway19 for your network. If you get ‘‘Request timed
out’’ messages, verify your physical connection to the LAN.
6. If you get good responses back from the local default gateway,
you may want to also check your connection to the Internet.
7. Ping the IP address of the router’s default gateway. If you get
good responses, you are able to reach the Internet. If you receive
‘‘Request timed out messages’’ and you own the whole network, you will need to troubleshoot further. If you are on a
company network, contact your network administrator.
8. DNS name resolution can be quickly checked if the Internet connectivity
test passed successfully. Ping an Internet connected computer by its host
name. For example, ping www.mywebsite.com. Receiving ‘‘Request timed
out’’ messages may not be an indication of a problem with DNS. Some sites
drop ping requests in order to combat denial-of-service attacks of their
site. What you would want to see is that the name has been resolved to
a numeric IP address. If so, then DNS appears to be working properly and
you should be able to connect to the site using your web browser.
9. If DNS resolution does not appear to be working, verify the address
you had entered on the Internet Protocol (TCP/IP) Properties. If there
are no typos, you may want to attempt to ping the IP address of
the DNS server. If there are no replies, you may want to attempt to
ping the secondary DNS IP address. If you get a reply there, you may
want to place the secondary DNS IP address in the preferred DNS
server address field and test again, pinging by Internet host name. If
problem persists, contact your ISP or your network administrator.

19 This

is the IP address inserted in the Internet Protocol (TCP/IP) Properties for the Default
Gateway field. A default gateway is normally the IP address of a router located on your network
that has access to the Internet.

11:04am

Page 180

Edwards

Chapter 4

■

c04.tex

V3 - 03/27/2009

11:04am

Operating Systems and Networking Software

This section configures a
Windows-based PC not only for
POP QUIZ
use on a Microsoft network but
Name two network operating systems that
also for any TCP/IP-based netare prominent in today’s networking world.
work, which includes the Internet as we know it today. There
will be changes coming such as
IPv6,20 but the basics will remain the basics. What is learned here is scalable to
any new nuances that may be coming into the world of networking.

4.2.1.1

File Sharing on a Peer-to-Peer Network

When we configured the NIC card on the PC to permit file sharing, we did
not expound on how this is accomplished in a Microsoft Windows world.
The strategy is to first determine what is needed to be shared between users.
Whole drives, including hard drives, floppy drives, CD-ROM drives, and
DVD drives, can be shared. However, any portion of the file system can be
shared down to the lowest subdirectory within a directory structure. So this
allows for drive, directory, and subdirectory file sharing, all of which can be
accomplished over the local network.
From My Computer, right-click on the drive that you are willing to share.
From the drop-down menu, select Sharing and Security. A new window will
open showing the properties for the drive (see Figure 4-8).

Figure 4-8 Windows XP drive properties
20 We

will cover this in Chapter 10, ”The Network Layer.”

181

Page 181

Edwards

182

Part I

■

c04.tex

V3 - 03/27/2009

Networking Nuts and Bolts

Notice the message about the security risk that is involved in sharing a
whole hard drive. You can proceed if you wish or you can back off to the
directory you want to share. Multiple directories can be shared on a hard
drive.
QUICK TIP
Proper planning can simplify sharing of directories over the network. Create a
single folder that you want to share. Under that folder you can create other
folders (subdirectories) that will be shared with the parent folder. The whole
directory tree under the shared folder will be shared when you allow sharing
on this folder.

One instance where it makes sense to share an entire drive is where
removable media is concerned. Floppy drives, CD drives, and DVD drives
can be both read and written to, as needed. The floppy drive is nowhere to be
found on today’s newer laptops, so if you need to generate a floppy disk with
information from your laptop, share the drive on the desktop to accomplish
that task. Granted, it may not be as fast as a directly connected floppy drive,
but it can get you by in a pinch.
Enabling file sharing is only
half of the task. You may want to
create user accounts on the PC.
RANDOM BONUS DEFINITION
This can be accomplished under
router — A network node that operates at
the User Accounts section of the
the Network layer.
Control Panel. For other computers to use the shared folder,
they will need to map a network
drive. This can be done from My Computer by selecting the Tools drop-down
menu and then Map Network Drive. This window is illustrated in Figure 4-9.
The format shown on this
window is \\server, which
POP QUIZ
would be the NetBIOS computer
name of the computer where
What can be shared using Windows file
the shared directory is located.
sharing?
An example would be \\joe pc.
However, with TCP/IP enabled
on the network connection, this
also may be an IP address of the computer where the shared directory is
located. The command format would be similar but with the IP address of the
computer is placed where the computer name had been. An example would
be \\192.168.5.154. The \share is the name assigned to the shared entity,

11:04am

Page 182

Edwards

Chapter 4

■

c04.tex

V3 - 03/27/2009

11:04am

Operating Systems and Networking Software

whether it is a drive or directory on the hard drive. The naming is fairly
arbitrary and the owner of the computer can use any name he or she pleases.
However, the owner must play nice and give the name to the user who would
be sharing the data contained in that directory. Without the proper shared
name, the share cannot be established. If a guest account or user account has
been created for that user, they will be prompted for the account prior to
gaining access to the shared data. However, for file sharing to work properly,
the computer with the shared directory must be powered on and connected to
the network before its shared resources can be accessed.

Figure 4-9 Windows XP Map Network Drive screen

4.2.1.2

Printer Sharing on a Peer-to-Peer Network

In today’s networking world there are network-ready printers that act as their
own print server. They can obtain a network IP address, be given a name, and
will allow themselves to be mapped to from other computers connected to the
network. This section does not deal with those printers but with the printers
that are locally connected to computer on the network.
These printers may be locally connected
to a network PC with a parallel port, serial
port, or USB port.21 To share a locally connected printer, select Printers and Faxes
ACRONYM ALERT
from the Control Panel. Select the printer
RAM — Random-Access Memory
to be shared by pointing to it and clicking
the right-mouse button. In the drop-down
menu, select Sharing. A new window similar to the window in Figure 4-10 will appear on the screen.
21 Extra

credit: What is the benefit and the disadvantage for each of the port types? (This is a
question that you will have to research — unless you already know).

183

Page 183

Edwards

184

Part I

■

c04.tex

V3 - 03/27/2009

Networking Nuts and Bolts

Figure 4-10 Windows XP Printer Sharing screen

Select the radio button to share this printer and enter a share name in the box
provided. Windows will attempt to enter a name that is being used locally, but
this can be changed as needed. For this example, it a high-speed laser printer
connected to Flo the secretary’s computer, and other users in the department
would like access to that print resource, so a share name may be something
like flo printer. Other computer users on the network can then go to Control
Panel and select Printers and Faxes and then Add a Printer. They may either
browse the network for Flo’s printer or enter the name directly, as discussed in
the file sharing section. For the sake of this example, the name may appear as
\\flo pc\flo printer, where flo pc is the server name of the computer and
flo printer is the share name for the laser printer sitting by her computer. If
needed, the IP address assigned to Flo’s computer can be used in place of a
server name.
QUICK TIP
The use of IP addresses in place of server names is indicative of static IP
address assignment. If the network is designed to use dynamic IP address
assignment, this could cause problems for users on the network since a
computer’s assigned IP address could theoretically change each time it is
booted up.

11:04am

Page 184

Edwards

Chapter 4

■

V3 - 03/27/2009

11:04am

Operating Systems and Networking Software

Remember that a shared resource in a peer-to-peer network
environment assumes that the
resource is available on the network. The computer providing
the source must be powered on
and connected to the network
for the resource to be shared.

4.3

c04.tex

POP QUIZ
Which printers connected to a
network-connected computer can be shared
with other users on the network?

Other Operating Systems

So far in this chapter, we have concentrated on the client aspect of networks
and the Internet. However, many computers on the Internet and within
the corporate environment are large computers running a wide range of
applications. Although there are many similar applications that can run on a
PC and offer the same type of service, they may not be equally able to handle
many users at the same time. Large computers were initially designed and
used to service multiuser environments, whereas the small computer or PC
was initially designed with the single user in mind. As a result, the operating
systems that control these large machines are much more robust when it comes
to handling a large number of simultaneous users.
This section will concentrate on the network aspects of these operating
systems and how they are used within both the corporate network environment
and the Internet.

4.3.1

Unix

Unix was first developed by AT&T Bell Labs as a multiuser operating system. It
was initially designed to handle many users connected simultaneously and all
sitting in front of character-based terminals. These terminals were connected
to terminal concentrators that were able to aggregate a number of users for
ease of communications with the computer the Unix operating system was
running on. TCP/IP had not been implemented and the Internet was in its
earliest planning stages.
Since its inception, Unix, because of its kernel design was able to be ported
to a number of different computer platforms from a variety of computer
manufacturers. Later, the operating system program was emulated and offered
by other software vendors and computer manufacturers. The discussion
in this section will cover the basics to get a Unix-based computer onto a

185

Page 185

Edwards

186

Part I

■

c04.tex

V3 - 03/27/2009

Networking Nuts and Bolts

TCP/IP network. Since these are usually specialized computers from many
manufacturers, it would be difficult to get into specifics for all the variations
and iterations, so consider this a familiarization with the requirements to make
a Unix-based computer network-able.
Unix is a flat file operating
system, which basically means
RANDOM BONUS DEFINITION
that most of the configuration
trap — A message that originates from a
files are in readable text. Confignetwork management client to a network
uration is accomplished using
management server to notify the server of a
one of the resident text procesnotable event.
sor programs that are part of
the utilities that come with the
operating system. The appropriate files can be edited as needed to configure the TCP/IP settings on the
computer. Usually, systems of this vintage have system administrators who
maintain and update the /etc/hosts22 file. The information that needs to be
modified includes the following:
The host name
The IP address assigned to the interface
The subnet mask being used for the network segment the computer
resides on
The IP address of the DNS server that is going to be used
The default gateway that is residing on the same network segment as the
computer
The version of Unix you are working with will determine which files and
syntax of commands will need to be used. Luckily, most iterations of Unix
have resident help in the form of the man pages. These pages are an online
manual and the common syntax is man , where  is the
command you need help with. You will be informed if the command does
not exist. When in doubt, issue the man command and you will get a complete
description of the command along with the various switches that are used by
the command.
Newer versions of Unix come with configuration utility programs that assist
with the network settings and configuration. Edits of the related network files
are automated for ease of use, but essentially it performs the same edits that
an administrator can do with a text editor.
22 The Unix /etc directory contains configuration files for devices connected to the computer.
The hosts file aids in host name to IP address resolution. For further information on the Unix
directory structure, including the full contents of the /etc directory, consult the operating
manual supplied with your Unix system.

11:04am

Page 186

Edwards

Chapter 4

■

c04.tex

V3 - 03/27/2009

11:04am

Operating Systems and Networking Software

The following are a few useful commands for troubleshooting network
issues on a Unix computer:
arp — Displays a table that shows the IP address to physical

MAC address relation for nodes on the same subnet with the
Unix computer. This is useful when there are connectivity issues
between the Unix computer and that host. If there is an arp entry
for the problem node, there is a possible Physical layer issue.
arp -a

ping — An important troubleshooting command that helps to determine
that the TCP/IP stack is configured properly on the Unix computer, that
the network interface is configured properly, that the default gateway
is reachable, and that domain name services are configured properly.
ping 127.0.0.1

If no response is received, you need to verify that TCP/IP services
have been loaded and are running on the Unix computer.
ping 
If no response is received, verify that TCP/IP has been bound to the NIC. Check that the operating system has been configured properly as far as the NIC’s hardware address and the proper interrupt request number. If the operating system is configured properly, check for a Physical layer issue. ping
This verifies that the subnet mask has ACRONYM ALERT been properly set in the TCP/IP configuration and that the request is sent SMTP — Simple Mail Transport Protocol to the default gateway correctly. If no response it received, check settings to verify that the default gateway is set correctly in the TCP/IP parameters after you were successful in pinging the default gateway. ping 23 This will verify that the DNS service is correctly configured on the TCP/IP stack. If no response is received, attempt to ping the configured DNS server using its IP address. If no reply is received, there may be a connectivity issue. Repeat the ping test to the default gateway. If that passes, verify the settings in the TCP/IP configuration. 23 Internet host name is the fully qualified domain name (FQDN) of the host server you are attempting to reach. An example of a FQDN for a host name would be www.google.com. 187 Page 187 Edwards 188 Part I ■ c04.tex V3 - 03/27/2009 Networking Nuts and Bolts netstat — A network status command that will display status and information on the network interfaces24 configured on the Unix computer. The following are some switches that can be used with the netstat command: -a — Displays information on all interfaces. -i — Displays configuration information. -n — Displays IP addresses. -r — Displays routing table information. ifconfig — Used to display information on the interfaces that are found on the Unix computer. These interfaces can be Ethernet or other types of interfaces. route — Used to add static routes to the Unix computer’s routing table. traceroute — A use- POP QUIZ ful tool to show the 25 nodes an echo request Which command can be used to verify the needs to pass through TCP/IP stack has been properly configured to reach its intended on a Unix computer? target. The target address may be either a numeric address or an alphanumeric Internet host name. traceroute
4.3.2 Linux 26 Linux has many similarities and commonalities to Unix. However, it was designed more for the desktop environment even though it will run on larger computers. The number of Linux variations is too many to mention, and each has its own piece of window dressing when it comes to configuration. Similar to Unix, Linux can be configured with a text editor, if necessary. The variables that are configured are part of a script that is loaded each time a Linux computer is booted. Therefore, changes in network configuration would require a reboot so that these scripts can be executed with the new variables 24 Network interfaces on a computer can be of the LAN variety (NICs) or interfaces for WANs, such as a WAN card for a T1 line. 25 Echo request is part of the ICMP protocol primarily utilized by the ping command. The ICMP components of a ping command are echo request (the ping to a target IP address) and echo reply (a successful response from that target). traceroute uses these components to verify the path by receiving and logging the network nodes that the echo request passed through on its way to the target IP address. 26 One of the Unix-like operating systems. 11:04am Page 188 Edwards Chapter 4 ■ c04.tex V3 - 03/27/2009 11:04am Operating Systems and Networking Software in place. The Linux distribution being used will determine the name of the script. In some distributions, the script responsible for initializing the kernel for networking may have the name rc.inet1, whereas the script that starts the networking services may be named rc.inet2. Again, the distribution and vintage of Linux being used may cause these file names to be totally different. You should consult the documentation for your Linux version prior to configuring or making network changes on the Linux computer. The networking information for the kernel runtime can be accessed and displayed through the /proc file system. The /proc file system is usually mounted when the computer is first booted. If it is not mounted, there will be a message stating that procfs is not supported by the kernel. If this is the case, the kernel will need to be recompiled with procfs support enabled. Most Linux distributions come with a set of binaries27 containing all the applications and utilities needed for networking support. These applications and utilities may change from time to time with updates to the kernel and the networking utilities. These updates and applications need to be recompiled in order to be used as part of the Linux operating system. The following are a few of the basic networking configuration and monitoring commands: hostname — Sets the name of the computer entered in the /etc/hosts file. hostname ifconfig — Allows the interface to be available to the kernel networking layer. This command is normally a portion of the network initialization script that is executed at system boot-up. ifconfig The first interface required to be activated is the loopback interface.28 The following ifconfig command configures this interface: ifconfig l0 127.0.0.1 27 Binary files are programs that have already been compiled for the system the program is to be executed on. Since Linux can run over many various platforms, application programs need to be compiled on the computer to execute properly. To save users time, many Linux OS providers have already compiled these programs for the platform they are and are considered to be included binaries with the operating system. An example of different platforms would be those that are built around the Intel family of microprocessors versus those computers that have been designed and built using the Motorola 68000 microprocessor family. 28 The loopback interface on a computer is a logical network interface which will allow for testing of applications requiring network connectivity. Using this adapter permits the testing of those applications even though the computer is not connected to a network. An example of this would be a computer that is running as a web server testing itself by launching a web browser and navigating to the loopback IP address of 127.0.0.1. The web browser will bring up the server’s own home page. A less sophisticated use is in checking the IP stack of the computer by pinging the IP address 127.0.0.1. If no response is returned, there is a problem with the IP stack of that computer. 189 Page 189 Edwards 190 Part I ■ c04.tex V3 - 03/27/2009 Networking Nuts and Bolts The following entry in the host table is inserted upon execution of this command: localhost 127.0.0.1 RANDOM BONUS DEFINITION wire speed — The maximum frame and data rate that is supported on a given interface. Configuration of an Ethernet interface is accomplished using the following command: ifconfig eth0 netmask Status of an Ethernet interface can be obtained by executing the following command: ifconfig eth0 route — Used to add or delete routes from the kernel’s routing table. route [add | del] [-net | -host] target [if] add — Adds a route. del — Removes a route. -net — Specifies it is a network route. -host — Specifies a host address. target — Specifies the address of either the network or host. if — Specifies the network interface the route should be directed to (optional). To add a default gateway, execute the following command: route add default gw
netstat — As in Unix, a useful command to verify the operation and status of the Linux network components. netstat [-nr, -i, -ta] -nr — Displays the kernel’s route table with IP addresses displayed in dotted numerical notation. -i — Displays interface statistics for currently configured network interfaces. -ta — Displays a list of both active and passive TCP sockets. This command option can also be modified to also show UDP (-u), RAW (-w), and Unix sockets (-x). arp — Displays the kernel’s ARP table. arp -a 11:04am Page 190 Edwards Chapter 4 ■ c04.tex V3 - 03/27/2009 11:04am Operating Systems and Networking Software Linux is a very robust and feature-rich operating system POP QUIZ that is under constant develTrue or false: The name Linux is a opment and improvement. The derivative of the words Unix lite. commands in this section are just a beginning when it comes to Linux. Much more investigation is required, and the information that is available from a wide range of sources is beyond the scope of this section and book. 4.3.3 Sun Solaris Sun Microsystems initially developed the Solaris operating system for their Sun SPARC workstations. It has been ported to X86 Intel-based computers and is distributed and supported by Sun Microsystems. Like Linux, it has similarities and commonalities with the Unix operating system. The latest release of Sun’s operating system is Solaris 10. Although Solaris-based workstations are capable of operating in a standalone (not networked) environment, the operating system provides strong networking tools to allow it to be interconnected not only to the local LAN but the Internet. Solaris does provide a number of installation programs that will configure the built-in installations. Enabling a network interface on a Solaris computer requires the following actions: 1. Install device drivers. 2. Reboot to reconfigure the system. 3. Assign an IP address on the interface. 4. Create a hosts file entry to map the IP address to the host name. 5. Configure the interface to pass traffic. The IP address is assigned to an interface when the IP address is entered into the hostname file located in the /etc directory. As with Unix and Linux, this can be accomplished with the use of a text editor. An interface is configured to allow IP traffic with the use of the ifconfig command. The command can also be issued to verify the operation of an interface and to monitor its health. Issuing the ifconfig -a command displays all active interfaces on the computer. Incorrect configuration of an interface will result in an error message being returned stating ‘‘no such interface.’’ To enable an interface, issue the following command: /usr/bin/ifconfig eri0 up 191 Page 191 Edwards 192 Part I ■ c04.tex V3 - 03/27/2009 Networking Nuts and Bolts To verify connectivity over TCP/IP with other hosts on the network, issue the following command, which will display the kernel’s ARP table: arp -a The flags that can be returned in the ARP table are as follows: P — Indicates a published address S — Indicates a static address U — Indicates an unresolved address M — Indicates a mapped address for multicast Solaris allows for manual tuning of protocol transmission parameters for increased performance. This can be accomplished with the use of the ndd command. Using ACRONYM ALERT ndd parameter options for TCP, UDP, IP, SRT — Source route/transparent bridge and ARP will display a list of parameter values related to that particular protocol. An example of this would be the issuing of the command ndd /dev/tcp \? to display a list of all the parameters that are currently related to TCP. Like Unix and Linux, Solaris uses the netstat command to display network statistics and to verify the operational status of network interfaces. netstat is capable of displaying the following statistics: Data collection by protocol type Statistics grouped by node address, which may be IPv4, IPv6, or Unix-based Data related to DHCP Multicast grouped interface data Details of the routing table Data associated to STREAMS29 State and status of all IP interfaces State of all active logical and physical interfaces, routes, and sockets netstat can display protocol statistics for packets of the following types: TCP, UDP, RAWIP, IPv4, IPv6, ICMPv4, ICMPv6, and IGMP. Each of these 29 STREAMS is a flexible programming model used for Unix communications services. It allows for the definition of standard interfaces for character input and output both within the kernel and between the kernel and the rest of the Unix system. It is a collection of system calls, kernel resources, and kernel routines. 11:04am Page 192 Edwards Chapter 4 ■ c04.tex V3 - 03/27/2009 11:04am Operating Systems and Networking Software packet types has specific parameters associated with it. Generally they display the total number of packets in and out and those that are in error. When monitored, these counters can be used to point out possible problem areas. Issuing a netstat -m command will display the system calls, standard libraries, and kernel associated with writing network applications that use the STREAMS package. Additional details on this function can be obtained by reading the man page for the streamio command. Sun Solaris version 10 can be obtained free of charge from the POP QUIZ download section of the Sun Microsystems web page. A verList some of the Solaris network commands sion with documentation can be that are similar to those found in Unix and ordered directly from Sun for a Linux. nominal charge. If you are interested in learning more about the configuration and maintenance of a Sun system, the X86 version can be loaded on any i86 Intel microprocessor-based computer. 4.4 Chapter Exercises 1. If you have a network-capable PC, try using a few of the network utilities discussed in this chapter. 2. Open a DOS window by running cmd from Start, Run. Enter the command ipconfig and note what is displayed. 3. Issue the command ipconfig /all and note what is displayed. 4. If your network allows your PC to access the Internet, execute this command tracert and hit the Return key. Note the results. You may want to repeat this with other Internet addresses. 5. To display information about all the interfaces on a Unix computer, which command would need to be issued? 6. What is used on the Internet to find the numeric address of a computer host that resides on the Internet? 7. True or false: Floppy disks are the fastest form of magnetic media. 8. True or false: AT&T is the sole provider for the Unix operating system. 9. Can you name at least one Linux distribution? 10. If a microprocessor designer wanted to allow his newest chip design to access a greater amount of memory space, what might he do to accomplish this? 193 Page 193 Edwards 194 Part I 4.5 c04.tex V3 - 03/27/2009 Networking Nuts and Bolts ■ Pop Quiz Answers 1. What function does an arithmetic logic unit (ALU) provide? The ALU performs mathematical operations on the data it is presented with. 2. Would it be advisable to cycle power to the computer while a ROM upgrade is in process? No. 3. True or false: The information contained within RAM is saved when the computer is powered off. False. 4. When a computer is first powered on, the first device it is most likely to read its initial instructions from is the ROM. 5. Name a device that you may find connected to a serial port. Generally serial devices are slow data rate devices such as keyboards, modems, pointing devices, scanners, etc. However, with the development of Universal Serial Bus (USB) high-speed serial ports, devices such as hard disk drives and printers can be used due to the increased data rates on these ports. 6. What is the acronym for a user interface that uses a point-and-click method of executing computer commands? Graphical user interface (GUI) 7. Name two network operating systems that are prominent in today’s networking world. Novell Netware Microsoft Windows networking 8. What can be shared using Windows file sharing? Drives Directories Subdirectories 9. Which printers connected to a network-connected computer can be shared with other users on the network? All of the ones designated for sharing. 10. Which command can be used to verify the TCP/IP stack has been properly configured on a Unix computer? ping 127.0.0.1 11:04am Page 194 Edwards Chapter 4 ■ c04.tex V3 - 03/27/2009 Operating Systems and Networking Software 11. True or false: The name Linux is a derivative of the words Unix lite. False — The correct answer is Unix-like. 12. List some of the Solaris network commands that are similar to those found in Unix and Linux. netstat ping traceroute 11:04am 195 Page 195 Edwards c04.tex V3 - 03/27/2009 11:04am Page 196 Edwards c05.tex V3 - 03/27/2009 11:05am CHAPTER 5 The TCP/IP Protocol Suite I dwell in Possibility. Emily Dickinson TCP/IP is the name that refers to the group of protocols that it encompasses. This group of protocols is known as the TCP/IP protocol suite. It’s called TCP/IP because of the two main protocols that are part of the group: TCP and IP. The TCP/IP protocol suite is also known as the Internet protocol suite, as TCP/IP is pretty much the backbone of the Internet (and the majority of all networks out there). There are many good books that cover the TCP/IP protocol suite. Some of these are multivolume, so that might give you an idea of the amount of information that is covered in the standard. TCP/IP can be considered the most widely used standard of the Internet, much as Ethernet is the dominant LAN standard. In addition to multiple standards, TCP/IP also includes any applications, tools, and transmission media used in the network to pass datagrams. As a matter of fact, RFC 1180, ‘‘A TCP/IP Tutorial,’’ states that the term internet technology is more appropriate than TCP/IP when defining the purpose of the standard. As we discussed in Chapter 1, ‘‘Introduction to Networking,’’ the processes and standards contained in the TCP/IP protocol suite are mapped to one of four layers.1 These layers are based on the four-layer model of DARPA. Every layer within the TCP/IP reference model is cross-referenced to the seven-layer OSI reference model. The TCP/IP protocol suite allows data communication to take place. No matter what the node is, who it was made by, which operating system software 1 Or five layers, depending on what school of thought one follows. 197 Page 197 Edwards 198 Part I c05.tex V3 - 03/27/2009 Networking Nuts and Bolts ■ is running, and where the node is located, TCP/IP makes it work. TCP/IP has kept up with the tremendous growth that the Internet (as well as networks in general) has experienced. The possibilities seem endless and may very well be. The quote we selected for this chapter really is appropriate for the TCP/IP protocol suite because anyone involved with any facet of the TCP/IP protocol suite should always dwell in the possibilities. This chapter covers the more well-known protocols and functions that make up TCP/IP. What do these technologies and standards do? What layer of the TCP/IP reference model does each fall into and why? What are the differences among IPv4, IPv6, and IPng? These are just a few questions that will be answered in the pages to come. 5.1 The TCP/IP Layers Developers of networking protocols adhere to a layered approach. Each layer is responsible for a different portion of the data communication that is occurring at any time. There are many protocols that are part of the TCP/IP protocol suite. Each protocol functions within a layer of the TCP/IP model, depending on its function. Figure 5-1 shows an example of the TCP/IP model, how it corresponds to the OSI model, and some of the more well-known protocols that are served at each layer. OSI TCP/IP Application Presentation Application Session Transport Transport Network Internet Data Link Network Interface Physical TCP/IP Protocol Suite Telnet FTP DNS SMTP SNMP TFTP NFS DNS DHCP TCP IP RIP ATM UDP IGMP ICMP OSPF Ethernet HDLC PPP Frame Relay Token Ring FDDI Figure 5-1 TCP/IP reference model layering The layers in the TCP/IP reference model roughly correspond to one or more layers of the OSI reference model. Protocols of the upper layers can focus on the layer they are a member of, without concerning themselves with the functions performed by the lower levels. This is huge during the development 11:05am Page 198 Edwards c05.tex Chapter 5 ■ V3 - 03/27/2009 11:05am The TCP/IP Protocol Suite of the protocol, as it enables developers to focus on the development at each layer, rather than worrying about an all-encompassing standard. The layers of the TCP/IP reference model and their responsibilities are as follows: Network Interface layer — The Network Interface layer corresponds to the Physical and Data Link layers of the OSI reference model. This layer is also often referred to as the Link layer or the Data Link layer. The Network Interface layer is responsible for the device drivers and hardware interfaces that connect a node to the transmission media. Internet layer — The RANDOM BONUS DEFINITION Internet layer corresponds to the Network layer of uplink port — Any switch port that is designed to connect to a backbone switch or the OSI reference model. network. This layer is also known as the Network layer. The Internet layer is responsible for the delivery of packets through a network. All routing protocols (RIP, OSPF, IP, etc.) are members of this layer. Nodes that perform functions at this layer are responsible for receiving a datagram, determining where to send it to,2 and then forwarding it toward the destination. When a node receives a datagram that is destined for the node, this layer is responsible for determining the forwarding method for information in the packet. Finally, this layer contains protocols that will send and receive error messages and control messages as required. Transport layer — The Transport layer corresponds to the Transport layer of the OSI reference model. Two primary protocols operate at this layer: Transmission Control Protocol (TCP), and the User Datagram Protocol (UDP). This layer serves the Application layer and is responsible for data flow between two or more nodes within a network. Application layer — The Application layer corresponds to the Application, Presentation, and Session layers of the OSI reference model. Users initiate a process that will use an application to access network services. Applications work with protocols at the Transport layer in order to pass data in the form needed by the transport protocol chosen. On the receiving end, the data is received by the lower layers and passed up to the application for processing for the destination end user. This layer concerns itself with the details of the application and its process, and not so much about the movement of data. This is what separates this upper layer from the lower three layers. 2 Based on the IP address that is assigned to the destination network or node. 199 Page 199 Edwards 200 Part I ■ c05.tex V3 - 03/27/2009 Networking Nuts and Bolts The design of the TCP/IP model was based on the original Department of Defense network model. The act of layering network protocols is known as protocol layering. Protocol layering ensures that data sent by one layer on the source side is the same data received at that layer on the destination side. This layered principle allows focus to remain on the functions of protocols at the layer and ensure that the data matches on each end. Most applications will use the client/server method of communication. One of the host nodes will act as the server, and the other as a client. Each layer will use a protocol or a group of protocols to transfer readable data from the source layer to the peer layer on the destination side. Figure 5-2 shows an example of which protocols would be involved to transfer an e-mail message from a source to a destination. Application Layer SMTP SMTP Protocol SMTP TCP TCP Protocol TCP Internet Layer IP IP Protocol IP Network Interface Layer Ethernet (driver) Ethernet Protocol Ethernet (driver) Transport Layer Ethernet Figure 5-2 TCP/IP layering in action As you can see, a user on one side of a communication session initiates an e-mail to be sent to the user on the destination side of the session. The Application layer protocol that is used in this process is the Simple Mail Transfer Protocol (SMTP). SMTP will use TCP as the Transport layer protocol, IP as the Internet layer protocol, and then use the Ethernet interfaces at the Network Interface layer to send the data to the media for transport to the other end. This works exactly the same way when there are multiple networks in the mix (see Figure 5-3). 11:05am Page 200 Edwards c05.tex Chapter 5 ■ V3 - 03/27/2009 11:05am The TCP/IP Protocol Suite SMTP SMTP SMTP TCP TCP TCP Router IP IP Ethernet (driver) Ethernet Protocol IP IP Ethernet (driver) Ethernet (driver) Ethernet Ethernet Protocol IP Ethernet (driver) Ethernet Figure 5-3 TCP/IP layering in multiple networks In this example, a router is connecting two different networks.3 Notice that the layers on each end, even though they are not local, are still able to recognize information from their respective peers, as though they are on the same segment. There you have it. That is how the layered model works. The next section discusses many of the protocols that make up the TCP/IP protocol suite. 5.2 Popular TCP/IP Protocols Now that you know the principles of protocol layering and how it relates to the TCP/IP protocol suite, it’s time to discuss the various protocols that operate at each layer. There are many more protocols that are part of the TCP/IP protocol suite. This section covers some of the more widely known (and used) protocols in use in many networks today. 3 That’s the really nice thing about a router. It does not care what type of network it connects to. It can be Token Ring, Ethernet, or many others. The layers don’t realize any of this as long as they can talk to their peer. 201 Page 201 Edwards 202 Part I ■ 5.2.1 c05.tex V3 - 03/27/2009 Networking Nuts and Bolts The Application Layer A lot of applications are supported by nodes that run TCP/IP. Many of these are commonly included with the operating system software running on the node. If they are not built into an operating system, these applications can readily be found on the Internet, often free of charge. The Application layer is not concerned with the movement of data from one point to another on a network. Its only concern is the details of the application to ensure that what goes out is what is interpreted on the other end. The following protocols are discussed in this section: Domain Name System Simple Network Management Protocol File Transfer Protocol Trivial File Transfer Protocol Simple Mail Transfer Protocol Network File System Telecommunications Network Protocol Secure Shell 5.2.1.1 Domain Name System A domain name is simply the name assigned to a node on a network. It is also the name that is assigned as a host name for a given URL on the Internet. For example, if you want to go to the Cable News Network (CNN) website, you would open a web browser application (for example, Firefox, Internet Explorer, etc.) and initiate an HTTP session for the domain name that is assigned to CNN:4 http://www.cnn.com In the example, cnn.com is the domain name that you want to reach because you know that is the domain name for the CNN website. So, why is DNS important? Well, instead of a direct answer to that question, let’s answer it this way: What is the IP address for the CNN website? If you know 4 This ACRONYM ALERT AFP — AppleTalk Filing Protocol example was probably too simple, so don’t get fooled into thinking that any website you want to go to will have a domain name that matches the site. It depends whether that domain name is owned by someone else and, if it is, whether the owner is willing to sell the domain name. During the initial Internet boom, a lot of people had the foresight to buy popular domain names and later sold them for a lot of money. 11:05am Page 202 Edwards c05.tex Chapter 5 ■ V3 - 03/27/2009 11:05am The TCP/IP Protocol Suite that one, you really are doing well, but most likely you do not know the CNN website’s IP address. If you have access to a computer that supports TCP/IP, you can find out what the address is. Open up a command-line session and initiate a ping to the domain name, and you will be able to see the IP address assigned to the domain name. Here is a ping that was run to the cnn.com domain name and the IP address that was returned: C:\>ping cnn.com Pinging cnn.com [64.236.16.20] with 32 bytes of data: Reply Reply Reply Reply from from from from 64.236.16.20: 64.236.16.20: 64.236.16.20: 64.236.16.20: bytes=32 bytes=32 bytes=32 bytes=32 time=88ms time=88ms time=87ms time=87ms TTL=51 TTL=51 TTL=51 TTL=51 Ping statistics for 64.236.16.20: Packets: Sent = 4, Received = 4, Lost = 0 (0% loss), Approximate round trip times in milli-seconds: Minimum = 87ms, Maximum = 88ms, Average = 87ms As you can see in the example, the IP address assigned to the CNN website is 64.236.16.20. Once you know the IP address, you can put that number where you would normally enter the URL in your web browser, and it should bring up the site. The need for DNS is simple. Humans speak in words, whereas computers speak in numbers. Bits and bytes are all the computers understand. This is why a node has to be assigned a number.5 Sure, humans can learn numbers and use them as well, but it would probably take a lot of conditioning to remember all the numbers in IP addresses that are assigned to nodes in networks worldwide.6 DNS is a database that maps host names to IP addresses. The database is referred to as a disRANDOM BONUS DEFINITION tributed database, as DNS inforstore-and-forward — A mode of switch mation is distributed among operation where frames are completely several servers. Each server will received before they are forwarded onto maintain the DNS information any of the output ports of the device. that is assigned the server to serve to clients within its own network. DNS uses the client/server model, and the protocol itself provides the facility for the servers to share this information with authorized clients. 5 Remember 6 Are back when we couldn’t send an e-mail to Brother Joel? you kidding? Jim has a hard enough time just remembering how old he is. 203 Page 203 Edwards 204 Part I ■ c05.tex V3 - 03/27/2009 Networking Nuts and Bolts DNS names are organized hierarchically,7 with an unnamed root at the top, then what are known as top-level domain (TLD) names next, followed by second-level domain, and, finally, one or more subdomains. The names assigned to nodes in the DNS hierarchical tree are often referred to as labels. This organized hierarchy is known as the DNS namespace. The DNS namespace sets the rules for how the labels are organized in the domain name. Figure 5-4 shows an example of the DNS namespace. Root Top Level Domain Second Level Domain Top Level Domain Second Level Domain Subdomain Subdomain Top Level Domain Second Level Domain Second Level Domain Subdomain Subdomain Subdomain Figure 5-4 DNS namespace hierarchy The DNS namespace hierarchy requires a different administrator on each level. This ensures POP QUIZ that the administration of a The Internet layer is also known as the particular branch in the DNS layer. tree does not become too cumbersome. At each level of the namespace, there is an administrative authority that provides updates to the database. The delegation of authority should ensure that no level of the namespace becomes too hard to manage. 7 There is that word again. 11:05am Page 204 Edwards c05.tex Chapter 5 ■ V3 - 03/27/2009 11:05am The TCP/IP Protocol Suite Authorities at each level must ensure the DNS server is updated as required. Whenever there is a new node added into the network, the authority adds this to the database. Any removed nodes are required to be updated as well. Not keeping up with these can cause real headaches to end users as well as additional traffic on the network. DNS servers are normally installed in a redundant fashion. Updates are made to the primary server and then are synchronized with the secondary server.8 This ensures there is not a complete failure of DNS services should the primary server fail. So, let’s see this in action, shall we? We are going to assume there is a company that sells widgets and has decided to use DNS resolution so that end users don’t have to remember all of the IP addresses they have to access. DNS name syntax for this company could be: widgets.co In this example, co is the top-level domain name, and widgets is the secondlevel domain name. Notice that in between widgets and co is a period (.), which is ACRONYM ALERT called a dot. The DNS name widgets.co μs — Microseconds would be pronounced widjits-dot-see-oh. Pay attention to the dot that separates the levels within the domain name structure. In any name, the dot separates the levels. You can quickly identify the TLD when you run out of dots. Now, let’s assume there is an additional subdomain level, and an authority has been assigned to assign names to nodes within the particular department (Payroll, Production, Planning, and Sales) nodes. The namespace would be updated to reflect this (see Figure 5-5), and the name syntax for each could be9 as follows: payroll.widgets.co production.widgets.co planning.widgets.co sales.widgets.co The Internet Assigned Numbers Authority (IANA) is responsible for maintaining the DNS root zone and is the authority for domain names, IP addresses, and other parameters as well as appointing the authorities that sponsor them. 8 The synchronization is handled by the secondary server. The secondary server will query the primary periodically to see if there are any updates and, if so, will perform the update to its record. 9 The authority for the level can assign almost anything that he wants. Normally the name would reflect some identification that reflects the users it serves. The name must be 63 characters or less; other than that, the sky is the limit. 205 Page 205 Edwards 206 Part I ■ c05.tex V3 - 03/27/2009 Networking Nuts and Bolts Sometimes the top-level domain names are specific for a particular group or organization. For instance, the top-level domain name for the country of France is .fr.10 Root .co .widgets .payroll .production .planning .sales Figure 5-5 An example of the hierarchical tree structure for the widgets.co domain Sometimes the top-level domain is not really assigned to a particular purpose and therefore is generic in nature. These types of domain names are called generic top-level domains (gTLD). Some of the more well-known gTLDs are .biz — restricted for use by businesses .com — intended for use by commercial organizations .edu — postsecondary educational institutions .gov — restricted for use by the United States federal, state, and local governments. .jobs — for sites related to employment .mil — the United States Military .net — miscellaneous11 .org — miscellaneous organizations 5.2.1.2 Simple Network Management Protocol Today’s networks are no longer the shared media environments they once were. As you learned in Chapter 3, ‘‘Network Hardware and Transmission Media,’’ a lot of different nodes are deployed in the networks of today. More often than not, there is traffic sharing between nodes and multiple protocols that regulate the flow of data in the network. All this growth requires a way to keep track of what is going on within the network. 10 Which 11 This is basically the country code. domain was originally intended for large network infrastructure support centers. 11:05am Page 206 Edwards c05.tex Chapter 5 ■ V3 - 03/27/2009 11:05am The TCP/IP Protocol Suite Determining traffic patterns RANDOM BONUS DEFINITION to ensure that the network keeps up with end-user demands is protocol — A set of algorithms, not an option; it is a necessity if communication formats, and processes used the network is to live to its full in the process of data transmission in a network. potential. Having the ability to monitor the network12 for any problems that may occur and getting notification when a problem has arisen is just as (if not more so) important. Once again, the technology opened up for the development of a protocol that would do these things. That protocol is the Simple Network Management Protocol (SNMP). SNMP is a protocol that runs between an SNMP manager and an SNMP client, also known as an SNMP managed system, for the purpose of sharing management information pertaining to the managed system. Software that runs on the managed system used to communicate system information with the SNMP manager is known as the SNMP agent. The information that is shared is determined by the information (known as managed objects) set in the management information base (MIB).13 Communication between an SNMP manager and an SNMP agent is handled in two directions. The SNMP manager can query the SNMP agent for system information, or the SNMP agent can report information to the SNMP manager. There are five Protocol Data Unit (PDU) types that are exchanged between an SNMP manager and an SNMP agent.14 These are the GetRequest, GetNextRequest, SetRequest, GetResponse, and Trap. The GetRequest, GetNextRequest, and SetRequest are all PDUs that are sent from the SNMP manager to the SNMP agent. The GetResponse and Trap are sent from the SNMP agent to the SNMP manager (see Figure 5-6). We discuss these in more detail in Sections 5.2.1.2.1 and 5.2.1.2.3. 5.2.1.2.1 SNMP Managers The SNMP manager is a workstation that is running SNMP manager software. In some environments, the SNMP manager function is shared by more than one manager, so the resources of one device are not completely consumed trying to monitor the nodes in its charge. System failover is another reason why you may want to have multiple managers in your network. 12 In a proactive manner. will often hear people refer to the management information base as ‘‘the MIBs.’’ 14 An easy way to remember who is responsible to send what message type is to remember that the requests are sent by the SNMP manager to the SNMP agent, requesting information. That leaves only the SNMP response, which are the responses by the SNMP agent to requests that were sent by the SNMP manager, and a trap, which is notification of a problem. 13 You 207 Page 207 Edwards 208 Part I ■ SNMP manager SNMP manager SNMP manager c05.tex V3 - 03/27/2009 Networking Nuts and Bolts GetRequest GetResponse GetRequest GetResponse GetNextRequest GetResponse Trap SNMP agent SNMP agent SNMP agent Figure 5-6 An example of SNMP’s five PDUs in action SNMP managers normally output audible alarms and also color-coded reporting in real time. SNMP managers enable you set the protocols and nodes that you want to keep an eye on. Information that is sent from the SNMP manager to the SNMP agents can be one of three message types: GetRequest — This message type is a request by the SNMP manager for information pertaining to a variable within a particular managed object. GetNextRequest — This message type is used to retrieve information that is contained in subsequent requests for information pertaining to a managed object. This helps speed up the retrieval process as the SNMP manager does not have to send a GetRequest for each variable needed. SetRequest — This message type is used by the SNMP manager to make a change to a variable within a managed object. 5.2.1.2.2 SNMP Managed Devices An SNMP managed device is any network node that has SNMP agent software running on it for the purposes of network management. ACRONYM ALERT UTP — Unshielded twisted pair 5.2.1.2.3 SNMP Agents The SNMP agent is the software that runs on the SNMP managed device. This software is what allows the managed device to release system information 11:05am Page 208 Edwards c05.tex Chapter 5 ■ V3 - 03/27/2009 11:05am The TCP/IP Protocol Suite to the SNMP manager. The information to be monitored is set by the SNMP manager and is known as the managed objects. Some of the information that can be gathered is port failure, traffic patterns, network unreachable, protocol failures, and many other things. Information that is sent from the SNMP agent to the SNMP manager can be one of two message types: GetResponse — This message type is a response to the requests that are sent by the SNMP manager. This can be anything from a value of a variable for a managed object to an error response (for example, if there is no value or if the SNMP agent does not recognize the managed object that the SNMP manager is requesting information about). Trap — This message type is used by the SNMP agent to report a change of state for a managed object, as well as reporting errors. Some examples of errors that may be reported by the SNMP agent include Link up — The link is up and operational. Link down — The link is down. Cold start — To start a node from the beginning (i.e., a reboot). Warm start — To resume from where a process had left off. OSPF neighbor state changes — In IP routing, the process of learning OSPF topology changes. Authentication failures — Data that is received that cannot be authenticated or verified. Hardware failures — The issue is caused by a problem with hardware. Traffic bursts — The transfer of large amounts of data, without interruption, to a destination node. 5.2.1.2.4 Management Information Base A management information base (MIB) is a database that contains manageable objects and variables of these objects pertaining to a network node, for the purpose of node management within a network. SNMP itself is not able to define details for the information it retrieves; that is what a MIB is there for. The reason to keep MIBs and SNMP as separate standards is simple. This allows the management station to monitor multiple nodes, many with a different set of MIBs specific to the node. A MIB is configurable and can be updated. If a node is upgraded to support new and/or approved standards, the MIBs can be updated on the manager to match what is available on the agent. 209 Page 209 Edwards 210 Part I ■ c05.tex V3 - 03/27/2009 Networking Nuts and Bolts The formal language used by SNMP is Abstract Syntax RANDOM BONUS DEFINITION Notation 1 (ASN.1, pronounced half duplex — A communication mode A-S-N-dot-one). ASN.1 specifies where a device can either transmit or how information can be mapped receive data across a communications channel, but not at the same time. so it can be readable by humans and data nodes as well. The purpose of this encoding of data is to assign names and variables contained within a MIB to a standard so they can be precisely read and recorded by administrators as well as SNMP supported nodes. A subset of ASN.1 is the Structure Management Information (SMI) standards, which define the relationship of MIB objects. The MIB structure is similar to the structure that is used by DNS. It is a hierarchical tree structure with an unnamed root at the top of the tree and then levels of object identifiers (OID). An OID is a series of sequential integers separated by dots. The OID defines the path to the sought object. Figure 5-7 shows an example of the OID for the MIB variables. Root iso iso (1) 1 iso.org org (3) 1.3 iso.org.dod iso.org.dod.internet iso.org.dod.internet.mgmt iso.org.dod.internet.mgmt.mib dod (6) internet (1) mgmt (2) Mib (1) 1.3.6 1.3.6.1 1.3.6.1.2 1.3.6.1.2.1 Figure 5-7 The OID structure for SNMP MIB variables 11:05am Page 210 Edwards c05.tex Chapter 5 ■ V3 - 03/27/2009 11:05am The TCP/IP Protocol Suite In Figure 5-7, you can see the OID string on the right side of the tree and the corresponding names for each level. All MIB variables will start with 1.3.6.1.2.1, which is assigned the named value of iso.org.dod.internet. mgmt.mib. 5.2.1.2.5 SNMP version 2 The Simple Network Management Protocol version 2 (SNMPv2) introduced improvements and additions to some of the areas in the original SNMP standard. These improvements include Improved security SNMP-manager-to-SNMP-manager communication Improved performance Confidential sessions Additional protocol support Improvements in the way Trap PDUs are handled SNMPv2 also introduced two new types of PDUs. The first one is called GetBulkRequest, which improved on the GetNextRequest PDU by giving the SNMP manager ACRONYM ALERT the ability to retrieve all of that consecutive TAG — Technical action group data in one request instead of one request in between responses. In other words, everything is handled in one request and return response. The second PDU type that was introduced by SNMPv2 is Inform, which allows an SNMP manager to receive and reply to traps sent to and from another SNMP manager. SNMP and SNMPv2 are not completely compatible. They use different message formats as well as handle protocols differently. There are some optional configuration strategies that will help these versions coexist within the same network. One of these optional strategies is called a bilingual network management system, where an SNMP manager will determine what version an agent is using and then will speak with that agent in the version the agent understands. The other strategy is through the use of a proxy agent, where an SNMPv2 agent can act as a middleman and translate communications between an SNMPv2 manager and an SNMP agent. 5.2.1.2.6 SNMP version 3 The Simple Network Management Protocol version 3 (SNMPv3) is considered the official standard and is the one that will be developed upon if there are any updates or enhancements needed at some point in the future. 211 Page 211 Edwards 212 Part I ■ c05.tex V3 - 03/27/2009 Networking Nuts and Bolts SNMPv3 introduces some very important support for securing the access to nodes in the network and also offers remote node configuration support. SNMPv3 ensures message integrity, authentication, and encryption to assist in preventing unwanted individuals from accessing important information from traffic between the managers and the agents. 5.2.1.3 File Transfer Protocol The File Transfer Protocol (FTP) provides the ability for users to access an FTP server and transfer files to and from the server. FTP is used by network nodes as well as end users for file transfer of large amounts of data.15 FTP is a really easy protocol to use. It provides an interactive interface for end users, authenticates and provides access controls based on the authorizations that have been given to the users, and enables the system administrator to determine the format of the stored data. The only thing that is required for file access with the FTP protocol is a node that is running FTP server software, and the users must have some sort of a client software application running on their workstations.16 The server needs to know the user credential information. The user needs know their user ID and password, as well as the name or IP address of the FTP server. Nodes that participate in an FTP session can be in the same building or across the world from one another. To connect to the FTP server, all you have to do is issue an ftp command. The following example opens an FTP session between a workstation and the widgetsinc.com FTP server. Once connected, the FTP server will print any messages that are configured on the server and then will request the login credentials. % ftp widgetsinc.co Connected to widgetsinc.co 220-FTP server ready 230- Have a great day! 230230-Access to this network and the information on it are the lawful 230-property of widgets.co and its employees. If you 230-are not an authorized user then you are not authorized 230-on this server. 230-User (widgetsinc.co:(none)): Previously we said that FTP provides an interactive interface for end users, provides user access control, and that the format of the data stored can be of various types. Now, let’s take a look at some of these functions. For the examples in the following sections, we used a Microsoft Windows PC via 15 Sure, 16 If you can e-mail files too, but try to e-mail a 100 MB file. the node is TCP/IP compliant, the utility should already be available. 11:05am Page 212 Edwards c05.tex Chapter 5 ■ V3 - 03/27/2009 11:05am The TCP/IP Protocol Suite the cmd.exe17 window for RANDOM BONUS DEFINITION all command-line operations. Additionally, there is a freeware full duplex — A communication mode FTP server application (Cerewhere a device has the ability to bus FTP server) that is available simultaneously transmit and receive data across a communications channel. for download and supported by most Windows-based PCs. This application can be downloaded at www.cerberusftp.com. We recommend that you use this application if you are interested in replicating some of the examples. End users can use an FTP client application to access a node that is running the FTP server software for the purpose of either placing files on the server (with the put command), or getting files from the server (with the get command). The directories on an FTP server can also be manipulated by the end user, provided the user had the appropriate credentials when they log in. We will talk more about user access in the next section; for now, all you need to know is that you can perform the following functions with FTP: Retrieve files Store files ACRONYM ALERT Create directories SA — Source address Remove directories Rename directories View hidden files and directories Issue miscellaneous commands to navigate the directory tree As with any command-line structure you may come across, FTP utilizes several commands to perform tasks while in an FTP session. The command structure can vary from operating system to operating system, but the function of the command remains the same. Table 5-1 lists some of the more common FTP commands. Keeping track of whether you are here or there is important when you are in an FTP session. Keep in mind that you will be working with files and directories on two nodes. If you are getting a file, you are pulling it off of the remote node and filing it away on your local node. Likewise, if you are putting a file, you are getting a copy of the file on your local node and saving it on the remote node.18 17 cmd.exe is a command-line interpreter for most Windows-based systems that are in use today. It is the command that allows a user to communicate with the OS. 18 This sounds straightforward, and it really is. It does get confusing at times when you have been working on an issue for a while and sleep deprivation sets in. 213 Page 213 Edwards 214 Part I ■ c05.tex V3 - 03/27/2009 Networking Nuts and Bolts Table 5-1 Common FTP Commands COMMAND FUNCTION ascii Sets the file transfer mode to ASCII. binary Sets the file transfer mode to binary. cd Changes to another directory. close Terminates a connection. delete Removes a file. get Places a copy of a file on the remote node into a specified directory on the local node. hash Used to monitor the file transfer process. For every 1028 bytes received, a # will be placed on the screen. help Lists available FTP commands. ? Gets information about commands. ls Lists the names of the files in the current directory. mget Used to copy more than one file from the remote node to the local node. mkdir Makes a new directory. mput Used to copy more than one file from the local node to the remote node. put Used to copy a file from the local node to the remote node. pwd Determine the directory path to the current directory. quit Terminates the FTP session. rename Renames a file or directory. rmdir Removes a directory and any subdirectories, if applicable. Now it’s time for a special treat. The following walks through the process of putting a file from the local node onto the remote node. 1. Once you have the name or IP address of the remote node (the FTP server), open up a session with the server, using an FTP client (in our case, we are using the command line). You should see some confirmation that you have connected, then the banner (if there is one) is printed, and you will be prompted to log in. C:\>ftp 192.168.1.104 Connected to 192.168.1.104. 11:05am Page 214 Edwards c05.tex Chapter 5 ■ V3 - 03/27/2009 11:05am The TCP/IP Protocol Suite 220-Access to this network and the information on it are the 220-lawful property of widgets.co and its employees. If you are 220-not an employee or an authorized user, then you are not 220-authorized to be on this server. 220 User (192.168.1.104:(none)): 2. Log in using the credentials that have been provided to you. Some users may have more rights on the server than other users. Most FTP server administrators also allow for anonymous logins. Anonymous logins are beneficial if you have customers, vendors, and partners you may want to share files with, but not give them full access, only access to the directories they have a need to connect to. Once you have logged in and provided the password, you will receive confirmation that you have been authorized on the server. User (192.168.1.104:(none)): jedwards 331 User jedwards, password please Password: 230 Password Ok, User logged in 3. Use the ls command to see what directories and files the current directory possesses. In the following example, note that there are two directories: ftproot and widgets. ftp> ls 200 Port command received 150 Opening data connection ftproot widgets 226 Transfer complete 4. If you determine that you want to change to the widgets directory, use the cd command. ftp> cd widgets 250 Change directory ok ACRONYM ALERT LLC — Logical Link Control 5. Use the ls command to see if there are any subdirectories; note the customers directory. Assume that you want change to that directory (with the cd command) and prepare to copy a file from our workstation to the remote node.19 ftp> ls 200 Port command received 150 Opening data connection ftproot 19 If you know the path name for the destination directory, you can change to that directory by listing the path (cd widgets/customers). 215 Page 215 Edwards 216 Part I ■ c05.tex V3 - 03/27/2009 Networking Nuts and Bolts ftp> cd customers 250 Change directory ok 6. To verify your current directory, you can issue the pwd command. ftp> pwd 257 "/widgets/customers" is the current directory 7. You can set the transfer mode to ASCII.20 ftp> ascii 200 Type ASCII 8. You can set the transfer mode to binary. ftp> binary 200 Type Binary 9. Now put the file in the directory on the remote node. In this example, you will transfer two files: transfer.doc and transfer2.doc. POP QUIZ What is the function of the FTP command ascii? ftp> put c:\transfer.doc 200 Port command received 150 Opening data connection 226 Transfer complete ftp: 24064 bytes sent in 0.01Seconds 2406.40Kbytes/sec. ftp> put transfer2.doc 200 Port command received 150 Opening data connection 226 Transfer complete ftp: 24064 bytes sent in 0.00Seconds 24064000.00Kbytes/sec. 10. Since you transferred multiple files, you can also do this with the mput command. Take note that there is a confirmation required between files. ftp> mput c:\trans*.* mput c:\transfer.doc? 200 Port command received 150 Opening data connection 226 Transfer complete ftp: 24064 bytes sent in 0.01Seconds 2406.40Kbytes/sec. mput c:\transfer2.doc? 200 Port command received 150 Opening data connection 226 Transfer complete ftp: 24064 bytes sent in 0.01Seconds 2406.40Kbytes/sec. 20 ASCII is the default mode. 11:05am Page 216 Edwards c05.tex Chapter 5 ■ V3 - 03/27/2009 11:05am The TCP/IP Protocol Suite 11. Finally, log out of the session with the quit command. This will close the session and display any messages, if configured. ftp> quit 221 Have a great day TIME FOR SOMETHING NICE TO KNOW The ? command and the help command do not require an FTP session to be established in order to run. If you type the command ftp, you initiate the FTP client. Once you have the FTP prompt, you can issue the help or ? command to see a list of FTP commands. You can also connect to the remote node using the open command. Here is an example of both these commands, and the output: C:\>ftp ftp> ? Commands may be abbreviated. Commands are: ! ? append ascii bell binary bye cd close delete debug dir disconnect get glob hash help lcd literal ls mdelete mdir mget mkdir mls mput open prompt put pwd quit quote recv remotehelp rename rmdir send status trace type user verbose ftp> open 192.168.1.104 Connected to 192.168.1.104. 5.2.1.4 Trivial File Transfer Protocol Why waste time with a protocol that is so trivial?21 The Trivial File Transfer Protocol (TFTP)22 is another popuRANDOM BONUS DEFINITION lar file transfer program. Since Session layer — Layer 5 of the seven-layer the protocol uses UDP (see OSI model, responsible for process-toSection 5.2.2.2), there is less chatprocess communication. ter than with the FTP protocol, which uses TCP (see Section 5.2.2.1). TFTP is mainly used 21 Okay, it’s a lame joke, but we could not resist. that not all nodes support TFTP. If a network is performing file transfer in a controlled environment, it is likely that TFTP is not used at all. 22 Note 217 Page 217 Edwards 218 Part I ■ c05.tex V3 - 03/27/2009 Networking Nuts and Bolts with the Bootstrap Protocol (see Section 5.3.4) to transfer node configuration files for nodes that do not have hard disk storage.23 TFTP is also utilized to transfer files to and from network nodes for the purpose of troubleshooting, configuring, upgrading, and so on. TFTP is a simple protocol that is small enough to be stored in a node’s ROM. It requires a TFTP client and a TFTP server in order to function. Since UDP is a connectionless protocol, the TFTP server allocates different ports in order to support multiple TFTP clients at any given time. Security parameters are limited with the TFTP protocol. A system administrator can provide user access to only certain directories, but there is a potential for a security problem in the network if the TFTP sessions are not monitored and maintained. TFTP does not have all the functions that are available with FTP. To understand why, keep in mind that TFTP is a simple file transfer protocol designed to transfer boot-up files for diskless nodes. You won’t be able to browse directories, make directory changes, list files or directories, and you will be limited to the files you have been assigned. TFTP commands are very similar to the FTP commands (keeping in mind that there are fewer options with TFTP). Table 5-2 contains a list of the most often used commands. Table 5-2 Common TFTP Commands COMMAND FUNCTION connect Sets the remote node and/or ports for file transfer. get Places a copy of a file on the remote node onto a specified directory on the local node. hash Displays hash marks (#) to monitor file transfer progress. mode ascii Sets the file transfer mode to ASCII. mode binary Sets the file transfer mode to binary. put Copies a file from the local node to the remote node. quit Terminates the TFTP session. rate Displays the transfer rate information. status Displays relevant information about the transfer. TFTP is connectionless. This means that a connection is not established prior to the transfer of data. When a user issues the tftp command or the connect command, the client does not actually make a connection; rather, it buffers the information to use when it initiates the file transfer process. Following are a few TFTP command examples from a cmd.exe window: 1. To view the commands that are available in the cmd.exe command line for TFTP, you simply initiate the tftp command. C:\>tftp Transfers files to and from a remote computer running the TFTP service. TFTP [-i] host [GET | PUT] source [destination] -i Specifies binary image transfer mode (also called octet). In binary image mode the file is moved literally, byte by byte. Use this mode when transferring binary files. host Specifies the local or remote host. GET Transfers the file destination on the remote host to the file source on the local host. PUT Transfers the file source on the local host to the file destination on the remote host. source Specifies the file to transfer. destination Specifies where to transfer the file. 2. To retrieve a file from the remote node and save a copy on the local node, use the get command. C:\>tftp 192.168.1.104 get /widgets/Users/dns.doc Transfer successful: 20480 bytes in 1 second, 20480 bytes/s 3. Finally, to place a copy of a file that is stored on a local node onto the remote node, use the put command. C:\>tftp 192.168.1.104 put c:\dns.doc /widgets/Users/dns2.doc Transfer successful: 6 bytes in 1 second, 6 bytes/s It’s as simple as that. Note that you have to know the full path for the file that you want to get and place on the remote node. This is because the TFTP protocol does not support directory path browsing. This makes it a little less simple than FTP, but if used mainly for transfer of files for diskless systems and system modification, it should easily serve the purpose of most networks. 219 Page 219 Edwards 220 Part I ■ 5.2.1.5 c05.tex V3 - 03/27/2009 Networking Nuts and Bolts Simple Mail Transfer Protocol The Simple Mail Transfer Protocol (SMTP) is a protocol used for the transfers of electronic mail (e-mail) between network nodes. SMTP sets the format of e-mail from the client running on one node to a server running on another. SMTP is not involved with the way an end user interfaces with an e-mail application or stores e-mail messages, when to check for new messages, or when to send messages, nor is it involved in determining what e-mail messages to accept or not accept on the destination node. SMTP is concerned only with how the e-mail messages are transferred across the shared medium. SMTP works with the Post Office Protocol version 3 (POP3) and/or the Internet Message Access Protocol (IMAP), which enables e-mail messages to be stored (queued) on a server. The client periodically queries the server to check for and retrieve new messages. Without POP3 or IMAP, some messages might have a hard time reaching a destination due to the limited ability to queue data on the receiving node. In summary, POP3 and IMAP receive e-mail messages, and SMTP sends them. Many SMTP server applications include POP3 support in the same package. Communication in SMTP is initiated by the client. The server will respond to a client query with a response code and an explanation. The server will also respond to other servers with response codes. Response codes can be used when troubleshooting e-mail transfer problems. Table 5-3 lists the server response codes and their meanings. The client also has a set of messages that it will send to the server. There are a total of five messages used by a client to send an e-mail message. These are HELO — Used by the client to identify itself to the server MAIL — Identifies the end user sending the message RANDOM BONUS DEFINITION collision — When simultaneous transmission is attempted by two or more nodes on a shared Ethernet LAN RCPT — Identifies the end user the message is being sent to DATA — Identifies the contents of the message QUIT — Terminates the session 11:05am Page 220 Edwards c05.tex Chapter 5 ■ V3 - 03/27/2009 11:05am The TCP/IP Protocol Suite Table 5-3 SMTP Server Response Codes SERVER RESPONSE CODE EXPLANATION 220 Ready to receive mail from the client 221 Server is closing the session 250 Message sent from the server to the client informing the client that a requested action has been completed 251 Message sent from one server to another that it is forwarding mail for a user whom the server does not recognize 354 Message sent to a remote server in response to a query from that remote server about whether it can send mail 421 Server is unavailable 450 Message sent by the server to inform the client that a message could not be sent because the destination mailbox was not available 451 Message sent by the server when there is an error in processing a request; when this occurs, the request is terminated 452 Server has run out of storage space and cannot accept the message 500 Syntax error with a command 501 Syntax error with a function of a command 502 Server is not configured to support the request 503 Requests from the client are out of sequence and cannot be understood 550 Message cannot be delivered to the remote server or mailbox; if local, the mailbox is not available 551 The mailbox is not local, and the server cannot forward the message due to configuration constraints 552 User has run out of storage 553 SMTP address format is not correct 554 Request failed — no specification as to why 221 Page 221 Edwards 222 Part I ■ c05.tex V3 - 03/27/2009 Networking Nuts and Bolts Following is a cleartext example of an SMTP session. We will assume that the client has already set up a connection request and is waiting for the response from the server (which is the response code 220 in the first line of the following example). The lines that begin with S: are messages from the server, and the lines that begin with a C: are messages from the client. S: 220 smtp.widgets.com SMTP Service ready C: HELO smtp.example.org S: 250 Hello smtp.example.org, I am glad to meet you C: MAIL FROM: S: 250 Ok C: S: C: S: RCPT TO: 250 Ok RCPT TO: 550 That is not a valid user C: S: C: C: C: C: C: C: C: C: C: C: S: DATA 354 Input mail. End data with . From: "Slick Johnson" To: Blah Blah Blah Date: Thurs, 15 Jun 2008 08:02:11 -0500 Subject: Example Hey Blah! I need 20,000 widgets. Sincerely, Slick . 250 Ok Please send ASAP. C: QUIT S: 221 Bye Notice how the five messages are organized in the SMTP transfer. Also, note that one of the intended recipients is not a valid user. 5.2.1.6 Network File System Developed originally by Sun Microsystems, the Network File System (NFS) protocol allows end users access to files that are stored remotely as if the files were local to the end user’s workstation. The original version of NFS used UDP as a transport protocol; however, with the release of NFS version 3 (NFSv3) in 1995, the protocol included transport via TCP. This made it more feasible to use NFS over a WAN, thus increasing the options available for networks that had implemented and utilized NFS. 11:05am Page 222 Edwards c05.tex Chapter 5 ■ V3 - 03/27/2009 11:05am The TCP/IP Protocol Suite Like all the Application layer protocols discussed so far, NFS is a client/ server application. Using NFS, end users are able to view, store, update, and manage files on a remote server. All that is required is that the originating node has an NFS client application running and the remote node has an NFS server application running. Files that are shared on the server node are mounted, or set as accessible, for the users in the network. Access is controlled based on the permissions or privileges that have ACRONYM ALERT been set for an individual user. Permissions CPU — Central processing unit are set based on what directories the user is authorized to access. Privileges can be read/write (user can modify the file) or read-only (user can view the file but cannot modify the file). An NFS server must have some background applications running, known as daemons,24 in order for the client to be able to connect to and utilize the services that are provided through the NFS protocol. Following are the daemons that need to run on the NFS server: nfsd — This is the NFS daemon, which receives and processes requests from the NFS client(s). mountd — This is the NFS mount daemon, which receives requests from nfsd and processes them. rpcbind — This is a daemon that provides a way for the NFS clients to see what ports the NFS server is using. MORE UNIX DAEMONS Here is a handy-dandy reference list of common Unix daemons and their functions. ◆ dcpd — The DHCP daemon, which allows for the dynamic configuration of TCP/IP data for nodes running the appropriate client application. ◆ fingerd — The finger daemon, which provides finger protocol access to the server. ◆ ftpd — The FTP daemon, which supports and services FTP requests from a node running the client application. ◆ httpd — The HTTP daemon, which provides web server support. (continued) 24 When you look at a node’s file system, you can usually tell which processes are daemons. Most of these are identified with a ‘‘d’’ at the end of the name of the process. For instance, the http daemon is labeled httpd. 223 Page 223 Edwards 224 Part I ■ c05.tex V3 - 03/27/2009 Networking Nuts and Bolts MORE UNIX DAEMONS (continued) ◆ lpd — The line printer daemon, which manages the spooling of print jobs. ◆ nfsd — The NFS daemon, which receives and processes requests from the NFS client(s). ◆ ntpd — The NTP daemon, which manages node clock synchronization. ◆ rpcbind — The RPC daemon, which takes care of remote call procedure conversions. ◆ sshd — The SSH daemon, which monitors for SSH request from an SSH client. ◆ sendmail — The SMTP daemon, which handles e-mail transport. ◆ syslogd — The system logging daemon, which logs system processes and system log messages. ◆ syncd — The synchronization daemon, which synchronizes file systems with system memory. NFS is more commonly used with nodes that are running a Unix-like25 operating system; RANDOM BONUS DEFINITION however, there are many other hub — A central interconnection device operating systems that can use used in a star-wired topology and implement NFS in an environment where it is feasible to do so. Users working in an NFS environment are able to access their home directories that are stored on the NFS server from any workstation that has access to the server. This is a huge benefit, especially for users who may migrate from workstation to workstation. Another benefit of NFS implementation is workstation resource sharing (not having to fit every workstation with the entire same storage medium and software requirements). 5.2.1.7 Telecommunications Network The Telecommunications Network (Telnet) protocol gives a user the ability to access and manage a remote node. Almost all nodes that are running TCP/IP will support the Telnet protocol. The Telnet client initiates a session with a node that is running the Telnet server application. 25 Unix-like is a term that is used to identify an operating system that is similar to the original Unix operating system. 11:05am Page 224 Edwards c05.tex Chapter 5 ■ V3 - 03/27/2009 11:05am The TCP/IP Protocol Suite The server runs telnetd, POP QUIZ which listens for a Telnet client request. Telnet is used mostly What does an SMTP server response code for system administration, man421 mean? agement, and troubleshooting, but can also be used to check the status of other server types in the network. To initiate a Telnet session, issue the following command: telnet If you are successful, you will either be prompted with a login prompt or you will be at the user interface for the node. It depends on the settings of the remote node. Optionally, you can initiate a Telnet session in a Windows environment by issuing the telnet command. This will bring you to the Microsoft Telnet prompt, where you can view a list of commands. You can also initiate your session with the open command. Following is the Windows Telnet client interface: C:\>telnet Microsoft (R) Windows 2000 (TM) Version 5.00 (Build 2195) Welcome to Microsoft Telnet Client Telnet Client Build 5.00.99206.1 Escape Character is ‘CTRL+]’ Microsoft Telnet> ? Commands may be abbreviated. Supported commands are: close display open quit set status unset ?/help close current connection display operating parameters connect to a site exit telnet set options (type ‘set ?’ for a list) print status information unset options (type ‘unset ?’ for a list) print help information 5.2.1.7.1 Network Virtual Terminal Because there are so many different operating systems, it’s important that a client and server can participate in a Telnet session regardless of which operating system they are running. This is done through the use of a virtual node known as a network virtual terminal (NVT). The NVT basically provides a 225 Page 225 Edwards 226 Part I ■ c05.tex V3 - 03/27/2009 Networking Nuts and Bolts way for the client to provide a mapping to the interface the end user is using, and the server will map to a terminal type that it supports. Data in the NVT environment is input to a keyboard and then output to a printer. Figure 5-8 is an example of an NVT. Keyboard Printer NVT Printer Keyboard Telnet Client Telnet Server Figure 5-8 An NVT example 5.2.1.7.2 Options and Option Negotiation If a Telnet client supports it, the client and server have the ability to negotiate the use of features known as options for the session. Options can be negotiated before a Telnet session is set up or at any time during the session. The following four control characters are used for option negotiation: WILL — Used when the sender wants to enable an option POP QUIZ What does the acronym NFS stand for? WONT — Used when the sender wants to disable an option DO — Used when the sender wants the receiver to enable an option DON’T — Used when the sender wants the receiver to disable an option Table 5-4 lists some Telnet option codes.26 Option negotiation can be initiated by the server and the client. Some options are specifically for a client (that is, the server doesn’t have a need to request), and some are for the server. 5.2.1.7.3 Modes of Operation Telnet servers and clients comply with one of three modes of operation: Half-duplex mode (the default) means that communication takes place in half-duplex. This in and of itself is why this mode is for the most part never used. Most nodes now support full-duplex, which means that communication cannot be handled in half-duplex 26 Currently there are more than 50 option codes. 11:05am Page 226 Edwards c05.tex Chapter 5 ■ V3 - 03/27/2009 11:05am The TCP/IP Protocol Suite mode. In this mode, echoing is performed by the client, and the client will not transmit new data until the line that was sent previously is complete and has been received by the remote node. Table 5-4 Option Codes OPTION CODE OPTION EXPLANATION 0 Binary Assumes that transmission is binary 1 Echo Repeats information received 3 Suppress go ahead Suppresses go ahead signaling 5 Status Lists the Telnet status 6 Timing mark Sets the timing mark 24 Terminal type Sets the terminal type 31 Window size Sets the window size 32 Terminal speed Sets the terminal speed 33 Remote flow control Sets the remote flow control 34 Line mode Sets to line mode Character mode is a mode where only O-N-E C-H-A-R-A-C-T-E-R at a time is transmitted. The server will provide an acknowledgment when it receives each character and the echoing is performed by the server. The client, in turn, will send an acknowledgment to the server as well. Line mode is the mode where full-duplex transmission occurs with data being transmitted a line at a time. In line mode, text that is entered by the user is echoed locally and only full lines of data are transmitted to the server. This greatly reduces the number of packets that are required to be transmitted across the network. 5.2.1.8 Secure Shell Protocol The Secure Shell (SSH) Protocol provides a very important function that Telnet lacks: the ability to protect the integrity of the data being transmitted by supporting encrypted connections between network nodes. SSH utilizes public key cryptography, which provides cryptographic keys to authenticate remote nodes and users. In public key cryptography, two keys are involved in the encryption/decryption process: the public key, which can be shared by multiple remote nodes, and a private key, which is a secret used to decrypt a corresponding public key. 227 Page 227 Edwards 228 Part I ■ c05.tex V3 - 03/27/2009 Networking Nuts and Bolts Nodes that support SSH have POP QUIZ both a public and a private key assigned to them. The private What is the purpose of Telnet option key is protected by a password, code 32? which is entered by the user. The private key corresponds with the public key, which matches the public key on the remote end. The remote node has a private key as well that will decrypt the information sent to a readable form for the remote user. SSH is used primarily as an encrypted form of Telnet. With SSH, you can log in and be authenticated so the session is less vulnerable to attack than is the Telnet session. SSH also provides other functions, which makes it a very appeasable application to support in a network. SSH servers listen for requests coming from an SSH client. The SSH daemon runs on the server node. There are many SSH variations in today’s networks. The most popular ones are OpenSSH and Putty.27 The most recent version of the SSH protocol itself is SSH version 2 (SSH-2), which has been submitted as a proposed Internet standard. 5.2.2 The Transport Layer The next layer of the TCP/IP reference model is the Transport layer. It is the layer that accepts requests from the Application layer, and it sends requests to the Network layer. Transport protocols operate at the Transport layer. The two most popular of these protocols are the User Datagram Protocol (UDP) and the Transmission Control Protocol (TCP) at the Transport layer, both of which we introduce in this section. Chapter 9, ‘‘The Transport Layer,’’ will discuss these in more depth. 5.2.2.1 Transmission Control Protocol We bet you are thinking to yourself that you must have heard about this protocol before. Well, you have heard of it. At the very least you have heard it mentioned in this book, and it’s a good possibility that you have heard of it if you have ever configured your computer to be 27 These RANDOM BONUS DEFINITION Media Access Control — The entity or algorithm used to arbitrate for access to a shared communications channel. and many others are open source applications, which can be downloaded from many different websites. An Internet search will point you to where you can download these. 11:05am Page 228 Edwards c05.tex Chapter 5 ■ V3 - 03/27/2009 11:05am The TCP/IP Protocol Suite connected to a network. You may not have known what it does, but you have heard of it. TCP is used to transport data. It ensures that data is placed in sequence (the order that it was sent in), that data arrives at its destination (or will force a retransmission if it didn’t), and it helps cut down on over-traffic in the network. To give you an idea of why TCP is important, take a look at Figure 5-9. H! H ! Ho Hi Tom! ? m om i ? ? ? i!T oHm T oT i H ? ? ? oi Figure 5-9 An example that proves why TCP is very helpful In the figure, you can see that a node wants to send the message ‘‘Hi Tom!’’ to a remote node.28 There are many different paths that data can take to get from the originating node to the remote node. Assuming that we are sending one character at a time, each character will take whatever path the routers tell it to take. Because the originating and the destination nodes do not know which path the data is taking, the destination node will have no way to put the data back together when it receives it, and therefore will most likely receive a jumbled mess. Note that the destination node receives all the data, but the message received is ‘‘i!T oHm,’’ which is nothing like the originating message.29 TCP is a connection-oriented protocol, which means that a TCP session must be established between a TCP server and a TCP client before any data transmission occurs. Most professionals use the analogy of a telephone when explaining the meaning of connection-oriented. When you make a phone call, you wait until someone answers the other end before you say hello, hey, how’s it going, or anything else that you called to say.30 This is exactly how TCP works. An originating node will contact a destination node to make sure they 28 For this example, it really does not matter what application is being used to send the message. All that is important is that you understand that the information is coming from the Application layer and is being sent to the Network layer. 29 Can you imagine what Brother Joel might think about this message? 30 Some phrases can be uttered that we can’t mention in this book. 229 Page 229 Edwards 230 Part I ■ c05.tex V3 - 03/27/2009 Networking Nuts and Bolts are available to get the message. Once confirmation is received that it is okay to send data, the transmission begins. TCP is also considered a reliable protocol because there are functions built into TCP that provide for various checks and balances to ensure the integrity of the data being transmitted. Some of the reliability functions are TCP is able to break down data that is received from the Application layer into segments. TCP places an acknowledgment timer on sent segments. When the timer expires, if the originating node does not receive confirmation from the remote node that the segment was received, the originating node will resend the segment. TCP maintains a checksum (within the TCP header and within the actual data payload) that is set on each end of the connection. The checksum is used to ensure that data arrives exactly as it was sent. If the receiving node notices that the checksum does not match (invalid checksum), the receiving node will throw the segment away. In throwing the segment away, the receiver does not receive the segment. This means that the receiving node does not send an acknowledgment, which causes the originator to send it again. TCP datagrams are not sent in order. They traverse the network over the best path possible (based on calculations made by nodes, which we discuss in several places throughout this book). TCP supports the ability for the receiving node to put all of the datagrams back into the correct order, once they have been received. TCP can recognize duplicate datagrams and can discard them when received. TCP supports what POP QUIZ is known as flow control. Flow conWhat does the acronym SSH stand for? trol is a way for each node to know how much buffer space they have available to receive data. This way no node will overwhelm the other node with more data than it can handle. Examples of applications31 that use TCP would be FTP Telnet 31 Notice that some protocols use both TCP and UDP (DNS, for instance). 11:05am Page 230 Edwards c05.tex Chapter 5 ■ V3 - 03/27/2009 11:05am The TCP/IP Protocol Suite SMTP DNS POP3 HTTP DNS IMAP 5.2.2.2 User Datagram Protocol Here is a bonus question for you. The User Datagram Protocol (UDP) is part of the Transport layer and is used to do what to data? That’s right! Just like TCP is used to transport data between nodes, UDP is also used to transport data within a network. That is about the only thing (at least functionally) that the two have in common. UDP does not guarantee that data is going to be delivered to a destination. Basically, UDP throws the data toward the destination and then moves on to its next task. This makes UDP a connectionless protocol. UDP is usually used to send short bursts of datagrams between nodes where reliabilRANDOM BONUS DEFINITION ity is not a big concern. UDP operating system — The application can get data to a destination software responsible for the proper quicker, as it avoids all of the operation of a given node. overhead required when all the checks and balances are occurring within TCP. Also, because UDP is connectionless, it can support broadcasting (sending messages to all nodes within a broadcast domain) and multicasting (sending messages to all nodes that are subscribed to the network). UDP provides an optional checksum that can be assigned to the UDP header as well as the data payload. This ensures that if any data that is sent over UDP requires a header and data payload checksum, the destination is able to do so. If any error checking is required, it will normally be performed by the application, not via UDP. Most voice and video applications transmit over UDP. If you have ever watched a video online that cut out or got choppy at times, this is because data was not being received. Recovery from these choppy moments can go unnoticed for the most part. If TCP were used in these instances, there would be delays that last much longer when packet loss is requiring retransmission of the data. Keep in mind that speed is the consideration when going with UDP, not reliability.32 32 You can always reload that video if you want to watch it again. 231 Page 231 Edwards 232 Part I ■ c05.tex V3 - 03/27/2009 Networking Nuts and Bolts Examples of protocols that use UDP include DNS BOOTP/DHCP TFTP SNMP RIP NFS UDP accepts data (the payload) from the Application layer. It then adds a UDP header and passes the header and the payload to the Internet layer, where it is encapsulated into an IP packet and is passed on to the Network Interface layer and over the transmission medium to the destination, where it makes its way up to the Application layer on the destination end of the connection. 5.2.3 The Internet Layer The final layer that we will be discussing in this chapter is the POP QUIZ Internet layer. Although we will discuss this layer in detail in Name the two popular transport protocols that we discussed in this chapter. Chapter 10, we wanted to provide a quick overview of some Internet layer protocols. This layer is responsible for ensuring that there is a path to a destination. It receives information from the Transport layer and ensures transmission to the destination node. Some examples of protocols that operate at this layer include Internet Protocol (IP) Internet Group Multicast Protocol (IGMP) Internet Control Message Protocol (ICMP) Address Resolution Protocol (ARP) Routing Information Protocol (RIP) Open Shortest Path First (OSPF) Border Gateway Protocol (BGP) Internet Protocol Security (IPSec) Although all layers of the TCP/IP reference model are important in their own right, the Internet layer is probably the most important one. It provides 11:05am Page 232 Edwards c05.tex Chapter 5 ■ V3 - 03/27/2009 11:05am The TCP/IP Protocol Suite the ability to route data to a destination based on an IP address. It manages the IP addressing structure for a network, and it also defines the datagrams that are transported to a remote node. 5.2.3.1 Internet Protocol The Internet Protocol (IP) is the most important protocol that exists within the Internet layer. IP receives data from one of the Transport layer protocols, packages it into a datagram, and then transports it to and from a given set of nodes. IP is a connectionless protocol, which means it does not establish a line of communication prior to transmitting.33 IP is also responsible for the IP addressing for network nodes. The network node that is responsible for getting data between different networks is a router. The router is responsible for receiving a datagram known as a packet and ACRONYM ALERT pointing the packet in the direction it needs FCS — Frame Check Sequence to go, based on the IP address the packet is looking for. IP addresses are learned by the router based on information from another router or information that it has discovered as it was passing packets to and fro. The information received for the purpose of routing packets is determined, calculated, and provided for by a routing protocol. IP addresses can also be configured and set statically (hard coded), but this is a tedious task to maintain. The dynamic option is a preferred method.34 Since IP is connectionless, the upper layers are responsible for any error checking. The most IP will do is drop a packet and then send a message to the source IP address telling them that the packet didn’t make it to where it was supposed to go. There are many protocols that work with IP and are placed into an IP packet for transmission. Some of these include TCP UDP ICMP There are a few versions of IP in use today. IP version 4 (IPv4) is the most commonly used version, but a proposed standard, IP version 6 (IPv6), is in use and 33 Here POP QUIZ Which layer of the TCP/IP reference model is probably the most important one? is more of that repetition that we mentioned in the front matter of this book. will find that there are times when static routes make the most sense. They can also help you get a route back up when you are troubleshooting an issue. Static routes can be your friend. 34 You 233 Page 233 Edwards 234 Part I ■ c05.tex V3 - 03/27/2009 Networking Nuts and Bolts is intended to eventually be the successor to IPv4. The main difference between IPv4 and IPv6 is the addressing. IPv6 allows for more addressing flexibility, as there is room for a larger address space. Both versions will probably be around for a long time, and there are ways to ensure that they can coexist, but eventually you will probably see a migration to IPv6. Have you ever heard of IP Next Generation (IPng)? IPng is nothing more than the unofficial name for IPv6. The name was coined early and replaced when the proposed standard was submitted. 5.2.3.2 Internet Group Multicast Protocol The Internet Group Multicast Protocol (IGMP) is a protocol that provides support for IP multicasting. IGMP provides a way for messages to be sent to multiple nodes. Nodes are grouped into multicast groups, so when a multicast message is destined for a group, only that group will receive the message. IGMP messages are transmitted in an IP datagram. Multicast routers (that is, routers that can support multicasting) use IGMP messages to keep track of what groups are connected to what interfaces on the router. When the operating system of the originating node initiates a program process that requires IGMP support, the node will send a report out of an interface in which the process joins the group. Processes can join groups over multiple interfaces. When there are no other processes running in a group, the node will no longer report the group. IGMP queries are sent out by a multicast router periodically to see if anyone has a process that might belong to a multicast group. This query is sent out of every router interface. When a remote node receives an IGMP query, it will respond with one report for each group that it recognizes as having a running process. There may be many remote nodes running processes that are tied to a multicast group. Each node is responsible for reporting process and group information. The times that these reports are sent are staggered so there are not too many nodes responding at the same time. For a router to acknowledge a multicast group, there must be at least one node that is a member of the group. 5.2.3.3 Internet Control Message Protocol The Internet Control Message Protocol (ICMP) is responsible for reporting conditions that need attention. When something goes wrong with IP, TCP, or UDP transmission, ICMP is there to let you know about it. Like ICMP, TCP, and UDP, ICMP messages are transmitted within an IP datagram. 11:05am Page 234 Edwards c05.tex Chapter 5 ■ V3 - 03/27/2009 11:05am The TCP/IP Protocol Suite Two versions of ICMP are in use today: ICMP version 4 (ICMPv4)35 and ICMP version 6 (ICMPv6). ICMPv4 was developed to work with IPv4, so with the release of IPv6 updates were required and ICMPv6 was born.36 The functions of each version are basically the same. ICMPvwhatever is there to pass messages. Following are the main reporting functions performed by ICMP: Error reporting Testing and troubleshooting Informational reporting IP and ICMP work very well together. As a matter of fact, you can consider ICMP the ‘‘right-hand man’’ of IP. While IP is busy packing up data and routing that data to a destination, ICMP is taking care of all the busywork. ICMP passes messages that help ensure IP can perform its job well. Many consider ICMP one of the simplest protocols there is. If you think about it, this is true. ICMP doesn’t have to give a lot of thought or calculation to do its job. All it has to do is pass messages. 5.2.3.4 Routing Information Protocol The Routing Information Protocol (RIP) is a dynamic routing protocol that is used in many POP QUIZ networks. It is a distance-vector What is the difference between IPng and protocol, which means that each IPv6? router will advertise the destinations it is aware of and the distance to each destination to neighboring routers. Many different implementations of RIP were in place when the protocol became an Internet standard. Although there were a few differences between RIP implementations in different networks, the differences didn’t cause many interoperability issues in production. A second version of RIP (RIPv2, or RIP2) was introduced and offered a few improvements over the original version of RIP. The most notable of these improvements was the support of variable length subnet masking (VLSM)37 and support for authentication. 35 ICMPv4 wasn’t always called that. It was called simply ICMP since its inception. The v4 was added later to separate it from ICMPv6. 36 ICMPng in and of itself is a pretty cool acronym. Not too many adopted the term, but at least one of the authors of this book would have adopted it (yes, we are talking about that author who thinks catenet is a cool term). 37 VLSM increases the efficiency of the utilization of IP addresses in a given network by allowing different subnet masks to be used for each subnet. This will be discussed in Chapter 10, ‘‘The Internet Layer.’’ 235 Page 235 Edwards 236 Part I ■ c05.tex V3 - 03/27/2009 Networking Nuts and Bolts RIP determines distances to a destination based on what is known as a hop count, which is the number of devices a packet must pass through on the way to a destination. The hop count increases each time a packet reaches a node along the path to its destination. The link taken by the packet from one node to another node is the actual hop. Figure 5-10 shows an example of hops38 in a network. Hop Hop Hop Hop Hop Hop Hop Hop Figure 5-10 Hops in a RIP-routed environment Now, let’s quickly review the operation of RIP. When a router first boots up, one of the first things it will do (once connectivity is established) is send a packet out of each interface requesting routing tables from each of the neighboring routers. In turn, each router will send the routing table to the router that requested it. As the router receives the routing table from the neighboring routers, it will send a response telling the neighbors it has received the requested routing table. The neighbors will respond with any updates they may have since they last sent the routing table. If there are no updates, the neighbors will validate that they know of the originating router. Once the preliminary routing table updates are performed, the routing table of each router will be broadcast to all other neighbor routers. This update occurs every 30 seconds. Updates known as triggered updates will occur whenever 38 This is not to be confused with the flower hops, which is a key ingredient in beer. There is a shortage of hops at the time of this writing, which makes the hobby of home brewing a bit more expensive than in years past. 11:05am Page 236 Edwards c05.tex Chapter 5 ■ V3 - 03/27/2009 11:05am The TCP/IP Protocol Suite there is a change with the hop count to a destination. When triggered updates occur, only the information that has changed is sent. 5.2.3.5 Open Shortest Path First The Open Shortest Path First (OSPF) protocol is a dynamic routing protocol that uses the link state between nodes to determine routing paths for packets. The link state is simply the state of the link to the next router (the neighbor). Routers in an OSPF environment do not check the distance from one point to another in a network. Instead, the routers monitor the state of a link to each of its neighbor routers (the router next door). The link states are logged into the link state database (LSDB), which is then shared with all the neighbors. LSDB information that is received is used to build the routing table for the router and then the information is shared with its neighbors. Although an OSPF system can be a single autonomous system, most often OSPF routers are assigned as members of OSPF areas. Each area is identified by a 32-bit identifier, much like an IP address. Routers in the OSPF environment are also assigned tasks they need to perform to ensure that the routing domain runs smoothly. Following are a few important terms you will need to know: Backbone area — The core of the entire OSPF network. The identifier that is assigned to the backbone area is 0.0.0.0. All areas are connected to the backbone area. Stub area — An autonomous system that only receives LSDB updates from routers within the same area. The stub area only receives external routes through the default route. Not so stubby area (NSSA) — A stub area that contains no external routes. The NSSA can retrieve external updates and send them to the backbone. Internal router — Any router that only shares information with routers in the same area. Backbone router — Any router that participates in the backbone area. Most backbone routers are ABRs as they share information between areas. There may be some routers in the backbone that are not ABRs, but these are still backbone routers as they are in the backbone area. Area border router (ABR) — Any router that is a member of more than one area. Autonomous system boundary router (ASBR) — Any router that shares link state with a router in another area is called an ASBR. Note that any router within the area can be an ASBR; this includes area border routers, backbone routers, and internal routers. 237 Page 237 Edwards 238 Part I ■ c05.tex V3 - 03/27/2009 Networking Nuts and Bolts Designated router (DR) — Any router that handles advertisements on multi-access networks. The DR is elected by a process among other routers. It is responsible for being the representative for the multi-access network to the rest of the network. It is also in place to ensure that data is not flooded due to the multi-access environment. Backup designated router (BDR) — Any router that takes over the responsibilities of the DR if the DR should fail. 5.2.3.6 Border Gateway Protocol The Border Gateway Protocol (BGP) provides for IP data communication between routers that are in different autonomous systems (AS). BGP routers share information with one another, providing paths that can be used to reach an AS. To prevent routing loops, BGP routers make a determination of the best path and any possible loops are pruned from the decision tree. An AS can be classified much as areas are in OSPF, including Multihomed AS — An AS that connects to more than one other AS. A multihomed AS does not participate in transit traffic. Stub AS — An AS that connects to only one other AS. A stub AS does not participate in transit traffic. Transit AS — An AS that connects to more than one other AS. A transit AS participates in local and transit traffic. Data traffic within an AS is either transit traffic (just passing through) or local traffic (traffic that starts or ends39 within the AS). Like RIP, BGP is a distance-vector protocol. However, instead of counting hops to a destination, BGP counts the number of autonomous systems it takes to get to a destination. BGP also supports policy-based routing. In other words, policy specifications are set by the system administrator and are used to allow BGP to determine the best route to a destination, ensuring all policies are strictly enforced. This means that even though there may be a quicker path to take to a destination, policies may prevent a datagram from going on that path. BGP sends what are known as keepalive messages to its neighbors to ensure that the neighbors are reachable. If they are not reachable, BGP will recognize this as a link failure. 5.2.3.7 Internet Protocol Security Internet Protocol Security (IPSec) is a suite of protocols that allow for security and encryption for IP datagrams. IPSec is designed to provide endpoint to 39 The alpha and omega of BGP traffic types. 11:05am Page 238 Edwards c05.tex Chapter 5 ■ V3 - 03/27/2009 11:05am The TCP/IP Protocol Suite endpoint datagram security (transport mode) for nodes that do not support security protocols.40 IPSec is also used in VPN environments (tunnel mode), which allows the gateway to the network to provide security and authentication services for the users and networks the node supports. IPSec provides several types of security for networks and the users of the networks. One of the biggest functions that came from IPSec is the ability to encrypt datagrams so that only the destination can read and understand them.41 IPSec also provides checks of datgrams to ensure that they have not been tampered with in transit. Finally, IPSec provides for the authentication of users, to ensure that anyone that should not have access doesn’t. AN UNRELATED MOMENT OF PAUSE Three friends were out driving one day. One was a network sales engineer, one was a network hardware engineer, and one was a network software engineer. All of the sudden the right rear tire blew out, and the car rolled to a stop. Since the car was full of problem solvers, the three friends jumped out of the car to survey the situation. The network sales engineer proclaimed, ‘‘The car just won’t do anymore; it is time to buy a new one!’’ The network hardware engineer gave it some thought and then said, ‘‘We need to try swapping the left tires with the right tires. If that does not fix it, then we need to swap the front tires with the rear tires. If we are still having problems at that point, we will have to replace the tires.’’ The network software engineer then piped in, ‘‘You guys are just wasting time. We need to get back in the car and drive some more to see if the problem will just work itself out.’’ 5.3 End of Chapter Hodgepodge We hope that you now have a better understanding of the TCP/IP reference model, some of the protocols that operate in each layer, and how each layer interfaces with each of the other layers. As you continue through the pages of this book, we will be revisiting a lot of these protocols and discussing some of the details that make each one tick. In this section, we will discuss some of the other processes that operate in a TCP/IP environment. Like many of the other functions and specifications that we have discussed in this chapter, we will be revisiting some of these in upcoming chapters. 40 These nodes may support security, but not at the level that a network needs the node to. when we were talking about key exchange? 41 Remember 239 Page 239 Edwards 240 Part I 5.3.1 ■ c05.tex V3 - 03/27/2009 Networking Nuts and Bolts There Is Hope for Diskless Nodes The Bootstrap Protocol (BOOTP) manages IP parameters on a given network. It assigns IP addresses for a pool of users. Not only that, it also provides for operating system initiation for remote diskless nodes. BOOTP is a network protocol that uses UDP for transport. When a node is booting up, there is a bootstrap process that initiates the execution of the node’s operating system. If a node is running a BOOTP client, the node will send a request to a BOOTP server for assignment of an IP address, along with any other startup assistance that the client node requires (and the BOOTP server supports). BOOTP is normally integrated into the node’s motherboard or NIC card. The Dynamic Host Configuration Protocol (DHCP) evolved from BOOTP. Several enhancements were provided with DHCP, although BOOTP is simpler to implement and maintain. A single DHCP server can provide IP addresses, subnet masks, gateway information, and more. When a node connects to the network, the DHCP client will broadcast a request for information from the DHCP server. The server will then send the requested information so the node can connect and operate in the network. BOOTP and DHCP are called communication management protocols. They can work separately or together (together is the most often implemented). DHCP can serve the requests that come from a BOOTP client. 5.3.2 A Little More Information on Routing Just when you thought we had finished with our discussion about routers, here we are back on the subject.42 Following are a few terms that we wanted to quickly touch on. Why not? We have to discuss them somewhere. Routing protocol — The protocol that performs functions that allow the routing of packets between routers. RIP, OSPF, and BGP are examples of routing protocols. Sometimes confused with a routed protocol, which is not the same thing. Routed protocol — A protocol that participates in transmitting data between nodes within a network. Telnet, SNMP, and IP are all examples of a routed protocol. Routed protocols are sometimes incorrectly termed routing protocols. Gateway — The entry point for an entity. A computer that provides access to a network area is a gateway. A network that provides access to another network is a gateway. Many applications have gateways that allow information sharing. The node that connects the LAN to the Internet (or any other network type) is a gateway. 42 We are far from finished with our discussion on routers. 11:05am Page 240 Edwards c05.tex Chapter 5 ■ V3 - 03/27/2009 11:05am The TCP/IP Protocol Suite Interior Gateway Protocol (IGP) — A routing protocol that operates within an AS. RIP and OSPF are IGPs. Exterior Gateway Protocol (EGP) — BGP is often called an EGP, although the EGP protocol was the predecessor to BGP for IP routing between autonomous systems. Static routing — IP routing information that is manually configured on a node by a system administrator. Dynamic routing — IP routing information that is learned by the node through a routing protocol, such as RIP. POP QUIZ What are the two IGPs that we discussed in this chapter? This concludes our discussion of routers for this chapter. 5.3.3 Sockets and Ports Are Not the Same Thing A couple of important terms that often get confused are socket43 and port. Note that we are referring to TCP and/or UDP ports, not to the physical interface of the node. A TCP or UDP port is a number assigned to the datagram header that is mapped to a particular process or application on a given node. A socket is the end-point of data communication flow on a network. TCP and UDP ports are basically an extension of addressing used by TCP/IP to ensure that data communication is tied to the correct running process. Each packet header that is transported over TCP or UDP has a source and destination port logged in it. The port number can range from 0 to 65535. Port numbers are divided into three sections. These are well-known ports (0 through 1023), registered ports (1024 through 49151), and dynamic and/or private ports (49152 through 65535). TCP/UDP WELL-KNOWN PORT NUMBERS Following is an example list of many popular well-known TCP and UDP port numbers. TCP well-known port numbers are identified by an assignment of 0 through 1023. This list is only an example to provide the port numbers for many of the protocols we have covered, along with a few that are just darn interesting. (continued) 43 Sockets are also often called TCP or UDP sockets (depending on the transport protocol), Internet sockets, or network sockets. 241 Page 241 Edwards 242 Part I ■ c05.tex V3 - 03/27/2009 Networking Nuts and Bolts TCP/UDP WELL-KNOWN PORT NUMBERS (continued) For a complete and current list, go to www.iana.org/assignments/ port-numbers. Port Number Description Applicable Protocol 0 Reserved TCP and UDP 1 TCP port service multiplexer TCP and UDP 5 Remote job entry TCP and UDP 7 Echo TCP and UDP 20 FTP – data TCP 21 FTP – control TCP 22 SSH TCP and UDP 23 Telnet TCP and UDP 25 SMTP TCP and UDP 53 DNS TCP and UDP 67 BOOTP/DHCP – server TCP and UDP 68 BOOTP/DHCP - client TCP and UDP 69 TFTP TCP and UDP 80 HTTP TCP and UDP 101 NIC host name server TCP and UDP 107 Remote Telnet service TCP and UDP 109 POP2 TCP and UDP 110 POP3 TCP and UDP 115 SFTP TCP and UDP 118 SQL TCP and UDP 123 NTP TCP and UDP 135 DCE endpoint TCP and UDP 143 IMAP TCP and UDP 161 SNMP TCP and UDP 162 SNMP trap TCP and UDP 166 Sirius TCP and UDP 179 BGP TCP and UDP 213 IPX TCP and UDP 220 IMAPv3 TCP and UDP (continued) 11:05am Page 242 Edwards c05.tex Chapter 5 ■ V3 - 03/27/2009 11:05am The TCP/IP Protocol Suite TCP/UDP WELL-KNOWN PORT NUMBERS (continued) Port Number Description Applicable Protocol 389 LDAP TCP and UDP 401 UPS TCP and UDP 500 ISAKMP UDP 513 Login TCP 513 Who UDP 515 Lpd TCP 520 RIP UDP 546 DHCPv6 client TCP and UDP 547 DHCPv6 server TCP and UDP 647 DHCP failover TCP 666 Doom (video game) UDP 989 FTP data over TLS/SSL TCP and UDP 990 FTP control over TLS/SSL TCP and UDP 992 Telnet over TLS/SSL TCP and UDP 1023 Reserved TCP and UDP Any application that provides a common and well-known service (SMTP, FTP, Telnet, etc.) will monitor for incoming requests on the well-known ports. Firewalls can ACRONYM ALERT be configured to allow or deny specific CRC — Cyclic redundancy check ports, thus enhancing network security. If a request comes in with a port that is not defined, the server will assign a port number for the duration of the application process. The socket is the combination of an IP address or node name and a port number. The syntax of a socket would be :< port number> An example of this would be the Telnet protocol, which uses port number 23 (for both TCP and UDP). If the host that is running the Telnet server has an IP of 10.10.10.10, the Telnet client would send a request to that IP for port number 23. The syntax would look like this: 10.10.10.10:23 243 Page 243 Edwards 244 Part I ■ c05.tex V3 - 03/27/2009 Networking Nuts and Bolts Any given port can have a single passive socket, which monitors for incoming requests, but can serve multiple active sockets, each serving a request from a different client. 5.4 Chapter Exercises 1. What are the four layers of the TCP/IP reference model? 2. Name four Application layer protocols that we discussed in this chapter. 3. Explain the structure of the DNS hierarchy. 4. What are the five PDU types that are used by SNMP? 5. What is the purpose of FTP? 6. Why does TFTP not perform many of the functions that FTP does? 7. What is a daemon? 8. What are the four control characters used by Telnet for option negotiation and their meanings? 9. TCP is a protocol -oriented protocol, whereas UDP is a 11:05am Page 244 Edwards c05.tex Chapter 5 ■ V3 - 03/27/2009 11:05am The TCP/IP Protocol Suite 10. What are the three main reporting functions that we said are performed by ICMP? 5.5 Pop Quiz Answers 1. The Internet layer is also known as the Network layer. 2. What is the function of the FTP command ascii? Sets the file transfer mode to ASCII. 3. What does an SMTP server response code 421 mean? Server is unavailable. 4. What does the acronym NFS stand for? Network File System 5. What is the purpose of Telnet option code 32? Used to set the terminal speed. 6. What does the acronym SSH stand for? Secure Shell 7. Name the two popular transport protocols that we discussed in this chapter. TCP and UDP 8. Which layer of the TCP/IP reference model is probably the most important one? The Internet layer 9. What is the difference between IPng and IPv6? None. Other than the names, they are the same protocol. 10. What are the two IGPs that we discussed in this chapter? RIP and OSPF 245 Page 245 Edwards c05.tex V3 - 03/27/2009 11:05am Page 246 Edwards c06.tex V3 - 03/27/2009 11:07am CHAPTER 6 Ethernet Concepts The system of nature, of which man is a part, tends to be self-balancing, self-adjusting, self-cleansing. Not so with technology. — E.F. Schumacher The term Ethernet is a catchall word used to describe the most common network architecture used in a majority of today’s networks worldwide. If you were to say to someone, ‘‘Describe an Ethernet cable,’’ 99 out of 100 would probably respond that it consists of unshielded twisted pair (UTP) cable that is terminated on each end with RJ45 plugs. That is mostly true in today’s network, but Ethernet technology has evolved from its early coaxial cable days to what it is today. All Ethernet networks, no matter the type of cable that is in use, are Carrier Sense Multiple Access with Collision Detection (CSMA/CD) networks that adhere to the standards described in IEEE 802.3. This is true for either coaxial or UTP cable Ethernet networks. Let’s review how Ethernet came about and how it evolved to its current emanation of Ethernet cable technology. N O T E The term Ethernet is derived from two words: ether and net. Ether is a medium that can be made from pretty much anything. This is evident in today’s network environment, where network signals can be carried over wire, fiber (fiber optic), or air (wireless). The word net may be short for network, but one of the authors likes the idea of visualizing a fishing net, where each node is tied to adjoining nodes, and there are multiple paths from one to the other. 247 Page 247 Edwards 248 Part I 6.1 ■ c06.tex V3 - 03/27/2009 Networking Nuts and Bolts The Beginning of Ethernet Technology From 1973 to 1975, Ethernet had its start at the Xerox Palo Alto Research Center (PARC). Xerox filed a patent application in 1975 with the U.S. Patent Office for a Multipoint Data Communication System with Collision Detection. Patent 4,063,220 described how multiple data processing stations distributed along a branched cable segment would be able to communicate with each other. It included descriptions of the cable the devices needed to send and receive data on that cable. It also included a packet description outlining both source and destination addresses along with data and error fields. In the experimental implementation of Ethernet, data rates were 3 Mbps, and the source and destination address fields were only provided 8 bits for addressing, which limited the number of devices that could be addressed on the network. There were 16 bits allocated for the packet type, which would be used to define a packet type that would be used within a particular protocol. N O T E Mbps means ‘‘megabits per second,’’ where mega is the value of a million. So 100 Mbps is 100 million bits per second. Remember that a bit is a single binary digit of either zero or one. Even if only one stream of zeros was being generated, there are still 100 million of them in a second. It may represent a whole lot of nothing, but in the network world they truly have value. One of the original inventors on the Xerox patent, Robert Metcalfe, left Xerox in 1979 to form 3Com to promote LAN development and the use of PCs as nodes on the Ethernet network. He was instrumental in convincing Digital Equipment Corporation (DEC), Intel, and Xerox to work together to promote Ethernet as a LAN standard. This standard came to be known as the DIX standard, after the companies (DEC, Intel, Xerox) who came together to create the standard. The DIX or Ethernet II standard describes a frame format that provides 48 bits each for destination and source addresses, along with 16 bits for the packet type. The standard also set the data rate at 10 Mbps. Figure 6-1 illustrates a DIX/Ethernet II frame. Destination MAC Address (6 bytes) Source MAC Address (6 bytes) Ethernet Type (2 bytes) Data Payload (46 to 1500 bytes) CRC Checksum (4 bytes) Figure 6-1 A DIX/Ethernet II frame The Destination and Source Address fields are 6 bytes in length and are usually presented as a group of 12 hexadecimal numbers. These addresses are 11:07am Page 248 Edwards c06.tex Chapter 6 V3 - 03/27/2009 ■ 11:07am Ethernet Concepts called the Media Access Control (MAC) addresses and are a unique Ethernet hardware address assigned to a network interface card (NIC). The DIX/Ethernet II standard has been superseded by IEEE 802.3. N O T E Hexadecimal number system is an easy way of illustrating 4 binary bits, which can have values from 0 to 15. The values 0 through 9 are presented as their actual value, while the units 10 through 15 are represented by the alpha characters A through F, respectively. The 16 (the root hexadeca means 16) values that can be contained in a hexadecimal number are 0, 1, 2, 3, 4, 5, 6, 7, 8, 9, A, B, C, D, E and F. Although Ethernet was originally designed to allow computers to communicate with each other over a coaxial cable as the broadcast transmission medium, twisted pair Ethernet cable systems have been under development since as early as the mid-1980s. The first network topology using UTP cable was StarLAN, and it was introduced with a data rate of 1 Mbps. However, StarLAN would eventually evolve into what became known as 10BASE-T, which is the predominant UTP cable in use today. Since the publication of IEEE 802.3 in 1985, there have been several amendments that provide for increased Ethernet rates. Table 6-1 lists the data rates that can be found in use today. Table 6-1 Ethernet Types and Speeds ETHERNET TYPE SPEED 10BASE-T 10 Mbps Fast 100 Mbps Gigabit 1000 Mbps Ethernet has emerged as the de facto network standard worldwide. It has withstood POP QUIZ challenges from other networkWhat was the first type of cable used to ing protocols over time, and as form an Ethernet network? a result, large numbers of products from a wide range of manufacturers are readily available and are able to successfully interconnect based on this standard. Due to the economies of scale, networking products have decreased in price while performance has increased. Ethernet allows for flexibility in network implementation that is easy to maintain and manage. The installed base for Ethernet networks 249 Page 249 Edwards 250 Part I ■ c06.tex V3 - 03/27/2009 Networking Nuts and Bolts is huge, guaranteeing that Ethernet will be around for some time to come. There will always be improvements inserted into existing networks, but they will not cause a total dumping of the current Ethernet network. 6.2 Ethernet Components We discussed how UTP cable evolved from UTP telephone wire used to create the StarLAN networks. It would stand to reason that some of the concepts would be carried over from the Telco influence in setting Ethernet standards. Ethernet components using UTP cabling fall into two categories: Data terminal equipment (DTE) Data communications equipment (DCE) This nomenclature is part of the long-standing serial communications standard EIA RS-232. RANDOM BONUS DEFINITION Much like that standard, the bridge port — A network interface on a Ethernet standard uses this bridge. framework as the basis in developing standards for the electrical signal characteristics for Ethernet cabling and signals. Figure 6-2 illustrates a DCE and DTE device connected with UTP cable. Receive + Transmit + Receive – Transmit – Transmit + Receive + Transmit – Receive – DTE DCE UTP Cable Figure 6-2 Interconnection of DCE and DTE Ethernet devices This figure represents the conceptual interconnection of two Ethernet devices using UTP Ethernet cable. You will notice that the cable appears to be straight across, although physically the + and − wires are twisted together within the jacket of the cable. This type of Ethernet cable is often referred to as a patch 11:07am Page 250 Edwards c06.tex Chapter 6 V3 - 03/27/2009 ■ 11:07am Ethernet Concepts cable or a straight-through cable because there is no crossover from receive to transmit circuits. N O T E Twisted pair wire does have a purpose. The pair of wires are twisted together in a uniform manner with a fixed number of twists per foot. Why should the cable be twisted in the first place? To look pretty? To keep the wires from drifting apart? Okay, the answer is: to combat the effects of electromagnetic interference (EMI). Electrical waves are all about us, now more so than ever with the plethora of cell phones and other mobile devices. When these waves intersect wire, they can induce minute fluctuations in voltage. No big deal, right? Just a little static on the line. Wrong! These signals could cause erroneous data to be read, so signal integrity is an absolute necessity. (How would you like it if your ATM card was swallowed before you could get your money out?) Now, do not go adding extra twists to your Ethernet cable thinking this is going to increase your immunity. In reality, you will alter the electrical characteristics of the wire and cause reflections within the cable, which is bad as EMI. Leave the cables alone and go pop some bubble wrap if you need to keep those idle hands busy. So, we have DCE and DTE Ethernet devices, but which is which? A good way to remember this is by recalling the early days of RS-232. The term data terminal equipment often referred to teletypewriters, whereas data communication equipment most often referred to modems. When PCs were introduced, the majority of telecommunications was accomplished via a modem. (Yes, we recall those days — the 300 baud handset devices where you squeezed your phone’s handset into the foam cuffs so it could receive the actual audio signals through the telephone.) N O T E A handset refers to a standard telephone like we had back in the olden days. The telephone wire was connected to the base, and the handset portion had a spiraled wire, which always managed to get so twisted that you found you could not talk on the phone unless your head was about a foot off the table where the base rested. The base contained the actual dialing mechanism, which allowed you to dial the number you wished to connect to. Yes, ‘‘dial’’ — where do you think the word originated? Surely, not from punching those minute buttons on the latest whiz-bang cell phone, which has given us a new set of human ailments such as ‘‘texting thumb.’’ As telephone technology evolved from mechanical dialing RANDOM BONUS DEFINITION mechanisms to touch-tone dialbandwidth — The data-carrying capacity of ing, modems also implemented a device or communications channel. those technologies. Even today’s modems — whether external or internal modems embedded in a laptop, PCMCIA modem card, or PCI 251 Page 251 Edwards 252 Part I ■ c06.tex V3 - 03/27/2009 Networking Nuts and Bolts modem card in a desktop computer — all support both dial and touch-tone dialing methodologies in their designs. N O T E What is meant by mechanical dialing? The old rotary phones had a dial with numbers and letters assigned around a dial mechanism shaped like a wheel with finger holes assigned the numbers 1 through 9 and 0 for either the number zero or Operator if that number was dialed first and by itself. A number was dialed by placing one’s index finger in the hole with the corresponding number that was desired and then in a circular motion moving the dial to the stationery finger-stop and releasing the dial to allow it to step back. As it stepped back, it sent a pulse on the wire to the home office, where stepping relays would increment to set up the circuit corresponding to that number. Switching theory was developed and used by the telephone companies in order to eliminate human operators who would actually make the circuit connection for the caller. The number selected would determine the number of pulses, which stepped the home office stepping relay to that number. You can just imagine how many relays were required to set up those switching offices. Today’s modems use a relay to pulse line the number of times required for the number to be dialed, and that is what is meant by the pulse setting on the modem. Touch-tone dialing was devised by the telephone companies to accomplish pretty much the same thing as pulse dialing. However, it uses a more modern technique of using distinct audio tones for each discrete number. If you ever listened to a modem dial with tone dialing, you know it sounds like automatons in sci-fi movies. PCs pretty much replaced teletypewriters as the device to use for telecommunications. POP QUIZ They were supplied with RS-232 An Ethernet network device that forwards serial ports. With a terminal data on the network would be considered emulation program, these PCs what type of Ethernet device? became the modern-day teletypewriter. We said that teletypewriters were DTE devices, so the PC with an Ethernet NIC is an Ethernet DTE device. Modems are DCE devices, and since they pass data along the network, devices like Ethernet hubs, routers, and switches are also considered to be DCE Ethernet devices. 11:07am Page 252 Edwards c06.tex Chapter 6 6.2.1 V3 - 03/27/2009 ■ 11:07am Ethernet Concepts DCE and DTE Cabling Considerations We mentioned that a straight-through cable was one where the wire from pin 1 would be connected to pin 1 on the other connector. Let’s discuss the RJ-45 modular plug that is used on any UTP Ethernet cable. Figure 6-3 represents how an RJ-45 plug would look if you held the plug with its gold contacts facing you. Pin 1 of the plug will be on your left, with pin numbers incrementing until pin 8 on your right is reached. The pin numbering is sequential.1 Pin 1 Pin 8 Figure 6-3 An RJ-45 modular plug UTP Ethernet cable consists of four twisted pairs,2 for a total of eight wires contained within an unshielded jacket. The wires are colored with four solid colored ACRONYM ALERT wires, each of which is twisted together AFP — AppleTalk Filing Protocol with its mate, which is mostly white with a colored stripe that matches the color of its solid colored mate. How and to what pin these wires connect to on the RJ-45 plug adhere to old telephone company standards and are contained within the TIA/EIA-568-A and TIA/EIA-568-B standards. Table 6-2 lists the wiring scheme for T568A wiring, and Table 6-3 lists the wiring scheme for T568B wiring. 1 Sequential is derived from the word sequence, which means one after the other. For those in the reading audience who find it difficult to grasp this concept, we shall be more precise in the pin numbering definition. Starting on the left with pin 1, the pin numbers increment in sequence: 2, 3, 4, 5, 6, 7, and pin 8, which is the last pin on the right. Now, if you tell us you can’t count, then we have a major problem here, and you need additional help, which is beyond the scope of this book. 2 Pair refers to the number two. So a twisted pair of wire would consist of two discrete wires which have been twisted together for . . . what? Noise immunity, good answer. 253 Page 253 Edwards 254 Part I ■ c06.tex V3 - 03/27/2009 Networking Nuts and Bolts Table 6-2 T568A Wiring Pin-out PIN PAIR WIRE COLOR ETHERNET SIGNAL 1 3 Tip White/green Transmit + 2 3 Ring Green Transmit – 3 2 Tip White/orange Receive + 4 1 Ring Blue 5 1 Tip White/blue 6 2 Ring Orange 7 4 Tip White/brown 8 4 Ring Brown Receive – Table 6-3 T568B Wiring Pin-out PIN PAIR WIRE COLOR ETHERNET SIGNAL 1 2 Tip White/orange Transmit + 2 2 Ring Orange Transmit – 3 3 Tip White/green Receive + 4 1 Ring Blue 5 1 Tip White/blue 6 3 Ring Green 7 4 Tip White/brown 8 4 Ring Brown Receive – A straight-through cable can be wired with either the T568A or T568B wiring scheme as long as both ends of the cable are wired exactly the same using the same wiring pin-out. A crossover Ethernet cable must have one plug wired with the T568A wiring scheme RANDOM BONUS DEFINITION and the other plug wired followApplication layer — The highest layer of the ing the T568B wiring pin-out. seven-layer OSI model. The purpose of a crossover cable is to interconnect to like devices, regardless of whether they are 11:07am Page 254 Edwards c06.tex Chapter 6 V3 - 03/27/2009 ■ 11:07am Ethernet Concepts DCE or DTE devices. The crossover is to have the transmit signals from one device terminate on the receive signals of the other device so they can pass data between them. A quick analogy is connecting two microphones together; the two parties could scream into them but neither could hear the other. Now, if we take one microphone and crossed over to a speaker and did the same for the other microphone, then parties would be able communicate without a problem.3 The same goes for Ethernet devices — just because there is some sort of Ethernet UTP cable strung between them does not mean they are ‘‘supposed’’ to communicate. So, when you are having problems getting two Ethernet devices to communicate, the first place to look is at the Physical layer (such as the cable being used). HELPFUL HINT Since for the most part Ethernet cables use RJ-45 jacks, which are mostly clear plastic, it is fairly easy to determine if a Ethernet UTP cable is either a straight-through or crossover cable. Take the two connectors on the ends of the cable and hold them against each other with both plugs oriented in the same direction. Scan the colors of each. They should look exactly alike on a straight-through cable. If it is a crossover cable, you will notice that the colored wires on pins 1 and 2 of one plug have moved to pins 3 and 6 of the other, with the reverse also being true. If for any reason the cables do match as described in this note, there is a likelihood it is a cable used for another purpose or it is supposed to be an Ethernet UTP cable but has been manufactured incorrectly. Do yourself a favor: if you find cables in your box of goodies that appear different from what has been described in this note, discard them in the nearest wastebasket. Many countless hours have been wasted fighting problems with bad cables, not only by people in general but by network administrators who should know better. For the frugally minded who cannot bear to toss anything away, our recommendation is to cut the ends off the cables so you will not be tempted to use them in your network. You may want to use them to tie up all those newspapers that have been collecting in the corner and bring them to a recycling drop-off in your community. 6.2.1.1 Interconnecting Like Ethernet Devices We have already discussed that Ethernet devices fall into two categories, DCE or DTE type devices. It has also been stated that interconnecting to like Ethernet 3 We fully acknowledge that his simple-minded analogy has very little likelihood of succeeding in the real world because there is a whole lot of electronics that needs to be added for it to actually work. The purpose of any analogy is to demonstrate in the simplest terms how something works. 255 Page 255 Edwards 256 Part I ■ c06.tex V3 - 03/27/2009 Networking Nuts and Bolts devices requires the use of a crossover cable. For example, two PCs with NIC cards can be directly interconnected with a crossover cable, as illustrated in Figure 6-4. Ethernet UTP Crossover Cable PC A PC B Figure 6-4 Two PCs interconnected via Ethernet In this simple figure, the two computers are able to communicate with each other over the POP QUIZ crossover cable. There must be If a cable is wired such that one plug is a some sort of networking protoT568A and the other is a T568B, it would col running on the PCs, such as commonly be referred to as TCP/IP, and some sort of applicable. cation that will allow the sharing of data or devices (which may be locally connected to either or both of them). Some operating systems, such as Microsoft Windows and Apple Macintosh, are ‘‘network-able’’ and include tools and utilities to facilitate data and device sharing over the network. The last example showed two Ethernet DTE devices interconnected, but how about DCE devices? We already mentioned that DCE devices are in the form of hubs, ACRONYM ALERT switches, and routers, so we know we BER — Bit error rate are dealing with that kind of device. Why would anyone want to connect those types of devices? To illustrate this, we will consider a few simple examples. The first example is a case where we have a stack of dumb,4 eight-port, passive hubs and there is a small office with 15 workers who need to be interconnected to a local server to share the resources available on that server. Figure 6-5 illustrates one method of how these passive eight-port hubs may be used to accomplish this. The three hubs are placed about the office for the ease of cabling between each other ACRONYM ALERT and the workstations connected to them. TTL — Time to live Since these hubs have eight ports, with one 4 Dumb means exactly that: dumb. There is no internal intelligence contained within the unit. 11:07am Page 256 Edwards c06.tex Chapter 6 V3 - 03/27/2009 ■ 11:07am Ethernet Concepts port dedicated for linking to the other hub, this leaves seven available ports for workstation connections. As you can see in Figure 6-5, two of the hubs have seven workstations each connected to them. That leaves one workstation and the server to be connected to the LAN. The hub that is used to connect these devices and the other two hubs has only used four of the eight available ports, so if needed there are four ports remaining for future expansion. You can see from the cabling legend that the workstations and the server are connected to the LAN with a patch or straight-through Ethernet UTP cable. The hubs are connected to each other using crossover cables since we are interconnecting like DCE Ethernet devices. Server with Shared Resources Straight Through Ethernet UTP Cable Crossover Ethernet UTP Cable Figure 6-5 A LAN created with passive hubs This scenario is not uncommon, and a few of you who may be familiar with cabling hubs today may be scratching your head. We remember the day when this was standard operating procedure for interconnecting passive hubs, so go with us on this one. Yes, there have been improvements in hub technology. One was actually adding what was called an uplink port, where a DTE port was added to the device to facilitate it being connected to another hub, with a patch cable eliminating the need to find a crossover cable, in case you forgot to purchase one when you purchased the hub. Another improvement is an uplink port with a switch dedicated to it that switches its receive and transmit 257 Page 257 Edwards 258 Part I ■ c06.tex V3 - 03/27/2009 Networking Nuts and Bolts circuits to match the cable and the port it was connected to at the other end. The most recent innovation in hub and switch design is that all ports on the hub are now auto-sensing and auto-switching. N O T E Auto-sensing is accomplished by electronic circuits that determine if the incoming wires to a signal pair of pins are connected to a transmitter or a receiver. Once the ‘‘sense’’ of the wire is determined, this information is passed to the circuits responsible for auto-switching. Auto-switching is circuitry added to a port to configure the port to which pins receive and transmit circuits should be connected to. If one set of pins is determined to be a receive pair, then the other set of pins must be the transmit pair. Receive and transmit are mutually exclusive in that one set of pins must be the receive circuit and the other must be the transmit circuit. If both sets of pins are the same, either receive or transmit, the device is defective. HELPFUL HINT Most Ethernet devices with RJ-45 jacks to accommodate Ethernet UTP cables have LED5 lights showing the link status. If there is no link indication, the first place to check is the cable. Both devices connected with the same cable should indicate link while connected. If you pull one end of the cable and the other device’s link light is still illuminated, you may not be connected to the correct device. In large LAN implementations, many times a cable is pulled to ensure that it loses link so one knows the port assignment is correct on both ends of the cable. We can see that look on your face. You are thinking that if devices can do auto-sensing and auto-switching, why do you have to learn the differences in cable types? The answer is, you may be correct if you are only doing new implementations and using stock cables you buy already assembled. However, there is a large installed base of legacy systems that have dedicated ports wired as either a DTE or DCE, so cable knowledge is essential. Let’s continue with another example. Remember, it still is not yet an autosensing/auto-switching world. Figure 6-6 ACRONYM ALERT shows a part of a larger installation at a HTTP — Hypertext Transfer Protocol corporate office. There are many user workstations, but for sake of illustration there 5 LED is the acronym for light emitting diode. It is actually a semiconductor device that will illuminate when a current is passed through it. Some are single colored while others are able to change color depending on how the device is electrically driven. 11:07am Page 258 Edwards c06.tex Chapter 6 V3 - 03/27/2009 ■ 11:07am Ethernet Concepts are only a few in the figure drawing. This figure may represent a floor or department location within a building. User Workstations Router Hub Router Internet Hub Patch Panel Server DMZ Patch Panel Premise Wiring Straight Through Ethernet UTP Cable Crossover Ethernet UTP Cable Figure 6-6 A larger LAN implementation There are three DCE devices in this drawing, two routers RANDOM BONUS DEFINITION and a hub that are interconnecmultimode fiber — An optical fiber that ted using crossover cables. Off allows signals to propagate in multiple the hub there is a server contransmission modes simultaneously. nected with a patch cable/ straight-through Ethernet UTP cable. The placement of the hub and server is considered a DMZ (demilitarized zone). The purpose of a DMZ is to regulate access to the networks it is connected to. In this scenario, there is a network of corporate user workstations that have access to a corporate server and the Internet. The routers within the DMZ have been programmed with policies that allow approved users from the Internet to have access to the corporate server but not to pass to any other networks connected to the DMZ. These routers and other equipment may be located in a data center on another floor from the users who need access to the server and the Internet. This is where premise6 wiring comes in. 6 Premise is the term used to represent a given locale like a home or building. Thus, premise wiring is the wiring contained within the building. 259 Page 259 Edwards 260 Part I ■ c06.tex V3 - 03/27/2009 Networking Nuts and Bolts Cable needs to be run from the data center to the floor where the user workstations are located. This is done by running Ethernet-grade7 cable, which is terminated on patch panels8 located in the data center and the wiring closet on the floor where the user workstations are located. HELPFUL HINT We have seen wiring closets that are neat and orderly, and others with wire strung everywhere and piled on the floor like a large bowl of my mother’s spaghetti and meatballs. (For more information on my mother’s secret recipe, read the note on it.) If you are a network administrator and want to do yourself a favor, please try to keep your wiring closets orderly and well labeled. You do not want to be called at all hours of the night or on vacation or even on your weekends off, and that will be the case each time someone is troubleshooting a problem and has no clue as to which cables go where. Do it right up front and you can truly have peace of mind. If not, your ears will be burning each time someone curses you for making their job harder. The patch panels are wired with Category 5e or Category 6 cable from panel to panel as straight-through cables. There is no crossover taking place within the long-run cables. If a crossover is needed, it will be taken care of from the patch panel to the device using an Ethernet UTP crossover cable. This is illustrated in Figure 6-6 with the router that is connected to the patch panel. Notice on the other patch panel that although the switch is a DCE Ethernet device, it is connected with a patch cable. This is because it connects to the router at the other end, which is connected to the patch panel with a crossover cable, so that only a single crossover is required. Double crossover9 cables will basically negate the crossover function, and the device link lights will not illuminate. 7 Ethernet using UTP cable was initially designed on the idea of using existing premise wiring that was in place for telephone communications. With improvements in speed on Ethernet circuits, a higher quality cable was necessary to support these new requirements. Today’s new cable installations should be using Category 5e or Category 6 cable, especially if Gigabit Ethernet is to be used. 8 Patch panels are an old holdover from the telephone company days. However, remember the basis of Ethernet over UTP was to use existing premise wiring, which was telephone UTP cable. It stood to reason if those cables are attached to patch panels, then patch panels would become part of the Ethernet UTP connectivity equation. 9 Double crossover is like a double negative: two negatives make a positive, so you don’t have the crossover. It may come in handy sometime when you find yourself up to the armpits in crossover cables but are unable to find that one badly needed patch cable. Now, how would you connect them? 11:07am Page 260 Edwards c06.tex Chapter 6 V3 - 03/27/2009 ■ 11:07am Ethernet Concepts The server and all the user POP QUIZ workstations are DTE devices connecting to other DTE You are interconnecting two Ethernet devices, so the cables used are devices, but neither device is showing a link straight-through (patch) Etherlight on the assigned port. List in order of net UTP cables. With the right likelihood where the problem might be. routing protocols and security policies in place, users at the user workstations are able to access the local corporate server as well as the Internet, while the corporate LAN is protected from unauthorized users from the Internet. AN UNRELATED MOMENT OF PAUSE — MAMA BRAMANTE’S SECRET SPAGHETTI AND MEATBALLS RECIPE The thought of all of the cables in a wiring closet made Rich think of his mother’s spaghetti and meatballs. Rich decided to share the recipe with you all: Well, the recipe is not under lock and key like you see in some of those commercials on TV, and no, the dog doesn’t know it either. The reason it is so secret is that my mother had the knack of making it without measuring ingredients other than with her watchful eye. I always said she could cook for five or fifty and it would always be the same, and it was. There is nothing like a mother’s cooking, eh? So, I am going to give you a list of ingredients, and you can mix up a batch. You may surprise yourself and it could be almost as good as my mom’s. My mother always started the sauce before the meatballs. (For you Italian readers out there, ‘‘sauce’’ is ‘‘gravy.’’) ◆ Sauce Steps: 1. Using a large pot, pour a liberal amount of olive oil to a depth of about a quarter of an inch and heat to a temperature that would fry whatever you place in it. 2. Slice up (slice, not dice) a medium-sized onion. Add the onion to the oil and brown to a dark crisp. Remove the onion from the oil and set aside. 3. Take some garlic cloves and slice them so you have these tiny garlic slabs. Add them to the oil and just brown (do not cook as long as the onions). 4. Once the garlic is brown, add two cans of peeled Roma tomatoes into the olive oil/garlic mix. Be careful that the oil does not splatter back. (continued) 261 Page 261 Edwards 262 Part I ■ c06.tex V3 - 03/27/2009 Networking Nuts and Bolts AN UNRELATED MOMENT OF PAUSE — MAMA BRAMANTE’S SECRET SPAGHETTI AND MEATBALLS RECIPE (continued) 5. Stir in one can of tomato paste and the fried onions. Stir for consistency and let simmer while making the meatballs. ◆ Meatball Steps: 1. Put about a pound or pound and a half of fresh ground beef into a large mixing bowl. 2. Grate in an amount of bread crumbs that is about a third of the hamburger volume. (Stale Italian bread allowed to thoroughly dry to a rock was used to make the bread crumbs. Not much was wasted when feeding six kids.) 3. Finely dice two garlic cloves and add to the mix. 4. Finely chop two or three sprigs of fresh parsley and add to the mix. 5. Grate in some fresh Parmesan or Romano cheese — about half a cup or slightly more. 6. Add salt (not too much, as the cheese is salty) and some ground black pepper. 7. Create a cavity in the mix and add three whole eggs into the mix. 8. Mix all the ingredients thoroughly so that the whole batch is consistent throughout. 9. In a large skillet, preheat olive oil to fry the meatballs in. Scoop up enough of the beef mixture to make a golf ball size meatball. Roll the meatball in the palm of your hand (wash your hands before and after this process) to form a firm ball that can withstand frying without falling apart. 10. Fry the meatballs to a deep brown crust on all sides before dropping them into the sauce. ◆ Spaghetti Steps: 1. Once everything is simmering in the large sauce pot, it is time to boil the water for the spaghetti. 2. Add a half teaspoon of salt to the spaghetti water and bring to a rapid boil. 3. Add a pound of spaghetti (smaller amount for a smaller gathering) to the water and stir in. 4. Keep an eye on the pot since rapidly boiling spaghetti has a tendency to foam up and overflow the pot. (continued) 11:07am Page 262 Edwards c06.tex Chapter 6 V3 - 03/27/2009 ■ 11:07am Ethernet Concepts AN UNRELATED MOMENT OF PAUSE — MAMA BRAMANTE’S SECRET SPAGHETTI AND MEATBALLS RECIPE (continued) 5. Once the spaghetti is cooked and is soft but firm to the bite (al dente), strain it in a colander.10 Make sure the spaghetti is well drained. ◆ Serving Steps: 1. Dump the colander of spaghetti into a large serving bowl. 2. Add some of the sauce (no meatballs) to the spaghetti and mix thoroughly to where the spaghetti has sauce on it but not is swimming in the sauce. I know there is a fine line to this, so add sauce slowly. 3. Once you are satisfied the spaghetti has sufficient sauce on it, fish out two meatballs for each diner and place in the bowl on top of the spaghetti. 4. Serve with freshly grated cheese on the side, a little vino, good company, and conversation. Congratulations! You have just served up Mama Bramante’s favorite dish to la famiglia. 6.3 Ethernet and IEEE 802.3’s Relationship to the OSI Model There is a close similarity between the ISO OSI model and IEEE 802.3 model, with the difference being at the Data Link layer of the OSI model, as illustrated in Figure 6-7. The Physical layer is the same in both models and is dependent upon the media11 being used. This layer deals with parameters such as cable pin-out, signal electrical ACRONYM ALERT characteristics, modulation encoding of the MAN — Metropolitan area network data being modulated on carrier signals, and data synchronization.12 Once it has been determined that the receive buffer has received a complete frame, the Data Link layer is signaled and the frame is passed up to that layer. 10 For those of you who are uninformed about cooking utensils, a colander looks kind of like a leaky bucket or a hemispherical pot shot full of buckshot holes. Not useful for holding water, but it sure comes in handy when draining spaghetti. 11 Media is in reference to the method of delivery of the data. Obviously in a wired network it depends on the type of cable and the NIC cards being used. However, other methods of delivery such as wireless and optical can be used. So media for the most part is how the data moves between data points. 12 Data synchronization refers to the capability to detect the start of a data frame from a stream of data bits and the fact that the binary pattern is a complete frame. 263 Page 263 Edwards 264 Part I ■ c06.tex V3 - 03/27/2009 Networking Nuts and Bolts IEEE 802.3 Reference Model Upper Layers OSI Reference Model Upper Layers Network Network Logical Link Control Data Link Media Access Control Physical Physical Figure 6-7 OSI’s relationship to IEEE 802.3 In the OSI reference model, the Data Link layer accepts service requests from the Network layer and sends service to the Physical layer. It is the layer responsible for data transfer between adjacent network nodes and has the capability to detect and correct errors that may occur on the Physical layer. Although the Data Link layer is responsible for data transfer over the Physical link, many data link protocols do not provide acknowledgments of a successful receipt and acceptance of a frame. Some data link protocols do not even provide for a checksum to detect errors in transmission. In these cases, frames received depend on higher-level protocols for frame flow control, acknowledgments, retransmission, and error checking. The IEEE 802.3 reference model divides the OSI model’s POP QUIZ Data Link layer into two sublayers, the Logical Link Control Into which two sublayers of the IEEE 802 sublayer and the Media Access reference model is the OSI reference model Data Link layer divided? Control sublayer. The Logical Link Control sublayer resides in the upper layer of the OSI Data 11:07am Page 264 Edwards c06.tex Chapter 6 V3 - 03/27/2009 ■ 11:07am Ethernet Concepts Link layer, whereas the Media Access Control sublayer is in the lower portion and provides the interface to the Physical layer. 6.3.1 Logical Link Control The IEEE 802 standard for the Logical Link Control resides in the upper portion of the OSI reference model’s Data Link layer and provides the same functions no matter what media is being used. The Physical layer can be Ethernet, Token Ring, or wireless LAN, of which the Logical Link Control sublayer is primarily concerned with providing flow control, error control, and what multiplexing protocols are being used over the Media Access Control sublayer. Logical Link Control flow control manages the data transPOP QUIZ mission rate between two network nodes to prevent one node With which functions is the Logical Link sending faster than the speed of Control sublayer mainly concerned? the receiving node. If one node is receiving data from multiple network nodes, it may not be able to receive as quickly as the sending node would like to transmit. Flow control depends on feedback from the receiving node to the sending node signaling possible congestion and its inability to receive data at higher speeds. In an Ethernet network, a receiving node that is unable to keep up with a sending node will transmit a PAUSE frame to halt transmission for a given period of time. The PAUSE frame for flow control can be used only on network segments that are running at full-duplex.13 6.3.2 Media Access Control The Media Access Control sublayer provides the interface between the Physical layer and the Logical Link Control sublayer. The Media Access Control sublayer is responsible for data encapsulation and frame assembly for sending frames, and de-encapsulation and error checking of received frames. It also provides addressing and a channel access control mechanism, which allows multiple nodes on a local area network to communicate. The Media Access Control address, or the physical address of the node device, is commonly referred to as the MAC address. It is an industry standardized unique address assigned to each network adapter at the time of manufacture. Although highly unlikely, there is a possibility of duplicate 13 We previously defined full duplex as the capability to send and receive simultaneously. It is logical that if a half-duplex node is currently receiving, it is unable to transmit until all the data is received. This makes a PAUSE frame unusable in half-duplex network segment. 265 Page 265 Edwards 266 Part I ■ c06.tex V3 - 03/27/2009 Networking Nuts and Bolts MAC addresses on a network segment due to the capability to overwrite a manufacturer’s previously assigned MAC addresses. HELPFUL HINT Although I have seen only one case of a duplicate MAC address on a LAN segment, I know it is possible. Depending on the network size, it can be a real nightmare. (Unfortunately, for the case I worked, it was a large network.) For whatever reason, the site in the case I worked decided that they would assign their own MAC addresses for every device in their network. Although they had full control and well-documented logging of MAC addresses, it took a while to find the offending node. Ultimately, knowing the MAC address of the device that was being adversely affected was helpful. Using a process of elimination that allowed for a digit being entered into a MAC address incorrectly aided in locating the culprit. If the site had not properly documented their MAC addresses and where they were assigned, the other option would have been to assign a new MAC address (which they preferred not doing) to the device that had not been previously assigned. I am sure they had good reasons to use their own MAC address scheme, and they attempted to document it well, which is a major plus. However, it is best to leave well alone and use the already assigned MAC address to identify the device on the LAN segment. Because Ethernet is a CSMA/CD (Carrier Sense Multiple Access with Collision Detection) network protocol, not only are all the network nodes on a network segment required to have unique physical hardware addresses, but there must be a provision for the control of the multiple access of more than one node at a time. The Media Access Control sublayer provides channel access control to allow multiple access.14 When multiple network nodes are connected to the same physical media, there is a high likelihood of collisions occurring. The multiple access protocol is used to detect and avoid packet collisions where multiple nodes contend for access to the same physical media. Ethernet and IEEE 802.3 are the most common standards used for CSMA/CD networks. CSMA/CD utilizes a carrier-sensing scheme. If a transmitting node detects another signal on the media while it is transmitting a frame, it ceases transmittal of that frame and immediately transmits a jam signal onto the media. All nodes on the network are aware a collision on the media has taken place and will 14 Multiple access allows more than one data stream to share the same Physical layer media. Examples of shared media networks are bus topology networks, ring topology networks, wireless networks, and Ethernet point-to-point links running at half duplex. 11:07am Page 266 Edwards c06.tex V3 - 03/27/2009 Chapter 6 ■ 11:07am Ethernet Concepts back off and not transmit for a period of time, which is calculated using a back-off delay algorithm. After the back-off delay has elapsed, the node will attempt to retransmit the frame, giving it a higher probability of success. The methods used for collision detection depend on the POP QUIZ media being used. On a wired Ethernet bus, it is accomplished When a collision occurs on the media, what by comparing the transmitdoes the transmitting network node do? ted data with the data being received off the wire. If it is determined that they differ, the transmitting station on that node recognizes that another node is transmitting at the same time and a collision has occurred. All transmitting nodes then cease transmission and use the calculated back-off interval before attempting to transmit again. The back-off algorithm is a calculation that randomizes the back-off interval for each transmitting node so that the probability of another collision is very low. HELPFUL HINT CSMA/CD is required in a half-duplex network environment. Although the protocol works well if all network node devices remain well behaved, a single ‘‘chattering’’ network node can cause all data flow on a network segment to cease. Of course, this is a malfunction, but it is within the realm of possibility. A quick sniffer trace15 of that network segment should out the culprit pretty quickly. With the movement to higher-speed full-duplex Ethernet devices, the need for CSMA/CD is diminishing, although it must be maintained for legacy network segments and devices. 6.4 Ethernet Frame Format Figure 6-8 illustrates the basic Ethernet frame format. Preamble Start of Frame Delimiter Destination Address Source Address Frame Length/ Type Data Frame Check Sequence Figure 6-8 The basic Ethernet frame format 15 Sniffer trace is a technical colloquialism referring to a packet capture. There are dedicated pieces of equipment to capture and display packets or you can load packet-capture software on a laptop. The sniffer trace will permit you to see the traffic that is on a network segment. 267 Page 267 Edwards 268 Part I ■ c06.tex V3 - 03/27/2009 Networking Nuts and Bolts The basic frame format illustrated in Figure 6-8 is required for all MAC implementations of the IEEE 802.3 standard. Some additional optional formats also are used to widen the basic capability of the protocol. Following is a list of the basic frame fields: ACRONYM ALERT OUI — Organizationally unique identifier Preamble — A 7-byte field consisting of alternating 1s and 0s to alert a receiving station that a frame is being received. It is a method used to aid synchronization between the Physical layer receiving circuits and the incoming data stream. Start of Frame Delimiter — A 1-byte field consisting of a field of alternating 1s and 0s ending with two consecutive 1 bits to signal that the next bit is the leftmost bit in the leftmost byte of the destination address. Destination Address — A 6-byte field that contains the address of the node that is to receive the frame. The leftmost bit in this field indicates if the frame is destined for a individual node address (0) or a group address (1). The second from the leftmost bit is an indicator if the address is a globally assigned address16 (0) or a locally administered address17 (1). The remaining 46 bits of this field contain the address value of the unique node address, a group of network nodes, or all nodes on the network. Source Address — A 6-byte field that contains the hardware address of the transmitting node, which is always a unique individual address where the leftmost bit of the field is always set to 0. Frame Length/Type — A 2-byte field that indicates either the number of bytes contained within the Data field of the frame or an alternate frame format type. If the Frame Length/Type has a value of 1500 or less, this value indicates the number of bytes contained within the frame’s Data field. If the field value is 1536 or greater, it is used to indicate the 16 A globally assigned address is the address assigned to the network interface at time of manufacture. These addresses are assigned in blocks to manufacturers and can be used to distinguish which device is from which manufacturer by the hardware address used on that network segment. This can be a valuable troubleshooting tool where large network installations are concerned. 17 A locally administered address is a MAC address that has been locally assigned by a network administrator. It overrides the default MAC address assigned to the network interface by the manufacturer. Without extreme care, there is a distinct possibility that duplicate addresses could appear on the local network. Duplicate addresses are a big no-no in the networking world. So, if you need to do this, be very careful or you could be in a lot of hot water. 11:07am Page 268 Edwards c06.tex Chapter 6 V3 - 03/27/2009 ■ 11:07am Ethernet Concepts alternate frame type that is being used for either a received or transmitted frame. Table 6-4 lists a handful of the common frame types. Table 6-4 A Few Common Frame Types FRAME TYPE PROTOCOL 0x0800 Internet Protocol Version 4 (IPv4) 0x0806 Address Resolution Protocol (ARP) 0x8035 Reverse Address Resolution Protocol (RARP) 0x809b AppleTalk 0x80f3 AppleTalk Address Resolution Protocol (AARP) 0x8100 IEEE 802.1Q Tagged Frame 0x8137 Novell IPX 0x86dd Internet Protocol Version 6 (IPv6) Data — This field contains the data that is RANDOM BONUS DEFINITION being sent within the network management — The process of frame. It can be any numconfiguring, monitoring, controlling, and ber of bytes of informaadministering a network’s operation. tion up to and equaling the maximum number of 1500 bytes that is allowed for this field. However, if the number of bytes is less than 46, a number of bytes must be added to pad the field to reach its minimum length of 46 bytes. The minimum frame size, per the IEEE 802.3 standard, which does not include the preamble, is 64 bytes. Frames of less than 64 bytes are discarded as frames from collisions, faulty NICs, or software-caused under-runs. Frame Check Sequence — A 4-byte POP QUIZ field that contains a 32-bit What is the maximum number of bytes that CRC (cyclical reduncan be contained in the Data field of an dancy check) checksum Ethernet frame? value, which is calculated and inserted by the sending network node and used by the receiving network node to validate the received frame. Both the sending and receiving nodes calculate the CRC value by using the data contained within the Destination Address, Source Address, Frame Length/Type, and Data fields. 269 Page 269 Edwards 270 Part I ■ 6.4.1 c06.tex V3 - 03/27/2009 Networking Nuts and Bolts Transmitting a Frame When a frame request is received by the Media Access Control sublayer from the Logical Link Control sublayer, it is accompanied by the data to be sent and the destination address where the data is to be delivered. The Media Access Control sublayer starts the transmission process by loading the data and address information into the frame buffer. The preamble of alternating ones and zeros, along with the start of frame delimiter, are inserted into their appropriate fields. Destination address and source address information is then added to the fields to which it is assigned. The data bytes received from the Logical Link Control sublayer are counted, and the number of bytes to be contained within the Data field is added to the Frame Length/Type field. The data from the Logical Link Control sublayer is inserted into the Data field, and, if the total number of data bytes is less than 46, a number of pad bytes are added until the number of data bytes is equal to 46. A CRC calculation is performed on the data contained within the Destination Address, Source Address, Frame Length/Type, and Data fields, and then appended to the end of the Data field. Once the whole frame is assembled and ready for transPOP QUIZ mission, the Media Access Control sublayer’s next operation What does the Frame Check Sequence field depends on whether it is operof an Ethernet frame contain? ating in half-duplex mode or full-duplex mode. If it is operating in half-duplex mode, it cannot transmit and receive simultaneously. Since IEEE 802.3 requires that all Ethernet Media Access Control sublayers support half-duplex, if the Media Access Control sublayer is operating in that mode, it is unable to transmit until any incoming frame is completely received. In full-duplex mode, this is not an issue, and the frame can be transmitted immediately. 6.4.1.1 Half-Duplex Transmission With the development of the CSMA/CD protocol, multiple network nodes are able to share a common media without the need for a centrally located bus arbiter, tokens, or dedicated transmission time slots to determine when they would be allowed to transmit on the media. N O T E Time division multiplexing (TDM) is a form of digital multiplexing where two or more bit streams are transmitted on a common communications medium. Although it appears as if they are simultaneous, they are actually sharing the time domain. The time domain is divided into a number of fixed time slots. Each data 11:07am Page 270 Edwards c06.tex Chapter 6 V3 - 03/27/2009 ■ 11:07am Ethernet Concepts stream is dedicated to a fixed time slot or channel. Although the same media is being shared, it is not the most efficient use of the media if all or some of the channels are not transmitting. If no data is being streamed on a channel for a particular time slot, it is still using up part of the bandwidth dedicated to it and cannot be used by other channels. Each portion of the CSMA/CD protocol can be summarized as follows: Carrier Sense — All network nodes continuously listen on the network media to determine if there are gaps in frame transmission on the media. Multiple Access — All network nodes are able to transmit anytime they determine that the network media is quiet. Collision Detection — When two network nodes transmit at the same time, the data streams from both nodes will interfere and a collision occurs. The network nodes involved must be capable of detecting that a collision has occurred while they were attempting to transmit a frame. Upon detecting that the collision has occurred, both nodes cease transmission of the frame and wait a period of time determined by the back-off algorithm before again attempting to transmit the frame. Although bit signals are propagated on a shared network POP QUIZ medium at the same rate, the amount of time it takes to transWhat is the name of the transmission mode that allows either transmitting or receiving mit a whole frame is inversely at different time intervals but never within proportional to the speed the the same time interval? interface is capable of transmitting it. This means that the time it takes to actually transmit a frame onto the network medium is less. By analyzing this, you can see that a worst-case scenario would be if two network nodes were at two extreme ends of the network media. Electrical signals travel at the same rate, but the amount of time to put a whole frame on the media is much ACRONYM ALERT less at higher interface speeds. In order to RTMP — Routing Table Maintenance Protocol detect that a collision has taken place, time is needed to travel to the far end of the network segment and back. To allow collision detection to occur within the transmission window of a sending network node, limitations were established for cable lengths and minimum frame length as higher interface speeds were developed. Table 6-5 lists these limitations. 271 Page 271 Edwards 272 Part I ■ c06.tex V3 - 03/27/2009 Networking Nuts and Bolts Table 6-5 Half-Duplex Operational Limitations PARAMETERS 10 MBPS 100 MBPS 1000 MBPS Minimum frame size 64 bytes 64 bytes 520 bytes Maximum collision diameter18 UTP cable 100 meters 100 meters 100 meters Maximum collision diameter with Repeaters 2500 meters 205 meters 200 meters Maximum number of repeaters in network path 5 2 1 6.4.1.1.1 Gigabit Ethernet Considerations Although the Gigabit Ethernet frame is similar to the standard Ethernet frame, it is slightly different in minimum frame length. As you can see in Table 6-5, the minimum frame size expanded from 64 bytes to 520 bytes for a 1000BASE-T frame. The Gigabit Ethernet19 frame is illustrated in Figure 6-9. Preamble Start of Destination Source Frame Address Address Delimiter Frame Length/ Type Data Frame Gigabit Carrier Check Extension Sequence Figure 6-9 The Gigabit Ethernet frame In order to maintain the same collision domain diameter, the developers opted to increase the minimum frame length to 520 ACRONYM ALERT bytes. The longer frame was obtained by SRB — Source routing bridging adding an extension to the frame after the Frame Check Sequence field. The Carrier Extension field is automatically removed by the receiving network node. The added frame length makes it possible for a frame collision to be detected because of the added time it takes to transmit a minimum-sized gigabit frame 18 Maximum collision diameter refers to the network media length from one transmitting network node to a receiving network node. Worst case is that each node is at the extreme end of a network segment. In wired network media, this equates to cable length and is linear, whereas in a wireless environment, it truly can represent a circle, where the diameter is the maximum distance from transmitter to receiver. 19 Gigabit per second capability is the capability to pass a billion bits per second on an interface. Remember, a bit is either a single binary 0 or a 1. Whatever the bit value is, there is a lot of stuff coming at you all at once. 11:07am Page 272 Edwards c06.tex Chapter 6 V3 - 03/27/2009 ■ 11:07am Ethernet Concepts onto the network media. The time is close to that of a 64-byte minimum-sized frame being transmitted on the network medium by a 10/100 half-duplex NIC. The standard for CSMA/CD Gigabit Ethernet added frame bursting, the capability of a Gigabit Ethernet NIC’s Media Access Control sublayer to transmit a burst of frames without releasing the access to the network media. This is possible since the time needed to place a minimum-sized frame on the network media is much less than the total propagation delay round-trip time of the frame traveling over the network media. Bursting is accomplished by allowing the transmission of a burst of frames within a time interval slightly greater than that needed for transmitting five maximum-sized frames. The media is kept occupied for the transmitting node by inserting frame carrier extension bits between the frames in the burst. Figure 6-10 illustrates a burst frame sequence. In Figure 6-10 you will notice that the first frame may have a POP QUIZ carrier extension added to it if it does not meet with the minWhat name is applied to the transmission imum frame size of 520 bytes. mode that allows multiple frames to be sent without the need to release the network Between frames or the frame media between frames? gap periods, the network media is kept busy with a continuous carrier by inserting carrier extension bits. For subsequent frames within a frame burst that do not meet the minimum frame size, a Frame Carrier Extension field is not needed since the frame gaps are being filled with extension bits while in the frame burst transmission mode. Frames will continue to be sent in burst mode until the burst frame limit has been reached. If there is a frame in the process of transmission when the burst frame limit has been reached, the frame is allowed to complete its transmission before the transmitting node releases the network media. Burst frame mode is only supported in Gigabit Ethernet. Frame plus Extension Frame Gap Frame Frame Gap Burst Carrier Duration Figure 6-10 The Gigabit Ethernet burst frame sequence Frame 273 Page 273 Edwards 274 Part I ■ c06.tex V3 - 03/27/2009 Networking Nuts and Bolts HELPFUL HINT Since frame burst mode is not supported in 10 Mbps or 100 Mbps Ethernet, it is not a good idea to add these types of network devices to a network segment that is running at gigabit speeds. If you need to mix these devices on the same network segment, you should not use burst mode on that network segment. 6.4.1.2 Full-Duplex Transmission Full-duplex transmission is the capability of a network node to transmit and receive simultaneously. It is a simpler method of communications than half-duplex since the need for collision detection is eliminated. However, it can only be attained in UTP networks or fiber optic networks, where transmit and receive circuits remain separated. The capability to send and receive at the same time effectually doubles the bandwidth of the network link between network nodes. The first cabling used for Ethernet networks was coaxial. Because this wired medium was being used for both transmission and reception, the CSMA/CD protocol was developed to permit a sending and receiving network node to communicate over the same cable. Moving from the coaxial wire network media to the UTP cable media, the half-duplicity of the coaxial cable was maintained with the use of hubs that simulated the coaxial cable. So the need to maintain the CSMA/CD protocol was carried forward from the coaxial wire network environment to the UTP cable environment using a half-duplex mode of communications. Full-duplex is a point-to-point method of communication, where the transmit circuit of one network node is directly connected to the receive circuit of another node, and vice versa. This is fine in a network where two network devices are connected directly to each other, but this is far from the capability to connect many network nodes together over a LAN. If hubs force network nodes into using half-duplex communications, how does one build a multinode network where the devices communicate using a full-duplex communications method? With the advent of Layer 2 network switches, full-duplex communications are possible on a multinode network. There is a difference between a ‘‘dumb’’20 hub and an ‘‘intelligent’’ switch. Hubs are actually considered part of the Physical layer because they are not decision-making devices. They basically provide the interconnectivity on the physical level for network nodes. 20 Hubs are sometimes called dumb or passive since they do not have any intrinsic intelligence to make a decision on how two nodes are to connect. They are always connected in half-duplex mode. 11:07am Page 274 Edwards c06.tex Chapter 6 V3 - 03/27/2009 ■ 11:07am Ethernet Concepts HELPFUL HINT Do not confuse terms such as switching hub or intelligent hub with true Layer 2 network switches. What is often being referred to in those terms for a hub is the capability to sense the pins for transmit and receive signals and configure the hub accordingly to accommodate the cable connecting the network node to the hub. Once the hub is configured, it still supports half-duplex communications. To run full-duplex on your local network segment, make sure the device you have selected is a true Layer 2 network switch. Layer 2 switches are more expensive than hubs, so there is a cost consideration. The name Layer 2 switch means exactly what it implies: it is a network device that operates within the first two layers of the OSI reference model. Of course, Layer 1 is the Physical layer, which implies that the construction of the ports of an Ethernet Layer 2 switch is designed with sockets that will accommodate UTP cables terminated with RJ-45 plugs. This physical attribute is no different from that of an Ethernet hub’s; they look almost alike but operate very differently. As the name implies, the Layer 2 portion is the Data Link layer of the OSI model, and that is the major difference between a hub and a switch. Hubs do not know or care about the hardware addresses of the devices that are connected to them. In a hub-interconnected network, the endpoint network nodes are responsible for knowing and deciphering the messages on the network media to determine if a frame is addressed to them. The Layer 2 switch uses this very information to electronically interconnect the ports that are connected to it using hardware source and destination addresses. The Layer 2 switch is not concerned with any other aspect of the frame other than being able to direct it to a port that corresponds to the hardware address of the device connected to it. In setting up this connection, the switch is able to maintain the network nodes connected to it to be able to communicate in full-duplex mode. In full-duplex mode, a frame can be transmitted as soon as POP QUIZ it is assembled. However, there is a requirement that the gap What does the term full-duplex mean? between successive transmitted frames be long enough for frame synchronization. Each transmitted frame that is transmitted must still adhere to Ethernet framing standards. 6.4.1.2.1 Full-Duplex Flow Control In the half-duplex mode of operation, a network node does not transmit unless the network medium is silent. It then transmits and while doing so attempts 275 Page 275 Edwards 276 Part I ■ c06.tex V3 - 03/27/2009 Networking Nuts and Bolts to detect any network collisions that may have occurred within its transmit interval. Since in full-duplex mode the transmit circuit is separate from the receive circuit, there is no need for collision detection. But how will a transmitting network node know when there is a need for a delay in transmission? A method of signaling between Media Access Control sublayers was devised to allow a receiving network to signal a transmitting network node that there is network congestion and to cease frame transmission for a period of time. This is referred to as flow control. To cause the cessation of frame transmission from a transmitting network node, the receiving network sends a PAUSE frame with a set delay time for the transmitting network node to wait before transmitting the next frame. If congestion is relieved after a PAUSE frame with a set interval is sent, the receiving network node may transmit another PAUSE frame with the time-to-wait value set to zero. Upon receiving this PAUSE frame, the transmitting network node may begin transmission once again. PAUSE frames are Media Access Control sublayer frames that have the Frame Length/Type field set to 0x0001 hexadecimal. The destination MAC address that is contained within the transmitted PAUSE frame is set to 01-80-C2-00-00-01. This reserved multicast21 address is a signal to the receiving switch that the frame is a PAUSE frame for a particular port and will not forward ACRONYM ALERT the frame to the other ports that are on the UI — User interface switch. A network node receiving a PAUSE frame will not pass the frame beyond the Media Access Control sublayer. The time-to-wait interval within a PAUSE frame is conPOP QUIZ tained within a 2-byte unsigned integer with a value between What is flow control used for? zero and all bits of the 2 bytes set to ones.22 Each unit of delay is equivalent to 512 bit times. In a 10 Mbps network, the bit time is equivalent to 0.0000001 seconds or a tenth of a microsecond. You can imagine how small these times are by factors of 10 in 100 Mbps and Gigabit Ethernet networks. In 21 Multicast is the capability to transmit a frame to all network nodes on the network. Upon seeing that the address is set for a multicast broadcast, a node on the network will receive the frame since it was intended to be received by all network nodes on the network. 22 Two bytes or 16 bits of ones are represented by 1111111111111111 binary, FFFF hexadecimal, or 65,535 decimal. These are all equivalents. However, there will be times in networking or digital circuits where the bit position carries a different connotation than simply a value. Usually these values are represented by a binary bit stream and are more an indication of position or time than just a value. 11:07am Page 276 Edwards c06.tex Chapter 6 V3 - 03/27/2009 ■ 11:07am Ethernet Concepts a 10 Mbps network, the minimum delay would be 51.2 microseconds, which is quicker than you can blink an eye. So you can see that for major congestion, the wait to send delay will have a greater value than the minimum of one. HELPFUL HINT Full-duplex and flow control are available for all network speeds of 10 Mbps, 100 Mbps and 1 Gbps. However, on any one particular link between a network node device and a switch, the transmission speed, duplex mode, and flow control all need to match. This is on a link-per-link basis. so it is possible that there can be links of various speeds, duplex, and flow control on differing ports within the same switch. Unless you are certain you know the configuration on a switch, it is not a good idea to swap ports blindly unless you are certain the ports are set identically. If switch ports are set to autonegotiate, they should be able to self-configure and settle on the method of communication to be used over the network link. 6.4.1.3 Autonegotiation Autonegotiation is the capability of a NIC to negotiate the communication parameters that are to be used between it and the port it’s connected to. The negotiation between peers only happens on a direct link between the two network nodes. The two devices can have different capabilities but will negotiate upon the duplex and the highest transmission speed the two network interfaces are capable of. Devices of 10 Mbps, 100 Mbps and gigabit speed can be matched on the same network link if needed. The maximum speed that can be attained on any one network link would be the maximum speed of the slowest network interface. An example of this would be if a 10 Mbs interface set to half-duplex is plugged into a switch port that is set to autonegotiate. Assuming that the switch has the capability to perform at 100 Mbs at full-duplex, it would negotiate the port settings down to 10 Mbs at half-duplex, which is below its rated capability. This allows for flexibility within the network environment where the switch has been placed, but is not really beneficial for network performance. Autonegotiation has its place and at times can be very beneficial, so that network administrators do not have to configure each port every time they want to swap a port. Another example would be if one end of a network link has a 100 Mbps network interface RANDOM BONUS DEFINITION and the other end has a gigabit Physical layer — The lowest layer of the interface connected to it. If both seven-layer OSI model. interfaces were set to autonegotiate, they would ideally settle 277 Page 277 Edwards 278 Part I ■ c06.tex V3 - 03/27/2009 Networking Nuts and Bolts upon 100 Mbps at full duplex. However, this is assuming that the two network node devices play nice and can settle on that speed and duplex. Depending on manufacturer and the network interface being used, a link may need to be set permanently to a speed and duplex due to the inability of the two devices to negotiate a speed and duplex that works for both of them. There may be instances where both interfaces do negotiate a speed but for some reason one interface settles upon half-duplex while the other settles upon full-duplex. On the surface everything may appear to be working as planned. However, performance over the link may be affected and communications seem slow. Mismatch in duplex is not uncommon and at times goes unnoticed until major network degradation is noted. It is possible when two network node ports are interconnected that it appears that one network interface may have failed. The two devices will not bring up the link. There are a couple of ways to attack this problem. One is to hard-set both ends to a speed and duplex that you know they are capable of and see if you can send data across the link. The other method is to have a third network node device that you know is reliable connect to each to see if the link will come up with either device connected. This test is not conclusive, but if both devices can link with the known device, the culprit may be that autonegotiation between the two network interfaces is not working. There is a possibility that two network node interfaces may appear to autonegotiate propPOP QUIZ erly and can operate for an What is autonegotiation? extended period of time without any problems. Then it is noticed that some network performance problems have arisen. Traffic over a particular link seems to degrade, comes back, and then degrades again. This can be an indication that the autonegotiation between the two network node interfaces may be flapping.23 If these network ports are set to autonegotiate, it would be best to manually configure them for the highest common speed and duplex and then monitor the link to see if performance picks up. If not, it can be an indication of bad cabling or possibly one network node interface may be having problems. 23 Flapping (or flopping or flipping) generally describes an unstable network interface link. This is perhaps an offshoot of the old digital design days when flip-flops were used to maintain a particular state. Flip-flopping has wiggled its way into our society to mean something that is either indecisive or changes state whimsically. A good example of this would be today’s politicians. 11:07am Page 278 Edwards c06.tex Chapter 6 V3 - 03/27/2009 ■ 11:07am Ethernet Concepts HELPFUL HINT Some devices indicate link status and/or speed, but few indicate whether the link is running half- or full-duplex. You may want to become familiar with the network devices being used in that network segment. This will allow you to use monitoring tools to determine if speed and duplex for the link are set properly for the two network node interfaces that are connected on the link. Many network node devices do provide software tools for monitoring and measuring performance of the ports on the device. These software tools are usually a part of the software suite that came with the network device and can be used not only for configuration but also for troubleshooting. 6.4.2 Receiving a Frame The receiving of a frame is the same no matter what type of network interface is in use. The electrical signals are received from the network media and loaded into a frame receive buffer. The major difference is between half-duplex network interfaces and full-duplex interfaces. A network interface that is strictly a half-duplex interface can use the same frame buffer for both transmitting and receiving a frame. However, full-duplex interfaces need to be capable of both transmitting and receiving at the same time, so a receive frame buffer is needed as well as a transmit frame buffer. When a frame is received by a network interface, it is loaded into the receive frame buffer and the destination address is compared to see if it matches the unique MAC address of the network interface or network group address or if the frame is a broadcast frame. If there is an address match, the frame length is checked along with the Frame Check Sequence field. The Frame Check Sequence field is checked against the checksum, which was calculated as the frame was received from the Physical layer. If this matches, the Frame Length/Type field is checked to determine the frame type of the frame that was received so it can be properly be parsed and passed to the appropriate upper layer. Once the frame has been unloaded from the receive buffer and passed up the ISO POP QUIZ reference model to the upper When a frame is received, what is the first layers, the network interface is criteria that is checked? then ready to receive another frame from the Physical layer. If a frame does not pass the proper framing criteria, it is discarded and the interface is readied to receive the next network frame. 279 Page 279 Edwards 280 Part I 6.5 ■ c06.tex V3 - 03/27/2009 Networking Nuts and Bolts Traffic Optimization What exactly is traffic optimization? It connotes a lot of various things, but the gist of the term is overall improvement in network performance. In the earlier sections of this chapter, we discussed speed and duplex and how they can affect the performance on a particular network link. We can see that there are advantages of having certain network paths being faster than others. Links going between devices that aggregate numerous network nodes need to be faster and more reliable than those of a single workstation to a hub or network switch. Figure 6-11 illustrates a network consisting of many user network nodes interconnected with high-speed switches that have high-speed gigabit interfaces between them. The high-speed switches in this figure are to aggregate the multiple workstations and allow them to stream network data unimpeded by congestion caused if the data links between the switches were of the same speed as those between the workstation and the switches. In this example, the workstations are connected to the switches using a 100 Mbps full-duplex link. The switches are interconnected with high-speed gigabit full-duplex links and provide a redundant path if needed. High Speed Ethernet Link User Workstations Server Farm Figure 6-11 A network segment with high-speed links User Workstations 11:07am Page 280 Edwards c06.tex Chapter 6 V3 - 03/27/2009 ■ 11:07am Ethernet Concepts The redundant24 path shown in this figure allows for any of the high-speed links ACRONYM ALERT to go down and still have workstations on both network switches to which they SNAP — Sub-network Access Protocol are connected be able to access the server farm. These servers can provide various services such as e-mail, mass data storage, and client/server applications The servers are interconnected over a high-speed data link with a gigabit NIC to eliminate congestion on any one server. This increases the likelihood that there would be less congestion on these data links but does not totally eliminate the possibility that congestion could occur. When administering large network installations it is imporPOP QUIZ tant to understand the traffic patterns that are present on What is the first step you should perform the network. Network efficiency before implementing a network? can be increased where needed. The idea is to balance the need versus what it will cost since there can be areas of overkill where the investment in network resources is underutilized and thus is not a wise decision. Careful planning can greatly aid in determining where more network resources are required and limit the amount of waste of underutilized network segments. Know the business environment in which the network you are administering is installed. A carefully thought-out network is easier to install, maintain, and troubleshoot and runs efficiently with higher reliability. 6.5.1 Traffic Shaping In the previous section, we discussed planning where high-speed links would be required. This approach is best-effort, and there is no differentiation of the type of traffic or if it is more important traffic than that of another transmitting network node. With real-time applications such as Voice over IP and videoconferencing, there is a need to give priority to these frames so they can be delivered in a timely fashion. What if there was a way to tag a frame so it would be given a priority over another frame that need not be delivered as quickly? If frames are marked, they can be queued so the frames with priority will be forwarded on to the next segment. A simplified diagram illustrates this in Figure 6-12. 24 Redundant path or redundancy in a network is the capability to provide multiple paths to various network resources to add fault tolerance. If one or more high-speed links go down, the network will either be unaffected or, at worst, be partially affected. It may not be able to have all of the network resources available to all of network users, but there will be areas of unimpeded network operation. 281 Page 281 Edwards 282 Part I ■ c06.tex V3 - 03/27/2009 Networking Nuts and Bolts Frames IN Frame Sorter Tagged Frames Untagged Frames Frame Decision Logic Frame Buffer Frames OUT Network Data Stream Figure 6-12 Frame prioritization Frames are tagged to identify them as frames that should be transmitted over the network with priority. As frames enter into a network node that is to transmit tagged ACRONYM ALERT frames with priority, they are checked for PDU — Protocol data unit a priority tag. A queuing system is used to keep both tagged and untagged frames in the same order as they are received. When the network node device is ready to transmit the next frame, a check is made by frame decision logic to see if there are any tagged frames to be sent with priority. If there are tagged frames, they will continue to be transmitted until there are no remaining tagged frames that need to be transmitted. When the tagged frames bin is empty, untagged frames will be transmitted until the next tagged frame arrives in the tagged frame bin. All frames are sent in the order they are received, with the tagged frames being transmitted before any untagged frames. 11:07am Page 282 Edwards c06.tex Chapter 6 V3 - 03/27/2009 ■ 11:07am Ethernet Concepts We have discussed the Layer 2 switch, but tagging requires POP QUIZ a higher level than that. Routers How is a frame given priority? are capable of operating at Layer 3 and can make decisions on tagged packets. However, there is a more recent development in the switching area — the Layer 3 switch (sometimes called the routing switch). Routing switches perform many of the same functions as routers, except they operate much faster. Conventional routers depend on software for the routing protocols and decision making. Routing switches implement the routing decision process in hardware, allowing higher throughput of frames. These network devices may be faster than routers as far as forwarding frames, but they are not as flexible or as programmable as a conventional router. 6.5.1.1 VLAN Tagging VLAN25 tagging was standardized in IEEE 802.1Q. The standard allows for 4 bytes used for tagging purposes to be inserted between the Source MAC Address and the Frame Length/Type fields. Any modification of a frame will destroy the Frame Check Sequence checksum, so after the frame is assembled with the 802.1Q tagging the checksum is recalculated and placed in the Frame Check Sequence field. Figure 6-13 illustrates the 802.1Q VLAN header. Tag Protocol Identifier (TPID) Priority Code Point (PCP) Canonical Format Indicator (CFI) VLAN Identifier (VID) 16 Bits 3 Bits 1 Bit 12 Bits Figure 6-13 The IEEE 802.1Q VLAN header TPID — The Tag Protocol Identifier is a 16-bit field containing the hexadecimal value of 0x8100 as an indicator that the frame is an 802.1Q tagged frame. PCP — The Priority Code Point is a 3-bit field26 that contains a value from 0 to 7 and is used to indicate the priority level of the frame. Zero is the lowest priority and 7 is the highest. 25 VLAN is an acronym for virtual local area network. Normally, a LAN is localized within a network segment. However, in a switched network environment, the member network nodes of a VLAN do not need to be located within the same local vicinity. They are identified as a group belonging to a particular VLAN. 26 The maximum value of 3 binary bits is 7: 111(binary) = 7 (decimal). The binary value positions are 4+2+1, which equals 7. This little exercise is for those readers who may find themselves ‘‘base-2 challenged.’’ 283 Page 283 Edwards 284 Part I ■ c06.tex V3 - 03/27/2009 Networking Nuts and Bolts CFI — The Canonical RANDOM BONUS DEFINITION Format Indicator is a 1-bit field when set to 1BASE5 — A baseband Ethernet system the value 0 to indicate operating at 1 Mbps over one pair of UTP cable. Also known as StarLAN. that the MAC address is in canonical format, which is always set to 0 for Ethernet switches. If a frame is received with the CFI set to the value 1, it should not be bridged to an untagged port. VID — The VLAN Identifier is a 12-bit field that specifies which VLAN the incoming frame belongs to. If this field is set to the value of 0, it indicates that the frame does not belong to a VLAN and that it is only a priority tag. The advantage of having network node devices that are part of a VLAN group equipped POP QUIZ with VLAN tagging is primarWhat does the acronym VLAN stand for? ily the capability to tag outgoing frames with a priority. This means that frames that require timely delivery are expedited over the network before less critical or best delivery frames. Another advantage is that network node devices can be grouped and are allowed to communicate across multiple LAN networks as if they were all on a single LAN network. The destination address is filtered by the switches and bridges in the network path and only forwards the frames to the ports that service the VLAN the frame belongs to. Because of the configurability of these switches, network management is made simpler, allowing for easy addition, removal, movement, or other configuration changes required on a VLAN port. HELPFUL HINT Layer 3 (or routing) switches seem so easy to manage and configure. We will again caution about the need for documenting your network well, unless you prefer to go through a multitude of switch configurations, port by port. It is even more imperative because of configurations where ports can be moved and juggled without physically going out and moving a cable on a port. Switch networking issues can be daunting on a large network, so there is no substitute for good network documentation. (continued) 11:07am Page 284 Edwards c06.tex Chapter 6 V3 - 03/27/2009 ■ 11:07am Ethernet Concepts HELPFUL HINT (continued) If you need to call for support on a problem, remember that the support engineer does not have a crystal ball27 to look into your network. He is going to rely on your ability to know your network and know it well. Support engineers do not like playing guessing games. It is a waste of their time and will add to your frustration level as your boss blows his hot breath on the back of your neck. Want to be a good network administrator? Document, document, document! 6.6 Chapter Exercises 1. What does the acronym CSMA/CD stand for? 2. What form of communications eliminates the need for collision detection? 3. When you choose not to configure an Ethernet port for speed and duplex mode, what are you relying on? 4. What is needed when setting up VLAN networking? 5. What is a source address? What is a destination address? 6. What is the maximum number of bytes the Data field can contain in an Ethernet frame? What is the minimum number of data bytes? 6.7 Pop Quiz Answers 1. What was the first type of cable used to form an Ethernet network? Coaxial cable 2. An Ethernet network device that forwards data on the network would be considered what type of Ethernet device? DCE (data communications equipment) 27 A crystal ball is a device a network administrator hopes the support engineer at the other end of the hotline has when he frantically calls for support. Alas, he does not possess one, so drop to your knees and start praying. Or you can take the easy way out and start documenting your network from initial installation through configuration changes, additions, and anything that modifies the network. 285 Page 285 Edwards 286 Part I ■ c06.tex V3 - 03/27/2009 Networking Nuts and Bolts 3. If a cable is wired such that one plug is a T568A and the other is a cable. T568B, it would commonly be referred to as Crossover 4. You are interconnecting two Ethernet devices, but neither device is showing a link light on the assigned port. List in order of likelihood where the problem might be. Cable type Defective cable Bad network interface 5. Into which two sublayers of the IEEE 802 reference model is the OSI reference model Data Link layer divided? LLC (Logical Link Control) MAC (Media Access Control) 6. With which functions is the Logical Link Control sublayer mainly concerned? Flow control Error control Multiplexing protocols 7. When a collision occurs on the media, what does the transmitting network node do? Stops transmitting 8. What is the maximum number of bytes that can be contained in the Data field of an Ethernet frame? 1500 bytes 9. What does the Frame Check Sequence field of an Ethernet frame contain? CRC calculation using the bytes of the Destination Address, Source Address, Frame Length/Type, and Data fields. 10. What is the name of the transmission mode that allows either transmitting or receiving at different time intervals but never within the same time interval? Half-duplex 11. What name is applied to the transmission mode that allows multiple frames to be sent without the need to release the network media between frames? Burst mode 11:07am Page 286 Edwards c06.tex Chapter 6 V3 - 03/27/2009 ■ 11:07am Ethernet Concepts 12. What does the term full-duplex mean? The capability to transmit and receive at the same time. 13. What is flow control used for? To stop a transmitting node from sending when congestion is detected. 14. What is autonegotiation? The capability of two network node peers to negotiate the speed and duplex used on the link they are connected to. 15. When a frame is received, what is the first criteria that is checked? Destination address 16. What is the first step you should perform before implementing a network? Carefully plan out the network. 17. How is a frame given priority? Tagging 18. What does the acronym VLAN stand for? Virtual local area network 287 Page 287 Edwards c06.tex V3 - 03/27/2009 11:07am Page 288 Edwards c07.tex V3 - 03/27/2009 11:19am CHAPTER 7 Not to Be Forgotten If you would not be forgotten as soon as you are dead and rotten, either write things worth reading or do things worth the writing. — Benjamin Franklin We are now at the end of the ‘‘Networking Nuts and Bolts’’ part of this book. So far we have discussed most of the predominate standards that are implemented in the majority of networks. We have discussed the popular LAN and WAN standards that you will most likely be involved with should you continue in your quest of network knowledge. What you have seen in this section of the book is only a portion of the technologies that are available and/or implemented in many networks. This chapter is going to provide an overview of some of the other standards and processes that are available and, for the most part, in use (if only in a small percentage of networks). The way we see it, it just wouldn’t be a good networking book if these weren’t at least mentioned.1 Some of the technologies in the following pages are of a dying breed, whereas others are just starting to grow. Whatever their status, these are standards that have been replaced by other standards, enhanced by revisions to the original standard, developed to support proprietary hardware and/or software products, or developed to support a new technology. When a standard is placed on the road to becoming obsolete,2 it is normally due to technology advancements that the standard cannot support. This does not mean you cannot use the standard, but it does mean there will be no further advancements to the standard and, for the most part, what you see is what 1 Although there are many good networking books out there that deal with even a single protocol. 2 The process of retiring a standard is known as placing it into an ‘‘end-of-life’’ status. 289 Page 289 Edwards 290 Part I ■ c07.tex V3 - 03/27/2009 Networking Nuts and Bolts you get (WYSIWYG).3 Some of the standards we will discuss are proprietary but are often implemented as the standard of choice, and some are newer technologies that are just experiencing ‘‘startup growth’’ and will probably prove themselves to be a major part of networks in the next decade. At the end of the chapter, we have provided an introduction to the structure of a datagram — what it is, how it works, and why it is important. This is to ensure that we keep that network knowledge flowing. 7.1 Can’t Get Enough of Those LAN Technologies In the last chapter, we discussed Ethernet, which is the most popRANDOM BONUS DEFINITION ular of LAN protocols in use today. Because of the advance100BASE-T — The term used to describe baseband Ethernet transmission of 100 ments and cost savings offered Mbps. by Ethernet, many other protocols have been retired (or are not as commonly used as Ethernet). In this section, we discuss a few LAN protocols that were once on the cutting edge, and may still be out there serving in some capacity. 7.1.1 Attached Resource Computer Network In Chapter 1, we defined a LAN as a data network that covers a small geographical area. This normally ranges from an area with just a few PCs to an area about the size of an office building or a group of buildings. Attached Resource Computer Network (ARCnet) is a protocol that was once very popular in LANs, and has even found a purpose in today’s Ethernet world. ARCnet is now used as an embedded standard to serve networks that control automation services, transportation, robotics, gaming, and other similar network types. Developed by the Datapoint Corporation in the late 1970s, ARCnet was designed to use token-passing bus technology over coaxial cabling. The physical topology of ARCnet is a star/bus topology (see Figure 7-1). ARCnet touted speeds of up to 2.5 Mbps4 and distances of up to four miles. ARCnet is considered the first truly commercially available LAN. Due to the low cost of the infrastructure and the simplicity in implementation and maintenance, ARCnet was very popular when it first arrived. 3 Pronounced ‘‘wizzy-wig.’’ later version of ARCnet was released in the early 1990s and was called ARCnet plus. It could operate at speeds of up to 20 Mbps. By the time ARCnet plus had come out, however, Ethernet was quickly becoming the standard of choice. 4A 11:19am Page 290 Edwards c07.tex Chapter 7 V3 - 03/27/2009 ■ 11:19am Not to Be Forgotten Figure 7-1 An example of an ARCnet topology ARCnet doesn’t have all the bells and whistles that are offered in networks today. It is a very simple technology that is easy to implement and run. A big drawback with ARCnet is that when an interface is brought into the network, the address of the interface has to be set by whoever is installing it. Most of the time, the address is set by jumpers or switches on the resource interface module (RIM)5 itself. ARCnet was designed to give Datapoint nodes the capability to share resources over the token bus, thus increasing the overall power of the attached nodes. Datapoint had originally intended to keep what became known as ARCnet fully proprietary because if the public bought their gear, they could tout resource sharing as a selling point. Datapoint had some problems with the design of the RIM chip, so they eventually contracted with Standard Microsystems Corporation (SMSC). SMSC successfully built the chip specifically for Datapoint, and in the final negotiations got the approval to sell a version of the chip to other vendors — and ARCnet was born. 7.1.2 StarLAN StarLAN technology is, for the most part, the predecessor to what we all know as Ethernet. Often referred to as 1BASE5 and developed in the early 1980s by AT&T, StarLAN provided a way 5 The POP QUIZ What was the name of the company that developed ARCnet? RIM is basically the ARCnet-supported NIC card. 291 Page 291 Edwards 292 Part I ■ c07.tex V3 - 03/27/2009 Networking Nuts and Bolts for nodes to communicate with one another over a telephone line. StarLAN operated at 1 Mbps and eventually supported speeds of 10 Mbps.6 1BASE5 actually came out after coaxial cabling came out supporting 10 Mbps. This is part of the reason that StarLAN never really got deployed in most LANs. Once 10BASE-T came out, the only time StarLAN was used was when someone needed a low cost infrastructure and speed was not a concern. Figure 7-2 shows an example of the StarLAN topology. StarLAN Hub All links operate at 1 Mbps Figure 7-2 The StarLAN topology StarLAN networks used UTP as a transmission medium and typically connected nodes to one another through at least one hub. StarLAN was able to also connect to multiple nodes without a hub by daisy-chaining them one by one upon the shared medium. The maximum number of nodes in a daisy-chain configuration was 10. Figure 7-3 shows an example of daisy-chaining. 7.1.3 Token Ring Token Ring network technology was developed by IBM in the late 1970s. IBM submitted the proposed standard to the IEEE LAN standards committee, which adopted the proposal and used the standard as the basis for the IEEE 802.5 standard. Token Ring topologies are a star physical topology and a ring logical topology, as shown in Figure 7-4. 6 By this time, however, 10BASE-T was out, which rendered this advancement moot. 11:19am Page 292 Edwards c07.tex Chapter 7 V3 - 03/27/2009 ■ Not to Be Forgotten StarLAN Hub Figure 7-3 Including a daisy chain in a StarLAN configuration Physical Topology Figure 7-4 A Token Ring topology 11:19am Logical Topology 293 Page 293 Edwards Part I ■ V3 - 03/27/2009 Networking Nuts and Bolts Token Ring networks pass a signal, known as a token, from one node to the next. The node that you receive the token from is the upstream neighbor. The node that you pass the token to is the downstream neighbor. Each node receives the token, takes action, and then passes the token to the downstream neighbor (see Figure 7-5). ive ce Re Se nd Ac n io t Ac tio n Re d ce n Se ive e Se eiv nd Re c Ac n tio tio Ac n Re ce ive nd Se 294 c07.tex Figure 7-5 Token Ring operations The actions that are taken are determined by whether the POP QUIZ node has control of the token. If a node controls the token, What technology is also known as 1BASE5? it transmits the token onto the ring to the downstream neighbor, which receives the token and then passes it on the ring to its downstream neighbor. The data is captured by each node, and once the token has made it back to the originating node, that node will remove it from the ring, thus freeing the ring up for the next token to be passed. The original Token Ring supported speeds of 4 Mbps and later came to support 16 Mbps. It didn’t take long for networks to upgrade to support the higher speed, especially as the demands on the LAN grew. There is an 802.5 approved standard for Token Ring, allowing up to 100 Mbps speeds, but this never really became popular.7 7 Anyone care to guess why? 11:19am Page 294 Edwards c07.tex Chapter 7 7.1.3.1 V3 - 03/27/2009 ■ 11:19am Not to Be Forgotten Token Ring’s Modus Operandi In a Token Ring environment, only one node can transmit data from itself at a time. The originating node is given the token in order to pass it on to the network. The node sets the Token bit from a 0 to a 1, which transforms the Token into a datagram known as a frame. The data is passed from node to node around the ring. Each node inspects the frame and forwards it to the downstream neighbor. Once a node inspects the data frame and recognizes its own address as a destination address, the node retains a copy of the data ACRONYM ALERT and sends the data on to the next node in TTL — Time to live line. The data continues around the ring, inspected by all nodes, and then returns to the originating node, which retrieves the frame from the token and sends a new token8 on to the next node. Once the token arrives at a node that wants to send data, the process begins again. 7.1.3.2 Token Ring Media Token Ring originally operated on STP cabling but converted to UTP cabling in the 1990s. This was greatly appreciated by the networking community, as it offered a cheaper and less bulky medium. MMF9 cabling was supported officially in 1998 when an approved amendment was written into IEEE 802.5, although in actuality a lot of networks were using it already. Token Ring 100 Mbps operation is conducted on the exact twisted pair specification that is used for 100 Mbps Ethernet. 7.1.3.3 The Format of the Token Ring Frame Token Ring uses one of three frame types. Token frames have the token bit set to 0 and have no data. Token data frames10 have the data payload contained within the frame (the token bit is set to 1). The abort frame carries no data and is used to stop its own transmission of data, or used to clear up data that is on the line. The fields contained within the token frame are fairly simple to understand, as shown in Figure 7-6. 8 Sending a ‘‘new’’ token simply means that the token bit is set back to 0, indicating an available token. 9 Quick refresher: In Section 3.2.1.3 we discussed the two types of optical fiber, multi-mode fiber (MMF) and single-mode fiber (SMF). 10 Also known as a token command frame. 295 Page 295 Edwards 296 Part I ■ c07.tex V3 - 03/27/2009 Networking Nuts and Bolts Number of Bytes 1 1 1 SD AC ED PRI 0 M RES Figure 7-6 An empty Token frame SD (start of frame delimiter) — This field lets the receiving node know when the frame begins.11 AC (access control) — There are four subfields in the access control field, all used to transmit information to the access control process within Token Ring. PRI (priority bits) — The priority bits show the priority level of the frame. 0 (token bit) — This bit differentiates the frame type. In Figure 7-6, the token bit is set to 0, identifying it as a token frame. M (monitor bit) — The monitor bit is used by a node that is known as an active monitor node. This bit is used to detect various errors. RES (reservation bits) — The reservation bits are used by a node to announce that it has data to send and needs to use the token as soon as it is available. Reservations are based on the priority level that has been set. ED (end of frame delimiter) — This field lets the receiving node know when the frame ends.12 The token data frame format is pretty much an extension of the token frame format. The first two fields are identical, but the third field is moved to the end of the frame (where it belongs). Several fields are in between that contain the data and the information that a node will need Ring. 11 There POP QUIZ What is the signal called that is passed in Token Ring from one node to the next? to send and receive frames on the Token has to be something identifying the beginning of the frame. you have to be clued in when the frame starts, there has to be some way to let you know that the frame is complete. 12 When 11:19am Page 296 Edwards c07.tex Chapter 7 V3 - 03/27/2009 ■ 11:19am Not to Be Forgotten Figure 7-7 shows the fields contained within the token data frame. Number of Bytes 1 SD 1 AC 1 6 6 FC Destination Address Source Address Data 4 1 1 FCS ED FS PRI 1 M RES Figure 7-7 Token frame with data attached SD (start of frame delimiter) — This field lets the receiving node know when the frame begins. AC (access control) — There are four subfields in the access control field, all used to transmit information to the access control process within Token Ring. PRI (priority bits) — The priority bits show the priority level of the frame. 1 (token bit) — This bit differentiates the frame type. In Figure 7-7, the token bit is set to 1, identifying it as a token data frame. M (monitor bit) — The monitor bit is used by a node that is known as an active monitor node. This bit is used to detect various errors. RES (reservation bits) — The reservation bits are used by a node to announce that it has data to send and needs to use the Token as soon as it is available. Reservations are based on the priority level that has been set. FC (frame control) — The frame control field is used to separate network management data frames from user data frames. Destination Address — This field contains the 6-byte network address of the node the frame is destined for. Source Address — This field contains the 6-byte network address of the node the frame originated from. Data — This field contains the data from the upper layer protocol that is being transmitted. There is a certain limit on the amount of data that can be included in the frame. At 4 Mbps, the limit is 4,528 bytes. At 16 Mbps, the limit is 18,173 bytes. At 100 Mbps, the limit is 18,173 bytes. FCS (frame check sequence) — This field is a checksum algorithm that checksums the frame from the FC field to the end of the Data field. 297 Page 297 Edwards 298 Part I ■ c07.tex V3 - 03/27/2009 Networking Nuts and Bolts ED (end of frame delimiter) — This field lets the receiving node know when the frame ends. FS (frame status) — This field is used by the RANDOM BONUS DEFINITION originating node to trunk — A name defining a bundle of links, detect whether there also known as aggregate links. were any errors during transmission. This includes: if the destination node copied the data; if there were any errors encountered; and even if the destination node recognized itself as the destination node. 7.1.4 Fiber Distributed Data Interface The Fiber Distributed Data Interface (FDDI) is a LAN13 and/or MAN technology. FDDI14 was the first such technology that could operate at 100 Mbps. FDDI is an ISO standard and is fully compatible with the IEEE 802 standards. Although FDDI could function as a LAN technology, it is cheaper and easier to use 100 Mbps Ethernet. When FDDI was developed, it was intended to provide higher speeds in LANs than the quickest rate that was available at the time: 16 Mbps Token Ring or Ethernet. FDDI is sometimes used to connect server farms and multiprocessors to the network. Most often you will find FDDI deployed within the backbone of the network, providing quick connectivity between other networks. 7.1.4.1 FDDI Does What FDDI Does FDDI was designed to operate over shared fiber media. POP QUIZ The fiber connected nodes in a ring similar to the IEEE 802.5 What information is contained in the Token Ring standard configuraDestination Address field in a Token Ring frame? tion. The difference is that FDDI uses a dual-ring topology over a shared fiber medium.15 Data traffic on a FDDI ring flows in a counter-rotating manner. This means that data on one of the rings goes in one direction while the other ring carries traffic in the opposite direction. The ring that actively carries data is the primary ring 13 Most networks use FDDI at the MAN levels. ‘‘fiddy.’’ 15 There is a newer standard for FDDI that allows the use of twisted pair cabling instead of fiber. This is called the Copper Distributed Data Interface (CDDI), discussed in Section 7.1.4.1.2. 14 Pronounced 11:19am Page 298 Edwards c07.tex Chapter 7 V3 - 03/27/2009 ■ 11:19am Not to Be Forgotten and the other is the secondary ring, which remains in an inactive status until needed. Figure 7-8 shows an example of the FDDI topology. Primary ring FDDI concentrator Secondary ring Primary ring FDDI concentrator Primary ring FDDI concentrator Figure 7-8 FDDI topology Notice that unlike Token Ring, which connects to a central MAU, there are concentrators16 that connect nodes to the FDDI topology. We will discuss the different concentrator types in Section 7.1.4.2. Other nodes that can be used within a FDDI ring are servers, routers, switches, and so on. As long as the node is able to support FDDI, it can be used for its intended purpose on the FDDI ring. The FDDI protocol supports optical fiber (FDDI) as well as copper cables (CDDI)17 as a shared medium. The operations provide the FDDI functions, with the difference being the medium type used. Both have advantages and disadvantages, which we will discuss in the next two sections. 7.1.4.1.1 Fiber Distributed Data Interface FDDI is the FDDI protocol over fiber optic cabling. Both MMF and SMF optical fiber medium types are supported in a FDDI environment. 16 Refer to Section 3.3.3.1 if you do not remember what purpose the concentrator serves in a network. 17 The official name is twisted pair physical medium dependent (TP-PMD); however, CDDI seems to be gaining in popularity. CDDI is a Cisco term, while TP-PMD is the ISO term. It seemed to us that it is easier to refer to this as CDDI for the purposes of this book, but you may need to know both acronyms when working in a professional environment (you don’t want to get caught saying, ‘‘Huh?’’ when someone asks you if your TP-PMD is running). As has occurred many times in the history of networking, terms come and go. What is important is that you understand what they are referring to. 299 Page 299 Edwards 300 Part I ■ c07.tex V3 - 03/27/2009 Networking Nuts and Bolts There are advantages in using optical fiber as the primary transmission medium: Performance Greater distances Faster transmission speed Reliability Data security Each advantage is due to the actual medium itself. Optical fiber uses light instead of electricity to carry data. This prevents the leaking of electrical signals, thus improving performance and the reliability of the transmission of data. This also increases security as there is no way to tap into the fiber optic cable. This ensures that, for the most part, only the individuals that are intended to see the data will see the data. 7.1.4.1.2 Copper Distributed Data Interface Copper Distributed Data Interface (CDDI) is the FDDI protocol over twisted pair media instead of fiber. CDDI is officially known as twisted pair physical medium dependent (TP-PMD) and is also known as twisted pair distributed data interface (TP-DDI). CDTP-PMDDI uses both STP and UTP cable types. The main advantage with copper is that it is cheaper and easier to install and maintain than fiber. Because copper cannot transmit the distances that fiber can, it is often used to connect nodes to the concentrator in the FDDI environment. Figure 7-9 shows an example of this. Fiber Optic Twisted pair Twisted pair Fiber Optic Figure 7-9 FDDI and CDDI together 11:19am Page 300 Edwards c07.tex Chapter 7 7.1.4.2 V3 - 03/27/2009 ■ 11:19am Not to Be Forgotten FDDI Node Types One of the really neat things about FDDI is there are options for how you can configure it. Will you use fiber or copper? How many nodes and concentrators should be supported? What types of concentrators should you use? FDDI offers a lot of choices for you. The four main node types in the FDDI environment are: Single attachment station (SAS) — Connects to the FDDI ring through a single connector. The connector has an input port and an output port. Data is received on the input port and is sent to the downstream neighbor via the output port. The SAS connects to a concentrator and then to the primary ring only. Single attached concentrator (SAC) — Like the SAS, the SAC concentrator connects to only the primary ring. The connection is made through another concentrator. Dual attachment station POP QUIZ (DAS) — Connects to the FDDI ring through What does the acronym FDDI stand for? two connectors (each with an input and an output port). Can connect directly to the ring or through a concentrator. Dual attached concentrator (DAC) — A concentrator that connects to both rings. 7.1.4.3 The FDDI Frame Format The FDDI frame format is very similar to the format of a Token Ring frame. FDDI uses either token frames or token data frames. Figure 7-10 shows an example of a token frame. Number of Bytes ≥2 1 1 1 Preamble SD FC ED Figure 7-10 An empty token frame Preamble — Provides a vehicle to ensure the receiving node is synchronized to receive the frame. SD (start of frame delimiter) — This field lets the receiving node know when the frame begins. 301 Page 301 Edwards 302 Part I ■ c07.tex V3 - 03/27/2009 Networking Nuts and Bolts FC (frame control) — This field is used to separate network management data frames from user data frames. ED (end of frame delimiter) — This field lets the receiving node know when the frame ends. The token data frame format is pretty much an extension of the token frame format. The first two fields are identical, but the third field is moved to the end of the frame (where it belongs). There are several fields in between that contain the data and the information a node needs to send and receive frames on the Token Ring. Figure 7-11 shows the fields contained within the token data frame. Number of Bytes ≥2 1 Preamble SD 1 6 6 FC Destination Address Source Address Data 4 1 1 FCS ED FS Figure 7-11 A token frame with data attached Preamble — Provides a vehicle to ensure the receiving node is synchronized to receive the frame. SD (start of frame delimiter) — This field lets the receiving node know when the frame begins. FC (frame control) — This field is used to separate network management data frames from user data frames. Destination Address — This field contains the 6-byte network address of the node the frame is destined for. Source Address — This field contains the 6-byte network address of the node the frame originated from. Data — This field contains the data from the upper layer protocol that is being transmitted. There is a certain limit on the amount of data that can be included in the frame. At 4 Mbps, the limit is 4,528 bytes. At 16 Mbps, the limit is 18,173 bytes. At 100 Mbps, the limit is 18,173 bytes. FCS (frame check sequence) — This field is a checksum algorithm that checksums the frame from the FC field to the end of the Data field. ED (end of frame delimiter) — This field lets the receiving node know when the frame ends. POP QUIZ What are the four main node types in the FDDI environment? 11:19am Page 302 Edwards c07.tex Chapter 7 V3 - 03/27/2009 ■ 11:19am Not to Be Forgotten FS (frame status field) — This field is used by the originating node to detect whether there were any errors during transmission. This includes: if the destination node copied the data; if there were any errors encountered; and even if the destination node recognized itself as the destination node. 7.2 As If You Haven’t Had Enough of These Sweet Protocols It was tough to decide what to include in this section. There are a lot of protocols and other services that you will need to know. For one thing, you will probably come across some, if not all, of them at some point. Additionally, many of the protocols were built upon some networking original protocols, so understanding their function and structure is helpful in understanding the more advanced protocols that have come out in recent years. The information in this section should really help you start piecing out how things are connected in today’s networks. It should also help you better understand the next two parts of this book (especially when you will be tasked to design your own network). This section is fairly long, but it simply made sense to put it all in here. After reading through this chapter, if you like what we did, you can thank author Jim. If you don’t like it, it was author Rich’s idea. 7.2.1 Digital Equipment Company Network The Digital Equipment Company (Digital)18 developed and released the first version of the Digital Equipment Company Network (DECnet) protocol in the mid-1970s. For years, Digital had been developing a series of minicomputers that were known as the programmed data processor (PDP)19 series. DECnet was developed to allow two PDP series 11 (PDP-11) nodes to connect to one another over a point-to-point link and share resources. 18 Many people in the industry refer to the Digital Equipment Company as ‘‘DEC’’ (pronounced ‘‘deck’’), but the official ‘‘short name’’ is Digital. 19 Digital decided to use the term programmed data processor (PDP) instead of what it truly was — a computer. This is because computers were known to be complicated and very expensive. To thwart the negative press the computer had developed, the term PDP was used and sold to a market that could not afford a computer. 303 Page 303 Edwards 304 Part I ■ c07.tex V3 - 03/27/2009 Networking Nuts and Bolts AN UNRELATED MOMENT OF PAUSE A reporter was given an opportunity to do an exclusive interview with a network engineer who had been sent to the International Space Station to upgrade the network. Reporter: ‘‘So, how do you feel now that you have been there for 3 days?’’ Engineer: ‘‘Lady, how would you feel if you were stuck in space, floating inside a grouping of about 120,000 parts all bought from the lowest bidder?’’ DECnet is not in and of itself a complete single standard; it’s a suite of protocols. As with most protocols that continue to have an end-user demand, DECnet has undergone several updates to the original protocols. Following is a brief overview of the DECnet phases: DECnet phase I — Allowed two PDP-11 series to communicate with one another. DECnet phase II — Increased support to networks of up to 32 nodes. The nodes did not have to be identical, but were requested to be able to interoperate with each other. Communication between nodes was done via a point-to-point link. File sharing was an important upgrade during this phase. DECnet phase III — Increased support to networks of up to 255 nodes. Communication was handled via point-to-point link, as well as multidrop links. Support was added to allow DECnet networks to communicate with networks of other types. Routing and network management were also supported at this phase. DECnet phase IV — Increased support of networks of up to 63 areas, supporting up to 1023 nodes each. Phase IV included Ethernet support as well as some hierarchical routing standards. Also, a client was developed for Microsoft DOS and some Windows platforms that allowed workstation support of the DECnet protocol. DECnet phase V — IOS standards were rolled into this phase, movACRONYM ALERT ing the protocol from a proprietary standard to an open standard. The SONET — Synchronous Optical Network name phase V was later changed to DECnet/OSI, identifying the compatibility with other OSI standards. Eventually, some TCP/IP protocols were added and the name was changed to DECnet-Plus. DECnet phase IV introduced a layered network architecture that is similar to the architecture outlined in the OSI reference model. The DECnet layered 11:19am Page 304 Edwards c07.tex Chapter 7 V3 - 03/27/2009 ■ 11:19am Not to Be Forgotten model is known as the digital network architecture (DNA). In the DNA model, each layer serves the layers above it and requests services from the layer beneath it. The structure and purpose of the DNA model are much like the OSI model, each layer being responsible for a function to support the protocol. Each layer is mostly based on the proprietary protocol, so some of the upper layers share functions within individual substandards. The DNA changed as well when DECnet phase V came about, due to the multiple open standard support that was now part of the protocol. Most of the upper layers support both the proprietary and the open standards that became part of the protocol suite. Note that you don’t have to know all the proprietary standards in the protocol suite; know only that it operates in a hierarchical manner. 7.2.2 Xerox Network Systems Xerox Network Systems (XNS), developed by the Xerox Corporation20 in the late 1970s and early 1980s, was a suite of protocols that supported a variety of functions. Although it was never a true competitor to TCP/IP, XNS was adopted by many vendors to run within their LANs.21 XNS also utilized a reference model that roughly matched the OSI reference model. There were a total of five levels22 in the XNS reference model: Level 0 — Roughly corresponded with the OSI Layers 1 and 2. Level 1 — Roughly corresponded with the OSI Layer 3. Level 2 — Roughly corresponded to the OSI Layers 3 and 4. Level 3 — Roughly corresponded to the OSI Layers 7 and 7. Level 4+ — Roughly corresponded to the OSI Layer 7. XNS used a routing protocol called the Internet Datagram Protocol (IDP), which was responsible for datagram delivery within a network as well as an addressing scheme for the routing of said datagrams. Because the format of the IDP packet differed23 from some other routing protocols, we wanted to break down the packet for you in Figure 7-12 so you can see the fields that are contained in the packet. Number of Bytes 2 2 11 CS L 4 T P Destination C T Network # 6 2 4 Destination Host # DSN Source Network # 6 2 Source Host # SSN Data Figure 7-12 The IDP packet format 20 That was pretty obvious, wasn’t it? was modified for several of these companies to suit the needs of their particular network. 22 Not layers. 23 For one thing, the IDP network address contains the following: a 4-byte network number, a 6-byte host address, and a 2-byte socket field. 21 XNS 305 Page 305 Edwards 306 Part I ■ c07.tex V3 - 03/27/2009 Networking Nuts and Bolts CS (checksum) — Used to determine the integrity of the packet upon receipt by the destination. L (length) — Identifies the length of the packet. TC (transport control) — This field actually contains two subfields. The first subfield identifies the current hop count for the packet. The other subfield identifies the maximum time the packet can live on the network. PT (packet type) — Identifies the format of the packet. Destination Network # (destination network number) — The 4-byte destination network identifier. Destination Host # (destination host number) — The 6-byte destination host identifier. DSN (destination socket number) — The 2-byte destination socket identifier. Source Network # (source network number) — The 4-byte source network number. Source Host # (source host number) — The 6-byte source host identifier. SSN (source socket number) — The 2-byte source socket identifier. POP QUIZ What are DECnet’s five phases? Data (data) — The payload! 7.2.3 Internetwork Packet Exchange The Internetwork Packet Exchange (IPX) protocol is normally found within networks with nodes running the Novell NetWare operating system. Novell NetWare was built to support the protocols that were a part of the XNS protocol suite. IPX is a datagram protocol used to route packets within a network. It is connectionless-oriented protocol (IP, for example) and therefore does not have to ensure a connection before it puts the packet onto the transport medium. IPX uses a distance-vector protocol (RIP, for example), making routing decisions based on hop counts. IPX RIP works similarly to RIP, but instead of using a hop count for distance determination it uses what is known as a tick. A tick is simply a measure of time (1/18th of a second) delay that is expected for a particular distance on the medium. If there are two routes to the destination and the ticks are the same on each path, the route with the lowest hop count is the one that will be chosen. 11:19am Page 306 Edwards c07.tex Chapter 7 V3 - 03/27/2009 ■ 11:19am Not to Be Forgotten IPX uses an IPX address for host/node identification. There are two parts to the IPX address. The first part of the IPX address is the network number: the remaining part is known as the node number. The network number is 4 bytes long (that’s a total of 32 bits for those of you who are counting).24 The node number is 6 bytes long (48 bits), which happens to match the length of the MAC address of the NIC. Why does it match? Because the MAC (IEEE 802) address is the number that is used for the node number part of the IPX address. Figure 7-13 is an example of the IPX address. Network– 4 bytes Node–6 bytes Figure 7-13 The IPX address Because the node has its own RANDOM BONUS DEFINITION MAC address, the only requirement you need to have an IPX workgroup switch — A switch used within address assigned to the node is a single department or workgroup. to plug it into an interface to the network. The node will send out a broadcast letting the network know it has joined the network. The appropriate router will then assign the network number to the node. The node now has identification and can send and receive IPX datagrams. IPX is simple to implement — it is basically plug and play. By now you have to be asking if there is anything complicated about IPX. The answer is no, but there is something you need to know about the IPX datagram format: there is not just a single datagram format. Why? Originally, IPX frame formats served well on the early Ethernet networks within a single network. But as networks grew and as LANs began communicating with one another, other standards were introduced and existing standards were improved, and IPX could not support communication with nodes outside of their known network number–which is why four Ethernet frame formats are used.25 Novell proprietary frame format — This is the original frame format that was used. It is often referred to as 802.3 raw (minus the LLC [802.2]). Figure 7-14 is an example of this. Number of Bytes 6 6 2 DA SA LNH 4 IPX Packet CRC Figure 7-14 The 802.3 raw frame format 24 If you are counting, or even thought of counting, then you get extra credit! Great job! router is responsible for translating and reformatting different formats so the destination can understand the information within the frame. 25 The 307 Page 307 Edwards 308 Part I ■ c07.tex V3 - 03/27/2009 Networking Nuts and Bolts DA (destination address) — The 6-byte destination MAC address. SA (Source address) — The 6-byte source MAC address. LNH (length) — This field identifies the amount of data contained in the data payload field. IPX Packet — This is the IPX datagram portion of the frame. The following subfields are part of the IPX packet: CS (checksum) — This field is normally not used. If it is used, then it is not compatible with the Novell proprietary format. PL (packet length) — The length of the IPX packet. TC (transport control) — The hop count (this is an incrementing field). PT (packet type) — Identifies the format of the data in the payload portion of the packet. DNN (destination network number) — The 4-byte destination network identifier. DHN (destination host number) — The 6-byte destination host identifier. DSN (destination socket number) — The 2-byte destination socket identifier. SNN (source network number) — The 4-byte source network number. SHN (source host number) — The 6-byte source host identifier. SSN (source socket number) — The 2-byte source socket identifier. Data — The payload! CRC (cyclic redundancy check) — This is a 4-byte value that is part of the frame check sequence (FCS), used to determine if a frame is intact at the receiving end. 802.3 frame format — This is the same format used by Ethernet, followed by the IPX data payload. Figure 7-15 is an example of this. Number of Bytes POP QUIZ Which operating system uses IPX? 6 6 2 DA SA 8137 4 IPX Packet CRC Figure 7-15 The 802.3 frame format DA (destination address) — The 6-byte destination MAC address. 11:19am Page 308 Edwards c07.tex Chapter 7 V3 - 03/27/2009 ■ 11:19am Not to Be Forgotten SA (source address) — The 6-byte source MAC address. LNH (length) — This field identifies the amount of data contained in the data payload field. IPX Packet — This is the IPX datagram portion of the frame. The following subfields are part of the IPX packet: CS (checksum) — This field is normally not used. If it is used, then it is not compatible with the Novell proprietary format. PL (packet length) — The length of the IPX packet. TC (transport control) — The hop count (this is an incrementing field). PT (packet type) — Identifies the format of the data in the payload portion of the packet. DNN (destination network number) — The 4-byte destination network identifier. DHN (destination host number) — The 6-byte destination host identifier. DSN (destination socket number) — The 2-byte destination socket identifier. SNN (source network number) — The 4-byte source network number. SHN (source host number) — The 6-byte source host identifier. SSN (source socket number) — The 2-byte source socket identifier. Data (data) — The payload! CRC (cyclic redundancy check) — This is a 4-byte value that is part of the frame check sequence (FCS), used to determine if a frame is intact at the receiving end. 802.3 with 802.2 frame format — The header of this format is the same format used by IEEE 802.3, then comes the LLC header, and finally the IPX data payload. Figure 7-16 is an example of this. Number of Bytes 6 6 DA SA RANDOM BONUS DEFINITION access priority — The priority used to determine access privileges on a shared LAN segment. 2 1 1 1 LNH DSAP SSAP CTRL Figure 7-16 The 802.3 with 802.2 frame format 4 IPX Packet CRC 309 Page 309 Edwards 310 Part I ■ c07.tex V3 - 03/27/2009 Networking Nuts and Bolts DA (destination address) — The 6-byte destination MAC address. SA (source address) — The 6-byte source MAC address. LNH (length) — This field identifies the amount of data contained in the data payload field. DSAP (destination service access point) — This field identifies which service access points26 the LLC information should be delivered to. SSAP (source service access point) — This field identifies the service access point the data originated from. CTRL (control) — This field contains information used by the LLC on the receiving node that identifies the LLC frame type. IPX Packet — This is the IPX datagram portion of the frame. The following subfields are part of the IPX packet: CS (checksum) — This field is normally not used. If it is used, then it is not compatible with the Novell proprietary format. PL (packet length) — The length of the IPX packet. TC (transport control) — The hop count (this is an incrementing field). PT (packet type) — Identifies the format of the data in the payload portion of the packet. DNN (destination network number) — The 4-byte destination network identifier. DHN (destination host number) — The 6-byte destination host identifier. DSN (destination socket number) — The 2-byte destination socket identifier. SNN (source network number) — The 4-byte source network number. SHN (source host number) — The 6-byte source host identifier. SSN (source socket number) — The 2-byte source socket identifier. Data (data) — The payload! CRC (cyclic redundancy check) — This is a 4-byte value that is part of the frame check sequence (FCS), used to determine if a frame is intact at the receiving end. Sub-network Access Protocol (SNAP) frame format — Uses the IEEE 802.3 standard header, LLC header, SNAP header, and finally the IPX data payload. Figure 7-17 is an example of this. 26 A service access point (SAP) is a label that is assigned to endpoints in a network. 11:19am Page 310 Edwards c07.tex Chapter 7 Number of Bytes 6 6 DA SA 2 1 1 1 LNH V3 - 03/27/2009 ■ Not to Be Forgotten 5 SNAP-H 11:19am 4 IPX Packet CRC DSAP SSAP CTRL Figure 7-17 The SNAP frame format DA (destination address) — The 6-byte destination MAC address. SA (source address) — The 6-byte source MAC address. LNH (length) — This field identifies the amount of data contained in the data payload field. DSAP (destination service access point) — This field identifies which service access points that the LLC information should be delivered to. SSAP (source service access point) — This field identifies the service access point that the data originated from. CTRL (control) — This field contains information used by the LLC on the receiving node that identifies the LLC frame type. SNAP-H (Sub-network Access Protocol27 header) — There are two subfields contained within this): VC (vendor code) — This identifies the vendor code of the source. ET (ether type) — This identifies the version of Ethernet being used. IPX Packet — This is the IPX datagram portion of the frame. The following subfields are part of the IPX packet: CS (checksum) — This field is normally not used. If it is used, then it is not compatible with the Novell proprietary format. PL (packet length) — The length of the IPX packet. TC (transport control) — The hop count (this is an incrementing field). PT (packet type) — Identifies the format of the data in the payload portion of the packet. DNN (destination network number) — The 4-byte destination network identifier. DHN (destination host number) — The 6-byte destination host identifier. DSN (destination socket number) — The 2-byte destination socket identifier. 27 SNAP is an extension of LLC. 311 Page 311 Edwards 312 Part I ■ c07.tex V3 - 03/27/2009 Networking Nuts and Bolts SNN (source network number) — The 4-byte source network number. SHN (source host number) — The 6-byte source host identifier. SSN (source socket number) — The 2-byte source socket identifier. Data (data) — The payload! CRC (cyclic redundancy check field) — This is a 4-byte value that is part of the frame check sequence (FCS), used to determine if a frame is intact at the receiving end. All of you Token Ring fans, don’t fret. IPX also can be encapsulated and transmitted on a Token Ring network. Figure 7-18 shows the format of the Token Ring frame. 1 1 1 6 6 S A F D C C DA SA 1 1 ARB — All routes broadcast 2 CTRL DSAP ACRONYM ALERT RIF and Data 4 1 1 FCS E F D S SSAP Figure 7-18 The IPX Token Ring frame format SD (start of frame delimiter) — This field lets the receiving node know when the frame begins. AC (access control) — There are four subfields in the access control field, all used to transmit information to the access control process within Token Ring. FC (frame control) — This field is used to separate network management data frames from user data frames. DA (destination address) — This field contains the 6-byte network address of the node the frame is destined for. SA (source address) — This field contains the 6-byte network address of the node the frame originated from. DSAP (destination service access point) — This field identifies which service access points28 the LLC information should be delivered to. SSAP (source service access point) — This field identifies the service access point that the data originated from. CTRL (control) — This field contains information that is used by the LLC on the receiving node that identifies the LLC frame type. 28 A service access point (SAP) is a label assigned to endpoints in a network. 11:19am Page 312 Edwards c07.tex Chapter 7 V3 - 03/27/2009 ■ 11:19am Not to Be Forgotten RIF (routing information) — This field assists in ensuring the Token Ring frame is sent in the correct direction. Data — The payload! FCS (frame check sequence) — This field is a checksum algorithm that checksums the frame from the FC field to the end of the Data field. ED (end of frame delimiter) — This field lets the receiving node know when the frame ends. FS (frame status) — This POP QUIZ field is used by the originating node True or false: IPX is not supported on a to detect whether Token Ring network. there were any errors during transmission. This includes if the destination node copied the data, if there were any errors encountered, and even if the destination node recognized itself as the destination node. 7.2.4 Point-to-Point Protocol The Point-to-Point Protocol (PPP) is really not a protocol at all; rather, it is a suite of protocols that work to allow IP data exchange over PPP links. Prior to the release of PPP, the standard that was being used for IP serial link transmission was the Serial Link Internet Protocol (SLIP). SLIP did a decent job of transmitting the IP data, but it wasn’t reliable, wasn’t secure, and really wasn’t able to support the performance demands of end users. Additionally, SLIP was used in LANs where the cabling wasn’t long at all — SLIP just couldn’t support communication over longer distances. PPP was developed to address these issues, as well as support serial communication for many network layer protocols, not just IP. To support the multiple protocol datagrams, PPP uses the following three main components: PPP encapsulation method PPP Link Control Protocol (LCP) PPP Network Control Protocol (NCP) 7.2.4.1 PPP Encapsulation Method PPP specifies a frame format that is to be used to encapsulate higher layer data. The format is based on the format used for the High-level Data Link Control (HDLC) protocol. HDLC is a synchronous Data Link layer protocol developed by the ISO and used as a reference for the PPP standard. 313 Page 313 Edwards 314 Part I ■ 7.2.4.2 c07.tex V3 - 03/27/2009 Networking Nuts and Bolts PPP Link Control Protocol LCP is the foundation protocol of the PPP protocol suite. It is the big kahuna in PPPland, supervising all the other protocols to ensure that they are performing the actions they are responsible for. LCP controls the PPP links. The processes involved in setting up and negotiating the rules for a link, managing the activity on the link, and closing the link when the data transmission is complete are all functions overseen by LCP. 7.2.4.3 PPP Network Control Protocol NCP is the control protocol that ensures the ACRONYM ALERT correct Layer 3 protocol is being used. NCP RFC — Request for Comments establishes which network layer protocol is required and then it sets the parameters needed to ensure that data can be recognized and understood at the endpoint. PPP supports multiple NCPs running on the same link, regardless of the type or which of the Layer 3 protocols is being supported. 7.2.4.4 Please, Tell Us More PPP has to set up a PPP link in order to communicate to the destination. The first node will test the link by sending an LCP frame. Once LCP has set up the link and all of the session parameters have been negotiated between the endpoints, NCP frames are then sent to set up and configure the parameters for the particular NCP type to be used. Once all these steps have occurred, packets can be sent. The link remains established until it is no longer needed or something external29 causes link failure. 7.2.4.5 PPP Frame Format We previously mentioned that PPP was designed based on the HDLC protocol. The frame format is the same for PPP and HDLC; however, PPP does not use all the fields. Therefore, some fields are set to a standard number for PPP.30 Figure 7-19 depicts the PPP frame format. Flag — The PPP Flag field is always set to binary 01111110. This field indicates the start point and end point of the frame. 29 In other words, PPP didn’t do it. reinvent the wheel? 30 Why 11:19am Page 314 Edwards c07.tex Chapter 7 V3 - 03/27/2009 ■ 11:19am Not to Be Forgotten BA (broadcast address) — This field is set to binary 11111111. CTRL (control) — This field is used by HDLC and is used for certain control parameters. The PPP control field is always set to binary 00000011. Protocol — This field identifies the protocol type for the information contained in the data payload. Data — The payload! FCS (frame check sequence) — This field is a checksum algorithm that checksums the frame from the FC field to the end of the Data field. Flag — The PPP Flag field is always set to binary 01111110. This field indicates the start point and end point of the frame. 1 1 1 2 Flag BA CTRL Protocol POP QUIZ What serial transmission standard was used before PPP came out? Data Up to 4 1 FCS Flag Figure 7-19 The PPP frame format 7.2.5 X.25 X.25 is a Network layer protocol standard that is maintained by the International Telecommunication Union – Telecommunication standardization sector (ITU-T). Used within packet-switched networks, X.25’s purpose in networking is to provide the rules on how connections between nodes are set up and maintained. X.25 protocols31 allow communication between different networks, regardless of what equipment and protocols they are running. Communication between the networks is actually handled through an intermediary (more on this in a little bit) at the Network layer. X.25 is a reliable connection-oriented standard of protocols. X.25 uses the following three main types of nodes (see Figure 7-20): Data terminal equipment (DTE) — Nodes that communicate on the X.25 network (these are the computers and nodes that connect the user to a network). Think of the DTE as the user nodes. 31 Did you notice the s? Yep — it’s a suite of protocols, not really a single protocol. 315 Page 315 Edwards 316 Part I ■ c07.tex V3 - 03/27/2009 Networking Nuts and Bolts Data circuit-terminating equipment (DCE)32 — A network access point (normally a modem or packet switch that is the interface to the cloud).33 Think of the DCE as the network nodes. Date switching exchange (DSE)34 — The nodes that are in the cloud. These nodes are responsible for passing data from DTE to DTE. DCE DTE DSE DCE DSE DSE DTE DSE DCE DTE DTE Figure 7-20 Deployments of the X.25 node types In X.25 data transmission operations, every DTE must have an association with a DCE. Don’t confuse DTE and DCE as being single standalone network nodes. DTE and DCE are actually the functions performed. As a matter of fact, a single node can provide multiple functions (for instance, a node can be both a DCE and a DSE. DCEs and DSEs are the nodes that route the packets through the cloud to a destination. Each and every packet that is transmitted may take a different 32 Also known as data communications equipment and data carrier equipment. is a term that defines the WAN infrastructure. Normally networks connect using a communication protocol (such as X.25). There is usually a switch that is the interface to the cloud. Once a packet hits the cloud, the provider is responsible for routing data to a destination. What goes on in the cloud stays with the cloud — meaning the endpoint networks don’t necessarily care how the provider is getting the data there, just as long as it gets there. 34 Also known as packet switching exchange (PSE). 33 Cloud 11:19am Page 316 Edwards c07.tex Chapter 7 V3 - 03/27/2009 ■ 11:19am Not to Be Forgotten path to get to the destination DCE and ultimately the destination DTE. Usually, the DTE connects to the DCE over some type of network, but two nodes can be connected directly. When there is a direct connection between nodes, then one of the nodes has to perform the functions of a DCE. The DTE is responsible for serving multiple sessions over a single connection to the DCE. RANDOM BONUS DEFINITION Each and every session first broadcast address — The well-known needs to be connected to the multicast address defining all nodes. DCE. Once the connections are established, the transmission of data can occur. Figure 7-21 is a basic diagram that depicts the session setup and processes. DCE 1 DCE 2 DTE 2 DTE 1 Figure 7-21 A basic X.25 network A session can be established in one of three ways (refer to Figure 7-21): The DTE can send a message to the DCE, letting the DCE know it has data to transmit. For instance, DTE 1 contacts DCE 1 and lets the DCE know it has data to transmit to DTE 2. This is known as a switched virtual circuit (SVC). A DCE can receive a message from another DCE, letting the DCE know that a DTE is requesting to send data to another DTE. For instance, DCE1 informs DCE 2 that DTE 1 wishes to pass data to DTE 2. The session can be left up at all times. In this scenario, as far as the DTEs are concerned, they can just pass the data to the destination DTE whenever they have data to send. No session setup is required. This is known as a permanent virtual circuit (PVC). 317 Page 317 Edwards 318 Part I ■ c07.tex V3 - 03/27/2009 Networking Nuts and Bolts THE X.25 PAD Some DTEs (for instance, dumb terminals) are not complex enough to understand full X.25 functionality. Therefore, they need a little assistance in communicating with the DCE. X.25 also supports a node type that performs just this function (helping the little guy out). The packet assembler/disassembler (PAD) is a node between the DCE and the DTE that is used to assemble packets, disassemble packets, and buffer data until the DTE is ready to receive. X.25 was developed and used before the OSI reference model was developed. To understand the protocol X.25, all you have to know is that (with only a few exceptions) operations can be mapped to the functions of the lower three levels (Physical, Data Link, and Network) of the OSI reference model. The three levels of the X.25 suite are as follows: 1. Physical level — This level corresponds to the OSI Physical layer. This includes defining all of the electrical and mechanical functions that are used by the physical medium. Some X.25 protocols operating at this level include: V.35 X.21bis RS232 2. Link level — This level corresponds to the OSI model’s Data Link layer. Functions that are performed at this level are the framing of packets, numbering packets, receipt acknowledgment, flow control, error detection, and recovery, etc. The X.25 protocol that operates at this level is Link Access Procedure, Balanced (LAPB). 3. Packet level — At this level, data is exchanged between X.25 nodes. The protocol that is used at this level is the Packet Layer Protocol (PLP). 7.2.5.1 RANDOM BONUS DEFINITION routing — The passing of data among various networks. X.25 Operations When an X.25 session is established, the session is assigned a virtual circuit number that is known to only the DTE and its associated DCE. The virtual circuit number is what is used to route the packets to the destination. The 11:19am Page 318 Edwards c07.tex Chapter 7 V3 - 03/27/2009 ■ 11:19am Not to Be Forgotten virtual circuit number is normally a shorted number, so the route lookup process is shorted (fewer bits and bytes to look at). The virtual circuit is nothing more than a path to a destination. A virtual circuit number reinforces the existence of a reliable path from one DTE to another DTE. As mentioned previously, there are two types of virtual circuits: switched virtual circuits (SVC) and permanent virtual circuits (PVC). The SVC is a circuit that is established as needed between DTEs. Each time a DTE needs to send data, the SVC will have to be set up before communication occurs and closed when the session terminates. The other type of virtual circuit, the PVC, is set up only once. It is used between DTEs that have a constant need to send data to other DTEs. Additionally, X.25 supports what is known as multiplexing, which means that it can carry multiple sessions over a single physical line. Each session would maintain ACRONYM ALERT its own virtual circuit, which will identify ATP — AppleTalk Transaction Protocol the destination DTE. Multiplexing is used when a single DTE has several processes that need to communicate with multiple destinations. Once data arrives at the destination, it is demultiplexed and sent to the appropriate DCE to be passed to the endpoint DTE. Figure 7-22 shows an example of how this works. Multiplexing De-Multiplexing Physical circuit Virtual circuits Figure 7-22 A multiplexing example 7.2.5.2 Link Access Procedure, Balanced The Link Access Procedure, Balanced (LAPB) is the X.25 Data Link layer protocol that ensures reliable, error-free packet framing and data communication management. LAPB employs the use of three message frame types: Information frame type — Frames of this type are known as I-frames. I-frames are used to pass upper layer data and some control data. I-frames perform packet sequencing, flow control, and error detection and recovery. 319 Page 319 Edwards 320 Part I ■ c07.tex V3 - 03/27/2009 Networking Nuts and Bolts Supervisory frame type — Frames of this type are known as S-frames. S-frames are used to pass control data, such as transmission requests, status reporting, I-frame receipt acknowledgements, and termination requests. Unnumbered frame type — Frames of this type are known as U-frames. U-frames are used to pass control data, such as session setup, error reporting, and session termination. LAPB frames include a header, the PLP data that is being passed to the other end, and a frame trailer. Figure 7-23 shows the format of the LAPB frame. Number of Bytes 1 1 1 Flag AD Ctrl Data 2 1 FCS Flag Figure 7-23 The LAPB frame format Flag — The LAPB Flag field indicates the start point and end point of the frame. AD (address) — This field identifies whether the frame is carrying a response or a command. CTRL (control) — This field details which frame type (I-frame, S-frame, or U-frame) is being used, the frame sequence number, and the frame function. Data — The payload! In LAPD, this is the PLP packet. FCS (frame check sequence) — This field is a checksum algorithm that checksums the frame from the FC field to the end of the Data field. This is where error checking and data integrity are monitored. Flag — The LAPB flag field indicates the start point and end point of the frame. 7.2.5.3 Packet Layer Protocol The Packet Layer Protocol (PLP) is the X.25 Network layer protocol that is used to direct the flow of packets between two DTE nodes over a virtual circuit. PLP can run in conjunction with other protocol standards (for instance, ISDN interfaces on a WAN or LLC within a LAN). There are five defined modes of operation for the PLP: Initial session setup mode — Used to set up an SVC or PVC between DTE nodes. 11:19am Page 320 Edwards c07.tex Chapter 7 V3 - 03/27/2009 ■ 11:19am Not to Be Forgotten Data transfer mode — Used to transfer data between DTEs. Idle mode — Used by SVCs to keep a session active when no data is being transmitted at the time. Session termination mode — Used to terminate a session and to clear the SVC. Re-initialization mode — Used to synchronize data transmission between a DTE and its associated DCE. 7.2.6 ACRONYM ALERT ISP — Internet service provider Asynchronous Transfer Mode Asynchronous Transfer Mode (ATM) is a standard maintained by the ITU-U. Its function is to pass fixed-size datagrams known as cells over an ATM network. ATM is a connection-oriented standard, which means the connection is up between nodes before data can be transmitted.35 Unlike pure packet-switched networks (IP, Ethernet, X.25, etc.), where the frames are of variable lengths, ATM provides cell-relay (transmission of data that is encapsulated into a fixed length cell) services on a packet-switched network. ATM uses nodes that are called ATM switches36 for the transfer of cells within a network. An ATM switch is not a switch in the Layer 2 meaning of the term. It is actually more like a router in functionality. 7.2.6.1 ATM Generic Cell Format ATM cells are a fixed 53 bytes in size (see Figure 7-24). The first portion of the cell is the header information and is 5 bytes long. The remaining 48 bytes are for the data payload. ATM cells are perfect for passing large amounts of data (streaming video, for example). The fixed length cells do not require the delays that can occur in synchronous data transmission because the variable length packets can cause long upload and download times. Asynchronous transmission, on the other hand, is a steady stream of cells. Number of Bytes 5 48 Header Data payload Figure 7-24 The ATM cell format 35 Repetition – repetition - repetition. nodes are tagged with the word switch by the marketing folks out there. It’s a buzzword that is often used to impress the customer base. 36 Often 321 Page 321 Edwards 322 Part I ■ 7.2.6.2 c07.tex V3 - 03/27/2009 Networking Nuts and Bolts An Overview of ATM Operations ATM is efficient and reliable. It offers transmission delay (there is no time lapse waiting for your turn), guaranteed to serve constant streams of data and patient enough to wait until data is ready to be passed. ATM networks contain nodes that are called ATM switches, as well as endpoint nodes that support ATM. ATM switches are responsible for passing data traffic to destination ATM switches and/or ATM endpoint nodes. Endpoint nodes are responsible for interfacing other network types to the ATM network. Examples of endpoint nodes include (see Figure 7-25): ATM channel service unit/data service unit (CSU/DSU) LAN router POP QUIZ Which protocol operates at the packet level of the X.25 model? LAN switch LAN workstation Router ATM switch ATM switch CSU/DSU LAN switch Figure 7-25 An ATM network 7.2.6.2.1 ATM: Virtual Paths, Circuits, and Channels Closely emulating the virtual circuit concept that is used in X.25, ATM uses what are known as virtual path identifiers (VPI) and virtual circuit identifiers (VCI)37 for the routing of cells in an ATM environment. The VPI/VCI pairing 37 Also known as a virtual channel identifier. A channel is basically the same thing as a circuit. 11:19am Page 322 Edwards c07.tex Chapter 7 V3 - 03/27/2009 ■ 11:19am Not to Be Forgotten is found in the ATM header and is used to map sessions that are active at any given time. The VPI is used by ATM switches to keep track of the paths to a destination. The backbone switches do not care about the VCI; it’s the interfacing nodes (nodes that are outside of the backbone) which include that in path definition. When a switch includes the VCI in its switching decisions, it considers the VPI/VCI pair as a single number. Different types of VPIs and VCIs are used in an ATM network: Virtual circuit types Permanent virtual circuit (PVC) — This is a static virtual circuit. Soft permanent virtual circuit (SPVC) — This is a dynamic PVC. Switched virtual circuit (SVC) — This is an ‘‘as needed’’38 virtual circuit. Virtual path types Permanent virtual path (PVP) — This is a static virtual path. Soft permanent virtual path (SPVP) — This is a dynamic PVP. The VPI and VCI sessions are identified in the header of the RANDOM BONUS DEFINITION ATM cell. THE VPI is a 12-bit identifier39 and the VCI is a end of frame delimiter — Used to indicate the end of the Data Link encapsulation. 16-bit identifier. Virtual circuits must be set up before any data transmission can occur. A virtual path is a group of virtual channels, which are bundled together and transmitted across the ATM network over a shared virtual path. Even though there may be multiple virtual circuits between ATM switches, the VPI and VCI pairing is used only by the endpoint nodes that are involved in the session (see Figure 7-26). Notice how this ATM multiplexing is very similar to the multiplexing processes in X.25. 7.2.6.2.2 ATM: Link Interface Types There are two primary types of link interfaces used in an ATM environment. The network-network interface (NNI) and the user-network interface (UNI). The UNI is the link that connects ATM endpoint nodes to an ATM switch. The NNI is the connection between ATM switches through the cloud. 38 This could also be ‘‘on demand.’’ bits of this can be used for generic flow control (GFC), when the communication is taking place between an endpoint node and an ATM switch. 39 Four 323 Page 323 Edwards 324 Part I ■ c07.tex V3 - 03/27/2009 Networking Nuts and Bolts Physical medium Virtual paths Virtual circuits Figure 7-26 ATM multiplexing Both interface types can be broken up into public UNIs and NNIs or private UNIs and NNIs. Private interface types are used to connect nodes within an ATM topology that is specific to their organization. The public interface types are used to connect nodes on a public network (available to everyone). 7.2.6.2.3 ATM Cell Header Format The format of the cell header that is used in the ATM cell is determined by the interface type being used. The UNI header (see Figure 7-27) is used for communication between an endpoint node and an ATM switch, while the NNI header (see Figure 7-28) is used for communication between ATM switches. Header GFC VPI Data payload VCI PT CLP HEC CLP HEC Figure 7-27 The UNI header format Header VPI Data payload VCI PT Figure 7-28 The NNI header format 11:19am Page 324 Edwards c07.tex Chapter 7 V3 - 03/27/2009 ■ 11:19am Not to Be Forgotten UNI header GFC (generic flow control; 4 bits) — Used to assist in identifying the nodes that are part of a shared ATM interface. VPI (virtual path identifier; 8 bits) — Used to identify the VPI portion of the VCI. VCI (virtual circuit identifier; 16 bits) — The circuit number used to associate the session’s virtual circuit. PT (payload type; 3 bits) — Identifies the data type in the data payload portion of the ATM cell. CLP (cell loss priority; 1 bit) — Often referred to as the discard bit, set by the sending node for cells that can be discarded if link congestion occurs. Also can be sent by nodes if there is a connection that is exceeding the bandwidth allotment for its session. HEC (header error control; 8 bits) — The checksum algorithm used for the information contained within the header only for error detection and control. NNI header VPI (virtual path identifier; 12 bits) — Used to identify the VPI portion of the VCI. VCI (virtual circuit identifier; 16 bits) — The circuit number that is used to associate the session’s virtual circuit. PT (payload type; 3 bits) — Identifies the data type in the data payload portion of the ATM cell. CLP (cell loss priority; 1 bit) — Often referred to as the discard bit, set by the sending node for cells that can be discarded if link congestion occurs. Also can be sent by nodes if there is a connection that is exceeding the bandwidth allotment for its session. HEC (header error control; 8 bits) — The checksum algorithm used for the information contained within the header only for error detection and control. 7.2.6.3 ATM Reference Model ATM is a protocol suite whose functions are described by a reference model. The ATM reference model uses layers that correspond to the Physical layer and a portion of the ACRONYM ALERT DRAM — Dynamic Random Access Memory 325 Page 325 Edwards 326 Part I ■ c07.tex V3 - 03/27/2009 Networking Nuts and Bolts Data Link layer of the OSI reference model. The layers that are part of the ATM reference model are as follows (see Figure 7-29): ATM adaptation layer (AAL) — Comparable to the functions of the OSI reference model’s Data Link layer. This layer is responsible for sorting higher layer data from the ATM processes. This layer combines its services with the service of the ATM layer. ATM layer — Comparable to the functions of the OSI reference model’s Data Link layer. This layer handles the relay of cells through the ATM environment. This layer is also responsible for cell multiplexing. Physical layer — Responsible for transmission of data on the medium. OSI Reference model POP QUIZ What are the three virtual circuit types used in ATM? ATM Reference model Application layer Presentation layer Higher layers Session layer Transport layer Network layer ATM adaptation layer (AAL) Data Link layer ATM layer Physical layer Physical layer Figure 7-29 A comparison of the OSI and ATM reference models 7.2.6.4 Traffic Management Several classes of service are defined for user data that is passed within an ATM network. These are as follows: Constant bit rate (CBR) — Data is passed constantly. The bandwidth required to pass the data is always available. 11:19am Page 326 Edwards c07.tex Chapter 7 V3 - 03/27/2009 ■ 11:19am Not to Be Forgotten Variable bit rate (VBR) — Data is passed often. The bandwidth required to pass the data is available, but there are limits on the amount of data that can be passed. The following two types of VBR are used: Variable bit rate real-time (VBR-rt) — This is used to pass real-time application data. Variable bit rate non-real-time (VBR-nrt) — This is used to temporarily store data in a queue when there is not enough available bandwidth to pass all of the data. It is used with applications that send data, but is not real-time. Available bit rate (ABR) — Data is passed when bandwidth is available. ABR supports congestion feedback so the sending node will know when there is too much congestion to pass data. Unspecified bit rate (UBR) — Data is passed if there is bandwidth available, and is dropped if there isn’t any available bandwidth. There are no guarantees about delivery. 7.2.6.5 ATM Adaptation Layer Types The AAL provides interface types that support the service class type that it is assigned to. The type of AAL to be used is determined by the sending node and the type announced when the initial call setup is sent. The AAL types are: AAL1 — Supports CBR transmissions. AAL2 — Supports VBR transmissions. AAL3/4 — Supports both connectionless and connection-oriented data transmission. This AAL type is used to transmit switched multimegabit data services (SMDS)40 packets. AAL5 — Supports both connectionless and connection-oriented data transmission. This AAL type is used to transmit non-SMDS packets. 40 SMDS RANDOM BONUS DEFINITION network layer — Layer 3 of the OSI reference model. is a connectionless telco service that supports various protocols and functions needed to transmit data over a high-performance packet-switched network. This protocol is outside of the scope of this book, so this footnote should provide all the information that you will need — a basic definition pertaining to the service. 327 Page 327 Edwards 328 Part I ■ c07.tex V3 - 03/27/2009 Networking Nuts and Bolts AN UNRELATED MOMENT OF PAUSE By now, we felt that you might be in need of a study break. To make your break a bit more enjoyable, here is a great peanut butter cookie recipe. Make a couple of batches to enjoy while you continue on with this book. If you are hyper-motivated, you can reread the section on X.25 while the cookies bake. That section is a good lead-in to the next section, ‘‘Frame Relay.’’ Ingredients: ■ 1 cup firmly packed brown sugar ■ 1/2 cup peanut butter ■ 1/2 cup softened butter ■ 1 tsp vanilla ■ 1 egg ■ 1 cup sugar ■ 1 1/2 cups flour ■ 1/2 tsp baking powder ■ 1/2 tsp baking soda ■ 1/2 tsp salt Preparation steps: 1. Preheat oven to 375◦ F. 2. Combine brown sugar, butter, and peanut butter in a large bowl. Beat on medium speed until well mixed. 3. Add egg and vanilla; continue beating until well mixed. 4. Reduce speed to low. 5. Add flour, baking powder, baking soda, and salt. Beat until well mixed. 6. Shape dough into 1-inch balls; roll in sugar. 7. Place the balls 2 inches apart onto ungreased cookie sheets; flatten balls in a crisscross pattern with fork dipped in sugar. 8. Bake for 8 to 10 minutes or until edges are lightly browned. Bon appétit! 7.2.7 Frame Relay Frame relay is a WAN protocol that operates as a packet-switched network. Like other packet-switched network protocols, frame relay uses the following: Multiplexing Variable length datagrams 11:19am Page 328 Edwards c07.tex Chapter 7 V3 - 03/27/2009 ■ 11:19am Not to Be Forgotten Frame relay is very similar to X.25, and is often considered the upgraded version of X.25. Because frame relay uses various WAN interface types (such as ISDN) to handle Layer 3 functions, and because communication media has improved, frame relay does not have to do the error checking and recovery that X.25 did. Because there is less chatter, frame relay is able to provide quicker and more reliable data transmission, which pretty much renders X.25 obsolete. Frame relay services operate at the Physical and Data Link layers of the OSI reference ACRONYM ALERT model. Originally designed to operate over CSMA/CD — Carrier Sense, Multiple Access with ISDN interfaces, it now supports transmisCollision Detection sion over broadband ISDN and ATM. 7.2.7.1 Frame Relay Node Types If you reread the section on X.25 while your cookies were baking, you will probably remember the X.25 node types are DTE, DCE, or DSE. In frame relay, you cut out the DSE and have the two node types that are used (see Figure 7-30): DTE DCE DCE DCE DTE DCE DTE Figure 7-30 DCE and DTE relationship in a frame relay environment DTE — Nodes that communicate on the frame relay network (these are the computers and endpoint nodes that connect the user to a network). Think of the DTE as the user nodes. 329 Page 329 Edwards 330 Part I ■ c07.tex V3 - 03/27/2009 Networking Nuts and Bolts DCE — These are the devices that are within the cloud that transports the data over a WAN. Because the DCEs in frame relay are able to handle the clocking and packet-switching services, there is no need for an intermediary device, like the DSE in X.25 7.2.7.2 Virtual Circuits . . . Again? Frame relay provides a connection-oriented service at the Data Link layer. Before data can be transmitted, the connection has to be up. The connection is associated with a unique data link connection identifier (see the next section). It is the DLCI that defines the virtual circuit between DTEs. Frame relay supports the multiplexing of virtual circuits to be established over a physical circuit. The frame relay virtual circuit types are: SVC — A temporary connection PVC — A permanent connection 7.2.7.3 Data Link Connection Identifier The identifier used to define a circuit is known as the data link connection identifier (DLCI). The DLCI is a value that is normally defined and assigned by the telco provider. The DLCIs are only important to the DTEs. The DCEs normally employ various methods and routes from circuit to circuit. In other words, the DLCI is what allows the data to be passed to the endpoint nodes outside of the cloud. POP QUIZ The DCEs make decisions based Frame relay is very similar to . on whatever technologies are in use by the telco. Because frame relay is a multiplexing WAN protocol, there can be multiple logical circuits passing data through the cloud over a single physical circuit. 7.2.7.4 Feckens and Beckens41 As much as we all may hate to admit it, network congestion occurs more often than we would like it to. It’s just a fact of life in a network. Fortunately, there are a lot of checks and balances in most networks that help to prevent errors and to detect and recover from them when they do occur. Within the frame relay cloud (the provider’s portion of the frame relay environment), there can be thousands upon thousands of transmissions passing 41 These are another pair of fun acronyms similar to catenet (although these are still in use). 11:19am Page 330 Edwards c07.tex Chapter 7 V3 - 03/27/2009 ■ 11:19am Not to Be Forgotten through from multiple organizational LANs. All of this data is passing through the same equipment to make its way through the cloud and to a destination. Because of all the end-user data passing through the nodes, congestion does occur. Frame relay has a couple of functions that help detect congestion and notify the DTEs that congestion is occurring. Additionally, the frame relay header provides an address field that reserves 1 bit for the FECN and one for the BECN. These functions are: Forward explicit congestion notification (FECN)42 bit — Within the address field of the frame relay frame header. Backward explicit congestion notification (BECN)43 bit — Within the address field of the frame relay frame header. In addition to the FECN bit and the BECN bit, there is also a bit that is used to indicate if the data is important or not. This field is known as the discard eligibility (DE) bit. If the DE bit is ‘‘set,’’ the DTE is notifying the DCEs that the frame is low priority and can be discarded if congestion is occurring. This gives the DCEs the capability to prioritize, dropping the data with less importance and only discarding the important data as a last option. The DTEs will retransmit the higher priority data if it gets notification from the DCEs that congestion is occurring. There are two additional bits in the frame relay frame header that can be set to notify a target node that there is congestion. BECNs are sent to the sending DCEs that there is ACRONYM ALERT congestion and FECNs are sent to the target FC — Frame control DCEs that there is congestion. Normally, the sending DCE will assume that there are problems if it receives so many BECNs in a certain time period (the number is set by the provider and the subscribing network). It will then cut down on the amount of data it is transmitting44 or will stop transmitting altogether. When the DTE stops seeing the BECNs, it will return to the way it normally performs. 42 Pronounced ‘‘fecken.’’ ‘‘becken.’’ 44 Normally, a frame relay provider will promise a minimum transmission rate for a virtual circuit. This is known as the committed information rate (CIR). Often, the provider will allow you to exceed the CIR and will try to pass the data on a best-effort basis. Should your edge router start seeing the BECNs repeatedly outside of the standards you have configured, the CIR should be checked and may need to be adjusted. It could be that multiple frames are being received by a router that has a lower CIR and cannot handle the level of traffic at the time (especially if all of the sending routers are exceeding the CIR). 43 Pronounced 331 Page 331 Edwards 332 Part I ■ 7.2.7.5 c07.tex V3 - 03/27/2009 Networking Nuts and Bolts Local Management Interface For the first few years that frame relay was in use, it didn’t really have any standards that ensured that the link was up between DTEs and DCEs. Several companies that were leaders in the networking and telecommunication fields banded together to come up with a signaling standard that would work with frame relay to assist in ensuring the link between a DTE and its associated DCE would remain up. What developed was an enhancement known as the local management interface (LMI). LMI is used to provide link status updates pertaining to PVCs between a DTE and the local DCE. One of the functions performed by LMI is status inquiries that are sent out periodically (normally 10 seconds) to test if a link is up. If the inquiry does not receive a reply, it assumes the link is down. These inquiries are known as keepalives. LMI also sends out updates pertaining to the status of all the links in frame relay network, provides information about PVC changes, and ensures that IP multicast is functioning. 7.2.7.6 Frame Relay Frame Format The standard frame relay frame format is also known as the LMI version of the frame relay frame. Figure 7-31 shows the fields contained within the frame relay frame. Number of Bytes 1 2 Flag AD Data 2 1 FCS Flag Figure 7-31 Frame Relay frame format Flag — The frame relay Flag field indicates the start point and end point of the frame. AD (address) — Included in this field is information pertaining to the DLCI. There are also 3 bits that are included in this field that are for the FECN, BECN, and the DE bit. Data — The payload! FCS (frame check sequence) — This field is a checksum algorithm that checksums the frame from the FC field to the end of the Data field. This is where error checking and data integrity are monitored. Flag — The frame relay Flag field indicates the start point and end point of the frame. 11:19am Page 332 Edwards c07.tex Chapter 7 7.2.8 V3 - 03/27/2009 ■ 11:19am Not to Be Forgotten Integrated Services Digital Network Integrated Services Digital Network (ISDN) is a data transport service that can be used over regular existing telephone lines. The ISDN service enables the telephone line to be digitized, allowing multiple data types to be passed over existing telephone lines. Additionally, ISDN can be used with digital telephone lines. ISDN is a baseband transmission standard, used to operate over normal copper lines. Broadband ISDN (B-ISDN) was designed to be faster and more reliable than ISDN. B-ISDN operates over fiber optics. As fiber optics are being rolled into more and more residences and businesses, many ISDN users are using the broadband service. ISDN provides two types of channels to be used for communication in the ISDN environment, the B channel and the D channel. The B channel is used to carry user data, whereas the D channel is used for signaling between the end user and the ISDN network. The B channel operates at 64 kbps, and the D channel operates between 16 and 64 kbps, depending on the interface rate standard that is being used. 7.2.8.1 Basic Rate Interface and Primary Rate Interface The following two services are used in ISDN to determine bandwidth availability between a source and a destination: Basic rate interface (BRI) Primary rate interface (PRI) The BRI service uses two B channels and one D channel.45 RANDOM BONUS DEFINITION Each B channel operates at 16 modem — A node used to pass data kbps. The BRI D channel opercommunication over an analog ates at 16 kbps as well. The PRI communications channel. service uses 23 B channels46 and one D channel.47 Each B channel operates at 16 kbps, whereas the PRI D channel operates at 64 kbps. 7.2.8.2 ISDN Nodes Several node types are used in an ISDN environment. Terminals are a node type that can be either an ISDN terminal type, known as a terminal equipment 45 This is referred to as 2B+D. in the United States and in Japan includes 23 B channels. Other parts of the world include 30 B channels. 47 This is referred to as 23B+D. 46 PRI 333 Page 333 Edwards 334 Part I ■ c07.tex V3 - 03/27/2009 Networking Nuts and Bolts type 1 (TE1), or a non-ISDN terminal, known as a terminal equipment type 2 (TE2). The next type of node is called a terminal adaptor (TA), which is used to interface a TE2 with the ISDN network. The next type of node is called a network termination device type 1 (NT1) and network termination device type 2 (NT2) (or a combination of both). Most ISDN networks will use the NT1. 7.2.8.3 The ISDN Reference Model ISDN standards span the first three layers of the OSI reference model. At the Physical layer, two different types of frames are used. Which one is used depends on whether the data is flowing from the user node (the terminal) to the ISDN network (TE frame) or from the network to the terminal (NT frame). Figure 7-32 shows the format of the TE frame, and Figure 7-33 shows the format of the NT frame. F L B1 EDA F F B2 EDS B1 EDS B2 LDL B1 LDL B2 = 1 bit = 1 byte Figure 7-32 The TE frame format F L B1 LDL F L B2 = 1 bit = 1 byte Figure 7-33 The NT frame format 11:19am Page 334 Edwards c07.tex Chapter 7 V3 - 03/27/2009 ■ 11:19am Not to Be Forgotten F — Framing bit, marks the beginning of the frame for synchronization. L — Load balancing bit. These are used to balance the frames signaling. B1 — B1 channel byte. This is B channel data. E — Echo bit. Echoes D channel data when line congestion is occurring. D — D channel bit. This is D channel data. A — Activation bit. Used to activate nodes. B2 — B2 channel byte. This is B channel data. S — Spare bit. F — Framing bit. When used, marks the beginning of the frame for synchronization. L — Load balancing bit. These are used when needed to balance the frames signaling. B1 — B1 channel byte. This is B channel data. D — D channel bit. This is D channel data. B2 — B2 channel byte. This is B channel data. S — Spare bit. The Layer 2 protocol used by ISDN is called the link access procedure D channel (LAPD), which functions like LAPB does for the X.25 protocol. Figure 7-34 shows the LAPD frame format. Number of Bytes 1 2 1 Flag AD Ctrl Data 1 1 FCS Flag Figure 7-34 The LAPD frame format Flag — The LAPD Flag field indicates the start point and end point of the frame. AD (address) — This field identifies whether the frame is carrying a response or a command. CTRL (control) — This field details which frame type (I-frame, S-frame, or U-frame) is being used, the frame sequence number, and the frame function. Data — The payload! In LAPD, this is the PLP packet. FCS (frame check sequence) — This field is a checksum algorithm that checksums the frame from the FC field to the end of the Data field. This is where error checking and data integrity are monitored. 335 Page 335 Edwards 336 Part I ■ c07.tex V3 - 03/27/2009 Networking Nuts and Bolts Flag — The LAPD Flag field indicates the start point and end point of the frame. Finally, two Layer 3 protocols are used by ISDN: ITU-T and ITU-T I.451. These proPOP QUIZ tocols take care of operations What are the four endpoint node types used at Layer 3, including setting in ATM? up sessions, establishing and maintaining connections, gathering information pertaining to remote nodes, and other functions. 7.2.9 AppleTalk AppleTalk is a protocol suite developed by the Apple Computer company to be integrated with Macintosh computers to allow users to share resources on a network. AppleTalk came into existence in the 1980s and was one of the first to implement the client/server network architecture. AppleTalk is a plug-and-play service that doesn’t require any intervention on the end user’s part to connect to a network. The first version of AppleTalk, known as AppleTalk Phase 1, was developed mainly for use in a local network segment. It was able to support a maximum of 135 client nodes and 135 server nodes. AppleTalk Phase 2 was developed to support routing outside of the local segment and could support a total of 253 nodes, regardless of whether they were clients or servers. The services provided and/or supported by AppleTalk span all the layers in the OSI reference model. Figure 7-35 compares ACRONYM ALERT the OSI reference model and the AppleTalk CIST — Common and internal spanning tree protocols that correspond to each layer. 7.2.9.1 AppleTalk Physical and Data Link Layers AppleTalk depends on the same media access protocols to exchange networking data. Each implementation has to work with the AppleTalk suite. At the Physical layer, AppleTalk data can be passed over fiber, twisted pair, and coaxial cabling. AppleTalk interacts with each implementation of a media access protocol to allow AppleTalk data to be exchanged. Following are some of the protocols used at this layer: EtherTalk — Used on Ethernet networks. The protocol that communicates between the network layer and the Physical layer is known as the EtherTalk Link Access Protocol (ELAP). 11:19am Page 336 Edwards c07.tex Chapter 7 V3 - 03/27/2009 ■ 11:19am Not to Be Forgotten TokenTalk — Used on Token Ring networks. The protocol that communicates between the Network layer and the Physical layer is known as the TokenTalk Link Access Protocol (TLAP). FDDITalk — Used on FDDI networks. The protocol that communicates between the Network layer and the Physical layer is known as the FDDITalk Link Access Protocol (FLAP). LocalTalk — This is the AppleTalk proprietary standard that is included with all Macintosh computers. This standard is supported on Macintosh nodes only. The protocol that communicates between the Network layer and the Physical layer is known as the LocalTalk Link Access Protocol (LLAP). OSI Reference Model AppleTalk Model Application AppleTalk Filing Protocol Presentation Session Printer Access Protocol AppleTalk Session Protocol AppleTalk Datastream Protocol Transport Name Binding Protocol AppleTalk Echo Protocol Routing Table Maintenance Protocol Datagram Delivery Protocol Network Data Link TokenTalk Link Access Protocol EtherTalk Link Access Protocol LocalTalk Link Access Protocol Physical Token Ring Ethernet LocalTalk Figure 7-35 The layers of the AppleTalk model 7.2.9.2 AppleTalk Network Layer The Datagram Delivery Protocol (DDP) is the protocol used by AppleTalk at the Network layer. The purpose of DDP in an AppleTalk infrastructure is to provide end-to-end datagram delivery. DDP uses sockets to identify a logical process on a node and as part of the address that is used in order to exchange datagrams. All the upper layers use sockets as well. 337 Page 337 Edwards 338 Part I ■ c07.tex V3 - 03/27/2009 Networking Nuts and Bolts All AppleTalk data is formatted to be exchanged in DDP packets over an AppleTalk network. DDP has two different packet types. The short DDP packet type is not used much anymore. It was developed when AppleTalk was limited to segments only. The extended DDP packet type is what is most commonly used.48 Another protocol used at this layer is the AppleTalk Address Resolution Protocol (AARP). Just RANDOM BONUS DEFINITION like the Address Resolution ProLayer 3 switch — A router. tocol (ARP) for TCP/IP, AARP maps network addresses to their associated data link addresses. 7.2.9.3 AppleTalk Upper Layers AppleTalk uses several upper layer protocols that were built off of the DDP protocol and therefore use DDP as the protocol of choice when information is being passed down to the lower layers for transport across the network. Transport layer protocols are used for flow control, circuit management, and error checking, detection, and recovery. The AppleTalk protocols included at this layer are: AppleTalk Echo Protocol (AEP) — The service provided by this protocol is an echo request or an echo reply. AppleTalk Transaction Protocol (ATP) — Used to pass transmissions between two sockets. Name Binding Protocol (NBP) — Maintains and manages the use of host names and socket addresses for nodes within the network. Routing Table Maintenance Protocol (RTMP) — Used to maintain and manage routing information. Session layer protocols manage communication sessions between Presentation layer processes. The protocols operating at this layer are: AppleTalk DataStream Protocol (ADSP) — A connection-oriented protocol that provides a data channel for the host nodes. AppleTalk Session Protocol (ASP) — Maintains and manages higher level sessions. Printer Access Protocol (PAP) — Maintains and manages virtual connections to printers, print servers, and other server types. 48 The extended DDP packet is the one most commonly used in new implementations. There is really no good reason to use the short DDP packet, as you need to plan for growth and that packet type limits where your data can be transmitted. 11:19am Page 338 Edwards c07.tex Chapter 7 V3 - 03/27/2009 ■ 11:19am Not to Be Forgotten Zone Information Protocol (ZIP) — Manages network numbers and AppleTalk zone names. The final two layers, the Application and Presentation RANDOM BONUS DEFINITION layers, use the services of the internetwork — A group of networks AppleTalk Filing Protocol (AFP)49 . connected to one another through a router. The Presentation layer provides services that are applied to data at the Application layer. Additionally, the Application layer interacts with Macintosh applications (which the OSI Application layer does not). 7.3 Chapter Exercises 1. True or false: The only type of node that is used on a FDDI ring is a FDDI concentrator. 2. What are the three levels of operation within the X.25 protocol suite? 3. In X.25, are used to pass control data, such as: transmission requests, status reporting, receipt acknowledgements, and termination requests. 4. What are the three main components used by PPP? 5. What is the difference between a DTE and a DCE in an X.25 network? 6. What are the Session layer protocols that are used in the AppleTalk protocol suite? 7. What does the acronym ISDN stand for? 8. What is the frame relay local management interface (LMI) used for? 9. What is a constant bit rate (CBR)? 10. 7.4 is the foundation protocol of the PPP protocol suite. Pop Quiz Answers 1. What was the name of the company that developed ARCnet? The Datapoint Corporation developed ARCnet in the late 1970s. 49 AFP is a file sharing protocol. 339 Page 339 Edwards 340 Part I ■ c07.tex V3 - 03/27/2009 Networking Nuts and Bolts 2. What technology is also known as 1BASE5? StarLAN 3. What is the signal called that is passed in Token Ring from one node to the next? A token 4. What information is contained in the Destination Address field in a Token Ring frame? The Destination Address field contains the 6-byte network address of the node that the frame is destined for. 5. What does the acronym FDDI stand for? Fiber Distributed Data Interface 6. What are the four main node types in the FDDI environment? Single attached station Single attached concentrator Dual attached station Dual attached concentrator 7. What are DECnet’s five phases? DECnet phase I DECnet phase II DECnet phase III DECnet phase IV DECnet phase V 8. Which operating system uses IPX? Novell NetWare 9. True or false: IPX is not supported on a Token Ring network. False 10. What serial transmission standard was used before PPP came out? Serial Link Internet Protocol (SLIP) 11. Which protocol operates at the packet level of the X.25 model? Packet Layer Protocol (PLP) 12. What are the three virtual circuit types used in ATM? Permanent virtual circuit (PVC) — This is a static virtual circuit. Soft permanent virtual circuit (SPVC) — This is a dynamic PVC. 11:19am Page 340 Edwards c07.tex Chapter 7 V3 - 03/27/2009 ■ Not to Be Forgotten Switched virtual circuit (SVC) — This is an ‘‘as needed’’50 virtual circuit. 13. Frame relay is very similar to . X.25 14. What are the four endpoint node types used in ATM? ATM customer service unit/digital service unit (CSU/DSU) LAN router LAN switch LAN workstation 50 This could also be ‘‘on demand.’’ 11:19am 341 Page 341 Edwards c07.tex V3 - 03/27/2009 11:19am Page 342 Edwards p02.tex V3 - 03/27/2009 12:41pm Part II The OSI Layers In This Part Chapter Chapter Chapter Chapter 8: The Upper Layers 9: The Transport Layer 10: The Network Layer 11: The Data Link Layer Page 343 Edwards p02.tex V3 - 03/27/2009 12:41pm Page 344 Edwards c08.tex V3 - 03/27/2009 11:22am CHAPTER 8 The Upper Layers Protocol is everything. Francois Giuliani1 The above quote is truly succinct, a real economy of words. This quote is not only true at the United Nations but also is easily applied to the networking environment. When you think of the mix of various equipment, wiring, networking operating systems, computer operating systems, programs running on servers as multiuser platforms, programs running on local computer workstations (which includes pretty much anything a person can hang off a network segment), the ability to communicate is essential. The United Nations uses translators to ensure that all the representatives from the many varied nations can understand the procedures. A network protocol also acts as a translator between the many subcomponents that we lump together under the word ‘‘network.’’ We would hate to think what a General Assembly meeting of the United Nations would look and sound like without the translators they employ. There is only one word that comes to mind: chaos. How would you ever be able to get anything done? The same goes for networks, except things move much faster than the world’s fastest talker can utter even a single word. So protocol is truly everything in the networking world. 1 Francois Giuliani worked at the United Nations for 25 years. At the time of his departure in March 1996, he was the director of the Media Division of the Department of Public Information (DPI). 345 Page 345 Edwards 346 Part II ■ c08.tex V3 - 03/27/2009 The OSI Layers This chapter investigates the RANDOM BONUS DEFINITION upper layers of the OSI reference model: the Application layer, hardware address — Synonymous with MAC the Presentation layer, and the address, physical address, and unicast Session layer. We will idenaddress. tify the ‘‘translators’’ being used so that information can flow smoothly and without error between these layers and eventually be sent over the network media to another network node and the device servicing that node. This is a top-down approach where users attempt to interact with the device they are using to communicate with another device and/or users somewhere over the net.2 8.1 Background Software programs use the upper layers of the OSI reference model to send and receive data over a network. Normally such programs are called applications and although they may interface with the Application layer of the OSI reference model, it does not necessarily need to be the case. In this chapter, ‘‘application program’’ and ‘‘Application layer’’ are not synonymous and refer to different aspects of computer usage. A computer user purchases an application program and loads it on to his or her computer’s hard drive. Basically, programs can be divided into two broad categories: locally run application programs and client/server-based application programs. As the name implies, a locally run application program executes program instructions and all data is maintained within the local computer, so there is never a need to utilize a network connection. A client/server application implies that a client computer and a server need to communicate if the application program is to run successfully. A client/server application in most cases requires a degree of interconnectivity for the application program to communicate with its counterpart server-based program. As this book is concerned with networking, the only application programs that have relevance are application programs that follow the client/server model. Figure 8-1 illustrates a client/server application program scenario. As you can see in the figure, a client computer communicates over the network with a server. Although they are working in conjunction within a certain application program, they run within their own realms. The server listens on the network, awaiting requests from client computers. When the server receives a request from a client, it fulfills it. The communication between a particular client computer and the server is considered a session. Servers only respond to 2 The ‘‘net’’ is in reference to any and all segments of a network, which can include in part or in whole any of the following: local network segment, the local LAN, intranet, or the Internet. 11:22am Page 346 Edwards c08.tex V3 - 03/27/2009 Chapter 8 ■ 11:22am The Upper Layers session requests in this environPOP QUIZ ment; they do not initiate the start of session. Once a data True or false: The Application layer is transfer to or from the server is where all the application programs you complete, it may request to terload on your PC are stored. minate the session. Depending on the server application being run on the server, the server may be capable of maintaining a number of simultaneous sessions with multiple client computers. Server applications that can maintain multiple sessions are usually referred to as multiuser applications. Server Realm Client Realm Client Application Program Server Application Program Network Protocol Stack Network Protocol Stack Network Physical Layer Figure 8-1 A client/server application The client realm involves not only the client computer and application program, but a user as well. The user initiates requests to the client computer via an input device (usually a keyboard, mouse, or both). The application responds back to the user in graphic images or text displayed on a screen or tone signals played back through the computer’s audio system. The application program requires user input in the form of commands and data in order for it to interact with the server application it is working in conjunction with in a particular client/server application. 347 Page 347 Edwards 348 Part II ■ c08.tex V3 - 03/27/2009 The OSI Layers Although client/server applications work in conjunction with each other, they ACRONYM ALERT are autonomous until a session is established between a particular client applicaAARP — AppleTalk Address Resolution Protocol tion workstation and the application server. The server application, in most cases, is constantly running on a server that is rarely shut down. For instance, a mail server is always available to receive messages from client workstations, process them, and direct them to another mail server where the recipient of that message has an account. Received messages from other mail servers destined for users on a particular mail server are stored on the server until the mail server is queried by a user to see if there are any messages. Mail servers or other application servers may also have to perform user authentication to ensure security and user privacy. An example of this would be when users launch a particular application on their client workstation, such as a mail reader. They may be first presented with a dialog message box to enter their user ID and password. Unbeknownst to the users, when they launched the client application it went out over the network and requested to establish a session with the server. The server at that point returned a response that security is required and requested that a user ID and password be provided for the connection to be established and maintained over the length of the session. Users at the client workstation enter their user ID and password, and if it matches the authentication parameters that the mail server is using for authentication, a mail session is opened between the client workstation and the mail server. The simple process of just logging on to a mail server requires interaction of the application program and the network stack3 to ensure that messages are properly transmitted over the network between the client workstation and the server within a predetermined protocol. Since TCP/IP (Transmission Control Protocol/Internet ProPOP QUIZ tocol) is the predominant network protocol in use within The predominant networking protocol run today’s networking world, the . over Ethernet networks is remainder of this chapter will refer to the network stack in terms of how it relates to the TCP/IP protocol suite. Most, if not all, of today’s computer operating systems provide a network stack that is compatible and easily interacts with applications that use TCP/IP to communicate over a network. 3 Usually in reference to the OSI model, ‘‘network stack’’ or simply the ‘‘stack’’ refers to layers within the OSI reference model that, in most cases, have been embedded within the particular operating system running on the computer in use. 11:22am Page 348 Edwards c08.tex Chapter 8 8.2 V3 - 03/27/2009 ■ 11:22am The Upper Layers The TCP/IP Model The TCP/IP model consists of four layers: an Application layer, a Transport layer, an Internet layer, and a Link layer. To accommodate a wide range of application programs that need to communicate over a network structure, encapsulation is performed between the layers to allow data to be moved independently of the application that produced the data. Figure 8-2 illustrates a conceptual view of the TCP/IP network stack. Layer UDP Header IP Header Frame Header Data Application UDP Data Transport Internet IP Data Frame Data Frame Footer Link Figure 8-2 The TCP/IP network stack/model The top level Application layer is the data portion of the network stack. It contains the upper level protocols that allow application programs to encapsulate data so that it can be passed down to the Transport layer. Since the OSI model Presentation layer and Session layer are combined with the OSI model Application layer to make up the TCP/IP network stack’s Application layer, any protocols needed within the OSI model for these layers are accomplished via the use of libraries4 within the TCP/IP model’s Application layer. The TCP/IP model Transport layer maps directly to the Layer 4 Transport layer of the OSI model, and the TCP/IP model Internet layer is usually mapped directly to the OSI model’s Network layer. However, the TCP/IP model’s Link layer covers both the OSI model’s Physical layer and Data Link layer. Application layer data is passed to the Transport layer, where a UDP header is applied and is framed with the data, as shown in Figure 8-3. 4 Libraries are collections of protocol routines for various protocol functions. 349 Page 349 Edwards 350 Part II ■ c08.tex V3 - 03/27/2009 The OSI Layers UDP Header Source Port (2 bytes) Destination Port (2 bytes) UDP Packet Length (2 bytes) UDP Data UDP Packet Checksum (2 bytes) Data Figure 8-3 A UDP packet As you can see, there is no address information other than the ports that that are being accessed. Since there is a lack of addressing and control, UDP is referred to as a connectionless protocol.5 With 2 bytes allocated for both the source and destination port addresses, this accommodates up to 65,536 port numbers. However, the lower 1,024 port address values are reserved for defined services and are considered to be the well-known port values.6 The UDP Packet Length field is 2 bytes in length and contains POP QUIZ the number of bytes of the whole packet, including header and True or false: UDP is a connection-based data. The UDP Packet Checkprotocol. sum field is also 2 bytes in length and is the checksum of the whole packet, including header and data. Unlike TCP, the Checksum field is optional, which brings into question its use for packet transport over the network. The choice between using UDP and TCP depends on the transport mode selected by the application program developers. A deciding factor may be speed, since UDP does not require further encapsulation and the overall packet size is smaller than TCP by 12 bytes. On a single packet basis, this seems like a small price to pay; however, in applications where large amounts of data are transferred over the network, there can be noticeable performance differences. A software developer may choose not to use UDP where reliability of the transfer is required. UDP has no means of guaranteeing packet delivery. To guarantee delivery requires further encapsulation and the packet is then passed to the Internet layer of the TCP/IP network stack. 5A connectionless protocol means that packets are streamed onto the network without any relation to one another. There is no means to connect packets that may have been fragmented or to determine if packets have been received out of order. 6 Well-known port addresses are reserved; however, the range above 1024 also has some predetermined services using a high-numbered port. An example would be radius server authentication using port 1812. 11:22am Page 350 Edwards c08.tex V3 - 03/27/2009 Chapter 8 ■ 11:22am The Upper Layers At the Internet layer, the UDP packet is encapsulated as data within the IP packet. Figure 8-4 illustrates the applied IP header. Bit 0-3 4-7 8-15 0 Version Header Length Type of Service 32 64 Identification Time to Live 16-18 19-31 Total Length Flags Protocol Fragment Offset Header Checksum 96 Source Address 128 Destination Address 160 Options 160 or 192 + Data Figure 8-4 The IP packet header You can see that additional information is added to the packet that can affect its delivery over the network. The bit order of the packet delivery begins with bit position 0. Streaming from left to right across the header, the first field encountered is the Version field. Since this packet complies with IP version 4 (IPv4), the value contained in this field is 4.7 The next field is the Header Length of the IP header. The value contained in this field is the number of 32-bit words that are contained in the header. This value also indicates the bit position of where the Data field begins. The minimum value for this field is 5. So, in a header containing five 32-bit words, the start of data will begin at bit position 160 (5 × 32 bits = 160 bits). The beginning of the Data field will be pushed back an additional 32 bits if the Options field is present. The Type of Service field was allocated to provide control over the packet’s delivery priority. In the past, this field was not utilized; in recent days, 7 Because this is a 4-bit binary field, the value in binary 4-bit notation would appear as 0100. 351 Page 351 Edwards 352 Part II ■ c08.tex V3 - 03/27/2009 The OSI Layers it has evolved into a Differentiated Services field (DiffServ). RANDOM BONUS DEFINITION DiffServ provides a method flow control — A function that prevents a of classifying network traffic sender of traffic from sending faster than for manageability and provides the receiver is capable of receiving. quality of service (QoS) guarantees across an IP network. This ability is essential for delivering time-sensitive packets for applications that require real-time performance. An example of a real-time application in wide use today is Voice over IP (VoIP). The Total Length field contains the value in the number of bytes of the total length of the IP packet datagram, which also includes the header. The minimum value this field can contain is 20, which is the minimum number of bytes in an IP header without any data. Since this is a 16-bit field, the maximum amount of bytes in the datagram is restricted to a theoretical limit of 65,536 bytes. However, most networks do not permit the transfer of super-sized packets without fragmentation. The customary size restriction for TCP/IP on an Ethernet network is 1500 bytes. Larger packets would need to be fragmented and delivered reliably so they can be reconstructed on the receiving network node. The next three fields, Identification, Flags, and Fragment Offset, are all used when fragmentation of a packet is required. A packet that is too large is broken into fragments, which are placed within a collection of packets to transfer the information within the original unfragmented packet. The Identification field is used to uniquely identify all IP packets that are fragments of a packet that needed to be fragmented before being placed on the network. The Flags field consists of 3 bits. The value of each field may either be a 0 or a 1, where 0 indicates ‘‘no flag’’ being present and 1 indicates ‘‘flag bit set.’’ In order of precedence, the most significant bit is reserved and always must be set to 1. The next bit is the do-not-fragment bit. When set, this bit signals that the packet is not to be fragmented. This can lead to packets being dropped if they exceed the overall packet size permitted by a receiving node. The only reason for use of the do-not-fragment flag is that the network node sending the packet knows that the network node that is to receive the packet does not have the capability to reassemble fragmented packets and sets the flag so upstream routers will not fragment the packet. The next flag bit is the more-frames bit, which indicates that more fragment packets are to follow this particular packet. The last packet containing a packet fragment segment will have this bit set to 0 to indicate that no other fragments are to follow this fragment. This bit is always set to 0 for all packets that don’t contain fragmented packet segments.8 8 If a packet does not contain fragmented packet segments, it is a packet unto itself and is considered an unfragmented packet. Whether to fragment a packet is determined by the amount of data that is be transmitted, since the header is for the most part of fixed length. 11:22am Page 352 Edwards c08.tex Chapter 8 V3 - 03/27/2009 ■ 11:22am The Upper Layers The Fragmentation Offset field contains the number of 8-byte blocks that the fragment data is offset from where it was located in the original unfragmented packet. The field is 13 bits long, so the maximum number of offset is 65,528.9 Since the maximum packet size is fixed at 65,536, the values of the offset, plus the 20 bytes required for the IP header, is greater than the maximum size of a packet. Thirteen bytes are more than adequate for this field. The Time to Live field is an 8-bit field that indicates how many seconds a packet can live on the Internet. With that many bits, it would equate to 255 seconds as a maximum or four and a quarter minutes. Imagine waiting more than four minutes per packet to see if they had arrived. Needless to say, the reason for the TTL timer is to prevent lost packets from traversing the Internet into infinity if they cannot find a home or until they end up being dropped somewhere along the way. These days this field is not used to display the amount of seconds but is a hop count.10 As a packet travels across the Internet, each network forwarding device it passes through decrements the TTL field by one before forwarding the packet along to the next network hop. The packet will continue to travel until the packet with a TTL set to zero arrives at the input of a network forwarding device. When a packet with TTL equal to zero is received by a network forwarding device, it will simply not forward the packet and it is dropped.11 When a packet is dropped, an ICMP (Internet Control Message Protocol) error is sent to the sender alerting it that the packet has been dropped. The typical message is that the TTL has been exceeded, which means the destination was not found. ICMP utilities include ping and traceroute and use error messages to allow a sender to know if a target address is reachable over the Internet. The Protocol field is an 8-bit field used to indicate the protocol of the data portion of the IP packet. These are pre-assigned ACRONYM ALERT values maintained by the Internet Assigned ATM — Asynchronous Transfer Mode Numbers Authority (IANA). Some of the most common protocols found in IP headers are a value of 1 for ICMP messages, a value of 6 for TCP messages, and a value of 17 for UDP messages. The Header Checksum field is a 16-bit field that contains the checksum of the header portion of the IP packet. The data portion carries the checksum of the protocol that is contained within it. When the packet is received, the checksum is calculated and compared to the value contained within the field. If values is derived by (213 –1) × 8 bytes per block, or 65,528 bytes. count is a method of counting the hops a packet traverses. As a packet is passed through a network forwarding device (e.g., a router), it is considered as a single hop. 11 What is meant by ‘‘dropped’’? Simply that the packet is ignored and not forwarded or analyzed any further. It just ends up in the sky, where all lost packets go. However, network administrators always like to know why a packet is dropped. 9 This 10 Hop 353 Page 353 Edwards 354 Part II ■ c08.tex V3 - 03/27/2009 The OSI Layers there is a checksum mismatch, the packet is dropped. Since the header includes the Time to Live field, which is decremented each time the packet crosses a network hop, the header checksum will need to change if it is to remain valid at the next receiving network node. Because of known decrementing of the TTL field and the possibility that a network forwarding device may fragment the packet before passing it to the next network hop, each network forwarding device must insert the new valid checksum value in order to not create a checksum mismatch at the next receiving network node. The Source Address field contains 32 bits of address information. The address is represented as four octets. Normally, IP addresses are annotated in what is called dot-decimal notation, such as: 192.168.16.1 Converting each octet into binary is represented as follows: 11000000.10101000.00010000.00000001 Binary address information in the Source Address field is represented as follows: 11000000101010000001000000000001 There are times when the source address of a packet is not the address of the sending network node. Various packet-forwarding network devices can perform a NAT function. Figure 8-5 illustrates a user workstation behind a router that is providing a NAT function. Source Address 74.123.17.33 Destination Address 38.214.37.10 Source Address 192.168.1.28 Destination Address 38.214.37.10 Internet 74.123.17.33 192.168.1.28 NAT Router 192.168.1.1 Source Address 74.123.17.33 Destination Address 38.214.37.10 Network Address 192.168.1.0 Figure 8-5 A private network behind a NAT router Web Server 38.214.37.10 11:22am Page 354 Edwards c08.tex Chapter 8 V3 - 03/27/2009 ■ 11:22am The Upper Layers In the figure, there is a private network12 with a network address of 192.168.1.0, and on that network is a router with NAT capability of taking packets from a device on the 192.168.1.0 network and routing them out to the Internet. A user workstation at 192.168.1.28 wants to access a web page from a web server over the Internet at 38.214.37.10. Since the NAT router is the default gateway for the 192.168.1.0 network, all traffic that is not destined for the local LAN is sent to it. The user workstation in its TCP/IP settings has the default gateway address of 192.168.1.1, which is the NAT router’s local network interface. The user workstation sends a request packet with a destination address of 38.214.37.10 with its own address of 192.168.1.28 in the Source Address field. Since the destination address is not on the local LAN, it is sent to the default gateway at 192.168.1.1. The NAT router accepts the packet from the workstation at 192.168.1.28 and determines that it is destined to another network device over the Internet. The router replaces the user workstation’s IP address with its own public interface13 address in the Source Address field of the packet. After the address is replaced, it computes a new checksum for the header and inserts it into the checksum field before sending the packet out its public interface at 74.123.17.33. The packet is routed over the Internet and arrives at the RANDOM BONUS DEFINITION web server residing at the public IP address of 38.214.37.10. Fast Ethernet — 100 Mbps Ethernet. The server determines that the request is destined for its address and notes that the source address is 74.123.17.33. The web server has no knowledge of the user workstation IP address of 192.168.1.28. The web server prepares a response using the public IP address of the NAT router as the destination address. When the response packet arrives at the NAT router from the web server, it uses its NAT translation table to send the packet to the requesting workstation. It accomplishes this by modifying the destination address to the workstation address of 192.168.1.28 and computing a new checksum for the IP header before sending the packet out its private address interface onto the local LAN. For all intents and purposes, the user workstation believes it is interacting directly with the web server. NAT has some advantages and disadvantages, but for most small local networks it works well and offers 12 Certain network addresses spaces have been determined by the Internet community to remain private. What this really means is that network forwarding devices on the Internet are not to forward any packet with a destination address that falls into the following ranges: 192.168.X.X, 172.16.X.X, and 10.X.X.X, where X denotes any number between 0 and 255. 13 There are two sides to every router that interfaces a private local LAN network and the Internet. Normally, the interface that is accessible over the Internet is referred to as the public interface or public interface address. 355 Page 355 Edwards 356 Part II ■ c08.tex V3 - 03/27/2009 The OSI Layers protection against unsolicited POP QUIZ network traffic ever making it through the NAT router to Describe what happens to a packet when it the local private network. If is passed through a NAT-enabled router. a packet’s parameters do not match the translation table’s known sessions, the packet is not processed and is dropped. The Destination Address field is pretty much self-explanatory. It is a 32-bit (4-byte) field containing the address information in the same format as the Source Address field. There is no difference in how the destination address is presented. In most circumstances the destination address is not messed with as the source address is with NAT. However, there are instances where the destination address may be translated and that is in special cases involving some sort of NAT router or a firewall. Actually, most routers used for the NAT function on outbound network traffic also have some capability to perform a port forwarding NAT. Notice that the web server in Figure 8-5 is directly connected to the Internet. That is certainly a possibility but is rarely found in today’s networking environment because of possible attacks on the server via the Internet. Figure 8-6 illustrates a network that offers services available on the Internet but is protected and hidden from users. Web Server FTP Server Firewall Internet Other Network Services VPN Router DMZ Network Private Network Figure 8-6 Port forwarding NAT As you can see, the network located behind the firewall is shielded to prevent users on the Internet from accessing these services directly. A firewall may be 11:22am Page 356 Edwards c08.tex Chapter 8 V3 - 03/27/2009 ■ 11:22am The Upper Layers a network device that is designed as a firewall for the inspection of packets as they are received, or it may be a router running a firewall application on it that provides the packet inspection. In any case, the firewall function requires packet inspection and a determination by the policies put in place by the network administrators of what to do with the received packet. If a packet is received and does not match any of the existing policies, it is dropped. The network behind the firewall may be a private network, but in this example it is shown as a DMZ14 network. Connected to this network are services that the Internet community is permitted to reach. In this example, we have a web server, an FTP server, and a VPN router. Obviously, the web server is where web pages can be accessed and is generally used only for queries to obtain information. The FTP server may be only for file downloads but if allowed may also be a place where users can upload files. An example where users from the Internet community at large can upload files to an FTP server is a website that allows user posting on the site or a photo lab site that prints users’ digital JPEG files on photographic paper. In the figure, there is a VPN15 router between the DMZ network and the private network. This device may be used as a remote access device for users who are remotely located but have permission to use the network service located on the private network. Usually VPN routers require user authentication, which can be performed locally on the VPN router, although it may depend on other authentication servers. For more information on this topic, see Chapter 14, ‘‘Network Security.’’ Back to our lovely red-brick firewall. We said that the firewall is responsible for inspecting the packets and using the policies installed by the network administrators to make a determination on what to do with the packet. To ensure that traffic is routed to the proper services, there must be port forwarding policies in place on the firewall. There are two ways this may be accomplished: either by changing the destination address and forwarding the packet on to the DMZ network, or, if the DMZ network addresses are routable Internet addresses, the packet may be inspected to ensure that only certain traffic is permitted to pass through the firewall. If the DMZ network uses addresses that are classified as nonroutable addresses, the only way traffic can be directed to the servers providing the requested services is by changing the packet’s destination address. In this example, the web and FTP services 14 DMZ is the acronym for demilitarized zone. In networking parlance, it refers to a network that may have some access by the public at large. The private network is protected by some sort of authentication process to only allow users with the proper credentials to reach the private network. 15 VPN is the acronym for virtual private network. Usually the acronym is applied to the device, but in reality it is not the network in itself. It provides access to the network using security authentication and encryption processes to ensure that the private network is accessed only by those authorized to use its services. 357 Page 357 Edwards 358 Part II ■ c08.tex V3 - 03/27/2009 The OSI Layers only receive traffic for those particular services. Although these services are shown as separate computers, many services can be supplied by a single server running multiple protocols. In this example, packets directed to port 80 for web services would be directed to the web server, while packets using ports 20 and 21 would be directed to the FTP server. Lastly, VPN requests would be directed to the VPN router, and there are a few VPN protocols that may be used, so for now we will just say any VPN service requests will be directed to it. The next field in the IP header is the Options field. As the name connotes, this is an optional field that follows the Destination Address field but is not used often. The last field in IP packet is the Data field, which is not part of the IP header so it is not used in the computation of the header checksum. The contents of the Data field are specified within the protocol header and can be any one of the IP protocols. Some of the most common protocols used in an IP packet are ICMP, TCP, UDP, and OSPF. OSPF (Open Shortest Path First) is a routing protocol used to route IP packets over the network. The last layer of the TCP/IP Model is the Link layer. This is a combination of physical hardware and software to frame the IP packet to transport it over whatever network medium is being used. So frame information depends on the type of network connectivity that is being used. In the case of Ethernet, the IP packet is encapsulated within the Ethernet frame. Figure 8-7 shows Ethernet encapsulation of an IP packet. Destination MAC Address Source MAC Address Ethernet IP Payload Type Frame Header IP Payload CRC Checksum Frame Footer Figure 8-7 Ethernet encapsulation of an IP packet The Ethernet frame header contains both the MAC (Media Access Control) destination and source addresses, each containing 12 bytes of addressing information. These addresses are unique and are directly associated with the physical network device. The last field in the Ethernet frame header is the Ethernet IP payload type. This is a 2-byte field and indicates the type of IP payload being transmitted by the Ethernet frame. Two of the most common IP payload types are 0x0800 for an IPv4 datagram and 0x080616 indicating that the frame is an ARP17 (Address Resolution Protocol). 16 The numeric representation with an ‘‘x’’ contained within it signifies that the number is a hexadecimal number. Each unit position is 4 binary bits in width. Thus, four hexadecimal numbers would contain 16 binary bits, or 2 bytes. If you still have difficulty grasping the concept of hexadecimal in relation to binary numbers, it is time for a review of number systems. 17 ARP is a mechanism for a transmitting network node to determine which network node is associated with a particular IP address. The network node assigned that IP address responds with its MAC address. 11:22am Page 358 Edwards c08.tex Chapter 8 V3 - 03/27/2009 ■ 11:22am The Upper Layers The Ethernet frame footer POP QUIZ contains the CRC checksum for the entire Ethernet frame. It conAt which layer of the TCP/IP model can the tains 4 bytes of checksum data, physical component of a network node be which is used to validate that found? the frame was received correctly by the network node it was forwarded to. So, if the minimum size of an IP packet is 46 bytes, the minimum size if an Ethernet frame is 64 bytes, with the addition of the 18 bytes of Ethernet header and footer. The maximum size of an IP packet is 1500 bytes, which makes the maximum Ethernet frame allowed onto an Ethernet to be 1518 bytes in total. For large data payloads, fragmentation must be used. We have worked our way down the TCP/IP model and now it is time to put the frame on the wire. Figure 8-8 conceptually illustrates the relationship between actual network elements and the TCP/IP network stack. Computer Computer Router Router Internet LAN A LAN B Application Application Peer to Peer Application Communications Transport Transport Internet Internet Internet Internet Link Link Link Link Local Area Network Local Area Network Internet Figure 8-8 The relationship between network elements and the TCP/IP network stack 359 Page 359 Edwards 360 Part II ■ c08.tex V3 - 03/27/2009 The OSI Layers Two LANs, LAN A and LAN B, have computers that want to communicate with each other using an application program that supports their capability to establish a session and communicate effectively. This is shown as a computer and router connected to each LAN. Each router is connected to the Internet, shown as a cloud since there is an unknown amount of network devices that may be in the path between the router on LAN A and the router on LAN B. The assumption is that if a frame18 is constructed properly, it can travel across many networks and through many devices in its path to reliably arrive at its predetermined destination. The application program running on both computers may be aware of the other’s network parameters, such as address and type of service, but it does not concern itself with the actual delivery of the data between the two peer computers running the application program. The only concern of the application program running within the TCP/IP’s Application layer is preparing the data so it can present it to the Transport layer in anticipation of having the data delivered to the computer residing on the other LAN. So a peer-to-peer application session between two computers over the Internet appears as if they communicate with each other only using the Application layer and the Transport layer of TCP/IP model. If application programs only concern themselves with getting the data properly packaged for the Transport layer, who does the rest of the actual delivery of the information? As illustrated in Figure 8-8, the lower two layers of the TCP/IP model are the Internet layer and the Link layer, which are directly responsible for reliably transporting the packet of information over the Internet. Since routing devices only need to be aware of addressing information, they only need to use the two lower layers of the model to effect the proper transmission of the information on its journey over the Internet. They are not concerned with data content since routing decisions are made on address and type of service. The Internet and Link layers are normally part of the operating system and the hardware that is installed on the computer. If ACRONYM ALERT we assume an Ethernet-based LAN, then DOS — Disk operation system the computer would require an NIC that is capable of providing an Ethernet connection to the LAN. This is what would be Layer 1 or the Physical layer of the OSI reference model. However, it is a portion of the TCP/IP model 18 Frame and packet are terms that are used interchangeably and are pretty much synonymous. Another term that may be tossed about from time to time is datagram. All these terms refer to some sort of encapsulation that includes the data to be transferred along with addressing and type of service being requested. It is how data can traverse the Internet from one computer to another. 11:22am Page 360 Edwards c08.tex Chapter 8 V3 - 03/27/2009 ■ 11:22am The Upper Layers Link layer. In order for the operating system to communicate properly with the NIC, device drivers are required that allow the software operating system to configure and control the physical components of the NIC. In a Microsoft Windows environment, this may be transparent to the user due to the capability of the operating system to recognize various pieces of computer hardware and automatically load the appropriate driver to communicate with the installed device. This portion of the TCP/IP model Link layer that includes device drivers maps to the Data Link layer of the OSI reference model. Once an NIC is installed in a computer and the device drivers are loaded so that the operating system is able to communicate with the device on a physical level,19 a network operating protocol needs to be bound to the card for it to communicate over the network with another network-connected device. In the case of TCP/IP, this is the address applied to the computer network interface along with its default gateway20 and the location of at least one DNS server. Most operating systems allow these parameters to be set manually, or the computer requesting the values can apply them automatically from a DHCP server that is servicing that network segment. Note that the routers illustrated in Figure 8-8 have their POP QUIZ Link layers connected to both the LAN and the Internet. In What determines the type of framing that is reality these would be two difto be used on a particular network segment? ferent interfaces and also of differing types of network connectivity. More than likely the router will have an Ethernet interface allowing it to be interconnected to an Ethernet-based LAN. The interface to the Internet is dependent upon the type of service the router is connected to. It may be a point-to-point T1 interface, a FDDI interface, or some other form of high-speed service to the Internet. So a router’s Link layer may consist of differing network hardware, device drivers, and Internet layer parameters to effectively transmit a data packet from the LAN to the Internet. 19 Physical level kind of implies actual hardware but includes software that allows the hardware registers be written to for data and control. It is the device driver that makes the translation from hardware-specific elements to the standardized routines within the operating system controlling network-based communications. 20 Default gateway has been mentioned more than once in this chapter. In a simple network, as illustrated in Figure 8-3, the address applied to the router on the LAN side would be considered to be a default gateway address. Basically, any packet with a destination address that is not located on the local LAN segment is forwarded to the address that is programmed into the default gateway address parameter field. 361 Page 361 Edwards 362 Part II 8.2.1 ■ c08.tex V3 - 03/27/2009 The OSI Layers TCP/IP Application Layer The Application layer of the TCP/IP model contains the RANDOM BONUS DEFINITION upper level protocols of the TCP/IP protocol suite, such as AppleTalk — A protocol suite developed by FTP (File Transport Protocol) Apple Computer. and SMTP (Simple Mail Transfer Protocol). Data is encapsulated and passed to the Transport Control Protocol for actual transmission on the network. The Application layer is dependent upon the lower layers to provide an effective and reliable means of network communications. The Application layer may be aware of the IP addresses and port numbers that are being used by the Transport layer, but it is that layer’s responsibility to encapsulate this information as it is passed to the Internet layer below it. Some of the more common Application layer protocols are listed in Table 8-1. 8.2.2 TCP/IP Transport Layer The two predominant protocols found in the TCP/IP Transport layer are UDP (User Datagram RANDOM BONUS DEFINITION Protocol) and TCP (Transmisendpoint node — A node that interfaces sion Control Protocol). The main with the user and the user’s communication difference between these protowithin a LAN. cols is that UDP does not guarantee delivery, and packets can arrive at the receiving network node out of order or duplicated, or not arrive at all. UDP is considered an unreliable delivery protocol whereas TCP is considered a reliable delivery protocol. TCP has the capability to detect missing, duplicated, and out of order packets and possesses mechanisms to request a packet be retransmitted if necessary. UDP relies on the use of ports for application-to-application communications. Since the port number is a 16-bit field in the UDP datagram, it can be anything between 0 and 65,535 or (216 –1).21 Port numbers may range from 0 to 65,535, but for the most part the first 1024 (0 to 1023 decimal or 0x03FF hexadecimal) are considered to be the well-known ports. The ports from 1024 to 49,151 (0x0400 to 0xBFFF) are registered ports 21 Why would the max port number would be 216 –1? True, the number 2 raised to the 16th power is equal to 65,536, so that is the maximum number of combinations that can be found when using 16 binary bits. However, one of those combinations is zero, so the −1 from the maximum value for the zero value and you end up the highest numeric value that can be attained with 16 binary bits is 65,535. 11:22am Page 362 Edwards c08.tex Chapter 8 V3 - 03/27/2009 ■ 11:22am The Upper Layers with the Internet Corporation for Assigned Names and Numbers (ICANN). Ports 49,152 to 65,535 (0xC000 to 0xFFFF) are considered to be temporary ports that clients can use when they communicate with servers. Table 8-1 Common Application Layer Protocols PROTOCOL PORT(S) DESCRIPTION DHCP 67 and 68 Dynamic Host Configuration Protocol provides the means for network clients to obtain an IP address, default gateway IP address, and Domain Name System server addresses. DNS 53 FTP 20 and 21 File Transfer Protocol is used to transfer files between an FTP client workstation and an FTP server. Port 20 is for data and port 21 is used for control signaling between server and client. HTTP 80 Hypertext Transfer Protocol is used to transfer hypertext information over the Internet. The most familiar application use for hypertext information retrieval is a web browser. IRC 19422 Internet Relay Chat is used for group communications over the Internet. Groups are referred to as channels and can also provide direct client-to-client chats and file transfers. POP3 110 Post Office Protocol version 3 is used to retrieve mail from a mail server by a mail reader application program. SMTP 25 Simple Mail Transport Protocol is used to send and receive mail messages between mail servers over the Internet. SNMP 161 Simple Network Management Protocol is used to manage and monitor network devices over the local network and Internet. Telnet 23 Telecommunications Network protocol is used over local networks and the Internet to establish terminal sessions between a client computer and a server. NTP 123 Network Time Protocol is used to synchronize time on a network by synchronizing network devices to a time standard found on the local network or over the Internet, BGP 179 Border Gateway Protocol is the main routing protocol of the Internet. It is responsible for maintaining a table of IP networks and makes routing decisions on path networking policies and rules. RIP 520 Routing Information Protocol is routing protocol run on local network segments to advertise route gateway addresses within the local network. 22 IRC 6669. Domain Name System server requests are used to convert a host name to an IP address so it may be found on the Internet. runs on the de facto standard port of 6667 and other nearby ports in the range of 6665 to 363 Page 363 Edwards 364 Part II ■ c08.tex V3 - 03/27/2009 The OSI Layers HELPFUL HINT As you’ll recall from the discussion on Network Address Translation (NAT,) a device that has NAT capability keeps a translation table. The device uses its own public interface address as the source address, while maintaining a cross-reference to the actual address of the requesting workstation. A technique known as port mapping maps the hidden source address to an unused port number. A workstation that requests a page from a web server must access the server using port 80 for the server to respond to the request. When the server receives the request, its only concern is the destination port, which must be port 80. So, what the source port number is makes no difference when servicing the request. The server simply sends the packet back to the requesting IP address, even though it is of a NAT-enabled router and not the actual workstation making the request. When the packet arrives at the NAT-enabled router, it examines the packet and finds that the destination port address correlates to a workstation on its private LAN in its NAT translation table. It modifies the packet with a new destination IP and port address, recalculates a new checksum, and then transmits it on to the private LAN. Therefore, knowing those temporary port addresses are available can come in handy when you’re using NAT. Port 0 is normally reserved, but its use is allowed as a valid source port in transmissions where the transmitting network node does not require a response from the receiving network node, which would be true in a case of a streaming application. Some common UDP network applications that are considered streaming applications are video teleconferencing, gaming, telephone using voice over IP (VoIP), and Trivial File Transfer Protocol (TFTP). Domain Name Services (DNS), an essential component of the Internet for the resolution of IP addresses to domain names, also utilizes UDP for its transmissions. Whereas UDP is connectionless, TCP is considered a connection-oriented protocol. This means that an end-to-end communication is required with the use of handshaking between client and server. Once the connection is established between the client and server, data can flow across that connection. Servers provide a multitude of services, including web, FTP, and Telnet. TCP utilizes a three-way handshake in establishing a connection. The server first must bind to a particular service and be available to all connections. This listening on a port is considered to be a passive open. Establishing a connection requires an active open on the server port. To do this the client sends a SYN (synchronization) packet with a random packet sequence number to the server. In response the client’s SYN the server replies with a SYN-ACK (acknowledgment) with the initial sequence number received from the client but incremented by one for the next sequence number it is expecting 11:22am Page 364 Edwards c08.tex Chapter 8 V3 - 03/27/2009 ■ 11:22am The Upper Layers to receive. Also in the packet is the server’s initial sequence number. The client then replies back to the server an ACK that contains its initial sequence number incremented by one along with the server’s acknowledgment number, which is the server’s sequence number incremented by one. After a successful SYN, SYN-ACK, ACK sequence between client and server, a connection23 is established. With the use of sequence numbers, it is very easy to determine packet order, duplicate packets, or missing packets. This provides TCP with the capability to provide error-free transmission. Applications requiring a high degree of reliability work best when they use TCP to set up communications over the network between a client and a server running that application program. HELPFUL HINT This section noted that certain applications utilize UDP for their transmission of data. An example of this is VoIP. However, telephone conversations are somewhat forgiving for lost audio packets. Voice quality can degrade rapidly when packet loss begins to increase. Depending on bandwidth usage on networks and with the addition of quality of service (QoS) for some traffic, UDP traffic may be affected because of its best-effort delivery method. With VoIP, this is manifested in choppy voice quality and dead air, which some users find intolerable. One way around this issue is further encapsulation, although it does add a degree of overhead to each packet. Some users opt for sending their VoIP data through a tunneling protocol, which is delivered using TCP/IP. To terminate a TCP connection, the protocol uses a FIN, POP QUIZ ACK sequence. When a network node desires to terminate the Which TCP/IP model Transport layer connection it sends a FIN packet, protocol is connection based? and the receiving network node sends an ACK in acknowledgment of receiving the FIN packet. This is considered a half open connection. The network node that has terminated its connection can no longer use the connection for data transmission, but the network node that has not sent its FIN packet can remain open and transmitting data. This sequence of FIN, ACK, FIN, ACK from both nodes is termed a four-way handshake sequence. Perhaps the most commonly used connection termination sequence is one network node sends a FIN packet and the other network node responds with a FIN-ACK combining the two handshakes into one. The network node that 23 Connection is sometimes synonymous with the word session, as in client server session. These words are sometimes used interchangeably to represent the SYN, SYN-ACK, ACK sequence. 365 Page 365 Edwards 366 Part II ■ c08.tex V3 - 03/27/2009 The OSI Layers initiated the termination sequence just responds with an ACK. This type of termination sequence is considered a three-way handshake. There is a possibility that both network nodes may send a FIN packet simultaneously and also will send their ACK packets at the same time. Since this sequence is done in parallel it is considered a two-termination sequence. 8.2.3 TCP/IP Internet Layer Some of the common services found at the Internet layer of the TCP/IP model are IP (Internet Protocol), ICMP (Internet Control Message Protocol), and IPSec (Internet Protocol Security). The primary protocol of the Internet layer suite of protocols is IP. Its main purpose is the delivery of packets between network nodes based solely on source and destination addresses since it is a connectionless protocol. Data from the upper layers is encapsulated within the IP datagram for delivery. IP is a best-effort delivery method and has no provision for out of order, duplicate, or missing packets. IP does not guarantee that the data payload has not been corrupted since the checksum it carries is only for the header, ensuring that it is error free. However, this does allow for quick discarding of packets whose headers have been corrupted. IP is responsible for fragmentation into multiple packets if POP QUIZ the data load it receives from the upper layers is too large to send True or false: The TCP/IP model Internet within a single packet. When layer IP protocol is a connectionless fragmentation is involved, the protocol. IP layer uses flags and offset to aid in the determination of packet sequence and their order. However, IP depends on the upper layers to ensure that the end-to-end integrity of the connection is maintained. ICMP is another integral protocol of the Internet layer. Its chief responsibility is to send a RANDOM BONUS DEFINITION message to the operating system bottleneck — A point in a data of a computer when a network communications path or computer error has been detected. These processing flow that limits overall messages usually report that a throughput or performance. requested service is not available or the other host could not be reached. Normally ICMP is a single-ended protocol since it not used to transmit messages between network nodes. However, there are some exceptions and the most common of these are 11:22am Page 366 Edwards c08.tex Chapter 8 V3 - 03/27/2009 ■ 11:22am The Upper Layers the ping and traceroute24 commands. These two tools require a reply from a receiving network node. If no reply is received, an error message is displayed. The ping utility is used to determine if a target network POP QUIZ node is available over the network. If it replies, the assumpWhat two ICMP applications can be used to tion25 is that the path is good verify the presence of an IP address on the between two network nodes. Internet or local network? The traceroute utility returns replies from each hop that it crosses to reach a particular targeted network node. Usually, it will try to reach a target in a given number of hops. The customary maximum hop count is 30 hops. It is a good indication if the packet is traveling in the right direction or not. 8.2.4 TCP/IP Link Layer We already mentioned that the TCP/IP model’s Link layer maps to the OSI model’s Data Link layer and Physical layer. The Physical layer components are the tangible pieces of hardware required to connect a computer to the network. It consists of the cabling, connectors, and NIC, which in most cases is installed in the computer. The hardware pieces are the lowest level of the TCP/IP model and make up the first level of the OSI model. Normally we do not think of hardware in terms of protocols. However, there are standards and specifications that hardware from different manufacturers must meet to be considered compliant with a standard. An example of this would be the electrical characteristics of cabling used for networking. There are also mechanical considerations such as size and form factor. The interconnection world is large, and manufacturers from all over the globe produce various components that all need to interconnect with products from other networking products manufacturers. So the protocol of the Physical layer is the standards and specifications that define various networking components. However, we know that the demarcation line between the Physical layer and the Data Link layer of the OSI model is at the Link layer of the TCP/IP model. It is the Network Interface Card. 24 traceroute is found mostly in Unix-based systems. In the Microsoft Windows world, the command is tracert. This is an accommodation to its predecessor MS-DOS, since commands and filenames could not be longer than eight characters. 25 The word ‘‘assumption’’ is used here since the fact that a reply is received does not guarantee that the host you desire to reach is actually the host that is replying. There is always a possibility of a duplicate address on a network. You will read more about this in Chapter 16, ‘‘Troubleshooting.’’ 367 Page 367 Edwards 368 Part II ■ c08.tex V3 - 03/27/2009 The OSI Layers An NIC card is a piece of hardware with electrical capabilities of sending intelligent ACRONYM ALERT electrical signals to another NIC card on the same network. The intelligence is contained ICMP — Internet Control Message Protocol within the bits and order that it places over the network medium, which in a lot of cases is wire based but may also be either fiber or air, in the case of wireless networking. The NIC contains registers and buffer space where the data and network control signals from the computer operating system are written to while sending packets to or reading packets from the network medium. Figure 8-9 shows a block diagram of a generic NIC. The diagram in Figure 8-9 is a representation of the basics of any type of NIC card. It is drawn to indicate that the card is capable of full-duplex operation because it contains both send and receive paths that are independent from one another, which would allow for simultaneous receive and transmit capability. To send a frame, the computer operating system needs to communicate with the card. Since these cards are functionally the same, the method used to communicate with a network interface is fixed by the operating system’s developer. It is up to the card manufacturer to either manufacture the card so it can be installed in a computer using generic N driver software or provide a tailored driver that would perform this function. Hardware interface software drivers26 are the link between operating system and the actual network hardware. Reviewing the block diagram, the computer bus interface component has to adhere to the architecture of the bus structure used within the computer. There have been many bus structures used since the spawning of PCs. In the earlier days, many were proprietary designs. As the industry evolved so did bus standards. One of the earlier standards was S-100, and cards of this type can be found in computer museums and in the cellars of computer aficionados. With IBM’s development of the IBM-PC, the bus standard that was rapidly adopted was ISA (Industry Standard Architecture). As computer capabilities began to expand so too did the bus architecture. The next evolution of the bus was the Extended ISA card or, simply, EISA card. Today’s bus standard is PCI (Peripheral Component Interconnect). So a network card or any sort of peripheral card needs the capability to be inserted into the internal bus of the computer it is being installed in. 26 Device driver is the common name for software that performs the hardware interface to the operating system. It is a piece of software code that allows the addressing and control of a hardware card installed in a computer. 11:22am Page 368 Edwards c08.tex Chapter 8 V3 - 03/27/2009 ■ 11:22am The Upper Layers To Computer Bus Computer Bus Interface Input Frame Control Output Frame Control Input Frame Buffer Output Frame Buffer Transmit Electrical Interface Receive Electrical Interface Network Interface Connector To Network Bus Figure 8-9 A block diagram of a generic NIC With the network card installed in the computer chassis and the appropriate device driver installed into the computer so that the operating system knows how to communicate with the NIC, the next step is to bind a network protocol to the card so data can be moved to and from the network. Depending on the operating system, differing methods can be used; consult your computer documentation. When all of that is completed, data can be sent and received from the network transparent to the workstation’s user. 369 Page 369 Edwards 370 Part II ■ c08.tex V3 - 03/27/2009 The OSI Layers Outgoing packets from an RANDOM BONUS DEFINITION application program flow down the network stack with each collision domain — A set of nodes layer encapsulated within the connecting to a shared medium among proper protocol. Once the frame which a collision can occur. Stations on the same shared LAN are in the same collision that is to be transmitted is domain. assembled and loaded into the output frame buffer, the output frame control prevents any further packets from being written into the output buffer until the frame has been completely sent. When the output frame buffer is cleared, the output frame control (through the device driver associated with this card) alerts the operating system that the card is ready to transmit another frame. On the receive side, the card monitors the network medium. When it has received a frame and it is completely in the input frame buffer and passes the checksum validation, the operating system is alerted (again via the device driver for the card) that a frame is ready to be passed up the network stack. As the packet passes through each layer, it is verified and checked as it is de-encapsulated. The input frame control is alerted that the frame is read and that another frame can be received. The last component to be discussed from the block diagram POP QUIZ of the NIC is the connector. Many people are already familList what is required for a network card to iar with the UTP RJ-45 connechave full-duplex capability. tors and plugs that are fairly commonplace on PCs, hubs, switches, and routers. However, depending on the medium being used, the connector will be different and adhere to the standards governing the usage of that type of medium. 8.2.4.1 TCP/IP Link Layer Protocols The three common protocols residing at the Link layer of the TCP/IP model are ARP (Address Resolution Protocol), RARP (Reverse Address Resolution Protocol), and OSPF (Open Shortest Path First). ARP and RARP are the complement of each other in resolving network addresses. ARP is used to find what hardware MAC address is associated with a particular IP address. It accomplishes this by sending out an ARP request packet as a broadcast to all nodes on its local network segment. The packet contains the IP address that the transmitting network node is seeking. The receiving nodes on the network that do not have the IP address being requested simply ignore the packet. The 11:22am Page 370 Edwards c08.tex Chapter 8 V3 - 03/27/2009 ■ 11:22am The Upper Layers network node that does have that IP address bound to its network interface responds with its MAC hardware address. RARP is a protocol that attempts to determine its IP POP QUIZ address by broadcasting on the local network segment with What is ARP used for? its MAC address. It expects a receiving network node to have an entry in its ARP cache that matches that MAC address with an IP address to transmit back a packet containing the IP address. With DHCP now in wide use, RARP has fallen into disuse. However, DHCP is a TCP/IP model Application layer protocol and does not reside at the Link layer. OSPF is a dynamic routing protocol used to move packets from network segment to network segment. Two network segments with a router in each that have a path ACRONYM ALERT between them can build and interchange RSTP — Rapid Spanning Tree Protocol route information. Figure 8-10 illustrates a network utilizing OSPF to pass network routing information. Area 0 Area 1 LAN A LAN B Figure 8-10 OSPF passing network routing information Notice there are two areas: Area 0 and Area 1. An area is a collection of network segments with routers and other network forwarding devices. For the sake of simplicity, these are shown as two large circles. Within each area there is a router to route traffic from that area to another area. Routers that border a network and pass routing information to another router within another area 371 Page 371 Edwards 372 Part II ■ c08.tex V3 - 03/27/2009 The OSI Layers are called area border routers POP QUIZ (ABRs). You will recall in the earlier discussion of routers in What is OSPF? this chapter we said they resided within the lower level of the TCP/IP model. The OSPF information passed between routers is used to update their routing information tables. The two routers only communicate OSPF information between them and do not pass that information into the network they control. So if a workstation on LAN A wanted to pass data to a server or another workstation on LAN B, it would send the packet to its default gateway. The packet will ultimately end up at the ABR for Area 0 and finding that the targeted address when compared to its learned routes in its routing table is destined for network node in Area 1, forwards the adjacent ABR for Area 1. The information that is used is the Link State Database (LSDB) routing data that is passed between the Area 0 and Area 1 ABR router. HELPFUL HINT The OSPF example used is very simplistic. Large networks have multiple areas where one ABR may be interconnected to many other ABR routers. The key to OSPF is to know that the updates exchanged between routers can be found within the router’s LSDB. Since this is a dynamic routing protocol, routes may pop up or age out as network nodes are inserted or removed from the network. 8.3 OSI Application Layer The OSI Application layer resides at Layer 7 at the top of the OSI model. It was mentioned that the TCP/IP Application model directly links to this layer. So the protocols listed in the discussion of the TCP/IP model Application layer are also contained within this layer of the OSI model. This is the layer that is directly responsible for interfacing with the application program a user is using on the computer. The most common use of a computer with Internet access is e-mail. The e-mail protocols residing at this layer are POP (Post Office Protocol), POP3 (Post Office Protocol version 3), and SMTP (Simple Mail Transfer Protocol). POP and POP3 are mail client–based in the form of user e-mail reader programs. SMTP is 11:22am Page 372 Edwards c08.tex Chapter 8 V3 - 03/27/2009 ■ 11:22am The Upper Layers e-mail server–based and is used to transfer mail from one mail server to another, so this layer is keenly aware of its communication peers. Mail clients know where their mail server is, and mail servers can establish a connection for the transfer of mail between them. Using the example of e-mail at the Application layer, the information the layer is concerned with is the identity of the sender and the identity of the recipient of that message and what application is available to assist in preparing the message to be sent. All e-mail users are pretty familiar with the address format used, e.g., john.doe@his company.com. There are two parts to the recipient address: the user name ‘‘john.doe’’ and the domain name ‘‘his company.com’’. The ACRONYM ALERT e-mail is formatted with sender address, MTU — Maximum transmission unit recipient address, and message and passed on to the local mail server servicing that sender. The mail server is concerned with both the domain name portion of the recipient’s address and the recipient’s name. The recipient’s name is used to identify the local mailbox for that user on the server. The application on the mail server is designed to use SMTP to send and receive mail from other mail servers. Most mail servers run a local post office where local users communicate locally over the local network using either POP or POP3 to retrieve mail from the local mail server. To send mail, users direct their outgoing messages to the SMTP service running on the mail server. Mail clients run POP at the Application layer to read mail and use SMTP to send mail. A mail server also runs two protocols at that layer, SMTP and POP and/or POP3. These protocols rely on the layers below them to actually get the message delivered and alert them when there is a message to pass up from the network. The Application layer is concerned with any syntax restraints such as the ‘‘@’’ sign in an e-mail message being required as a delimiter between recipient address and domain address. It is also the layer where security is applied for user identification and privacy. If quality of service is being applied to network communications, this is the layer concerned with determining the priority of a packet by its QoS27 tagging. N O T E Although there are many devices that are capable of QOS tagging of packets, there is no support for it over the Internet. The Internet is still a best-effort network. 27 QoS is the acronym for quality of service. We mentioned that the DiffServ field or the Type of Service field of the IP header is used for tagging packets to allow them to be transmitted along the network with a priority determined by how they are tagged. 373 Page 373 Edwards 374 Part II ■ c08.tex V3 - 03/27/2009 The OSI Layers This chapter covers only a POP QUIZ handful of the most familiar Layer 7 protocols. Many more True or false: The maximum number of protocols are available, considprotocols the TCP/IP Application layer can ering that the combination for have at any one time is two. port numbers is 65,536. Even with some protocols using more than one port, there is still a lot of them. You can obtain information on many protocols by reading their RFCs. RFCs are available over the Internet at www.ietf.org/rfc.html. 8.4 OSI Presentation Layer The middle layer of the OSI model upper layers is the Presentation layer, which occupies Layer 6 of that model. It has been mentioned that within the TCP/IP model, this OSI layer resides within its top Application layer. In the OSI model, it takes service requests from the Application layer and then issues requests to the Session layer below. Although we said that this layer resides within the TCP/IP POP QUIZ model Application layer, its components are more likely to True or false: The OSI model Presentation be found within the computer’s layer maps directly to the Transport layer of operating system. Within this the TCP/IP model. layer, incoming and outgoing data can be translated from one data format to another. This layer also offers the capability for data encryption and compression as well as decrypting and uncompressing data received. 8.5 OSI Session Layer The lowest layer of the upper layers of the OSI model is Layer 5, the Session layer. Like the OSI model’s Application and Presentation layers, it too can be found within ACRONYM ALERT the Application layer of the TCP/IP model. SNMP — Simple Network Management Protocol True to its name, it is the layer that is responsible for opening, managing, and closing a session between applications. It also provides the capability of restoring a session. It is the layer where authentication and permissions are granted. 11:22am Page 374 Edwards c08.tex Chapter 8 V3 - 03/27/2009 ■ 11:22am The Upper Layers The Session layer is where TCP SYN handshake sequences are provided for. Although the Session layer is responsible for checkpointing and recovery within the OSI model, it is seldom used by protocols of the Internet Protocol suite. Some of the protocols found within the Session layer are L2F (Layer 2 Forwarding Protocol) — Used to provide virtual private networks (VPN) over the Internet. L2TP (Layer 2 Tunneling Protocol) — Used to provide virtual private networks (VPN) over the Internet. NetBIOS (Network Basic Input/Output System) — In today’s networks is usually run over TCP/IP on the local network. It is a naming convention used to identify hosts on a Windows-based network. Although it is run over TCP/IP, its host name is not to be confused with the host domain name a computer may be given to resolve its name to an IP address. Those host names are registered with a DNS server and are not associated at all with a computer’s NetBIOS host name, which on larger networks is resolved by a WINS (Windows Internet Name Service) server. In small networks where WINS may not be available, WINS name resolution can be accomplished by editing the LMHOSTS file on the computer to correlate the NetBIOS name to an IP address. PAP (Password Authentication Protocol) — A simple authentication protocol to allow users access to network services. A major drawback to PAP is that passwords are passed in cleartext and can be easily captured. Since PAP is not secure, network administrators have been making use of CHAP (Challenge Handshake Authentication Protocol), which uses a hashing function to secure the password. MS-CHAP is Microsoft’s implementation of CHAP. PPTP (Point-to-Point RANDOM BONUS DEFINITION Tunneling Protocol) — Provides catenet — A collection of networks a means of creating a connected together at the Data Link layer VPN over the Interlevel. net. PPTP uses a standard PPP (Point-to-Point Protocol) session to its peer endpoint using the Generic Routing Encapsulation (GRE) protocol. A second session is then opened using TCP port 1723 to initiate and control the GTE session. Due to the need to have two simultaneous sessions opened, PPTP is not easily passed through a firewall. PPTP has lost favor and is being replaced by the L2TP and IPSec tunneling protocols. SSH (Secure Shell) — Allows for the secure exchange of data between two network nodes. It was designed as a replacement for Telnet and 375 Page 375 Edwards 376 Part II ■ c08.tex V3 - 03/27/2009 The OSI Layers other insecure protocols that were used for remote access over the Internet. These shells sent communications in cleartext, and passwords were easily compromised. SSH makes use of public key cryptography for authentication of the remote computer and allows the remote computer to also authenticate the user establishing the session. The Session layer provides for either half-duplex or fullduplex operation, synchronization points in the message stream, and error checking. POP QUIZ At which layer of the TCP/IP model is the OSI Session layer found? LAST BUT NOT LEAST As mentioned previously, you are encouraged to review the RFC documentation for any further information on protocols. Be aware, however, that any RFC is subject to variations in interpretation, and one implementation of a protocol may not be identical to another. A network administrator or member of the support staff must always be aware of this when integrating network pieces from different manufacturers. When there are interoperability issues, performance degradation issues, or functional issues, you may have to draw on the RFC to find which way to point the finger. 8.6 Chapter Exercises 1. List in order from highest to lowest the upper layers of the OSI model, also indicating their layer number. 2. An application that runs on a user’s workstation and communicates over a network with an appropriate application that is running on a server is considered to be what type of application? 3. Which protocol is considered to be a connection-based protocol? 4. What functionality can be used to disguise addresses from a private address space to be seen on the Internet? 5. List the three private address spaces that may be used and are considered to be not routable over the Internet. 6. Name an Application layer protocol that may be used to perform file transfers over the network. 7. What is the protocol that resolves IP addresses to hardware addresses? 11:22am Page 376 Edwards c08.tex Chapter 8 8.7 V3 - 03/27/2009 ■ 11:22am The Upper Layers Pop Quiz Answers 1. True or false: The Application layer is where all the application programs you load on your PC are stored. False 2. The predominant networking protocol run over Ethernet networks is TCP/IP 3. True or false: UDP is a connection-based protocol. False 4. Describe what happens to a packet when it is passed through a NAT-enabled router. A technique known as port mapping maps the hidden source address to an unused port number. A workstation that requests a page from a web server must access the server using port 80 for the server to respond to the request. When the server receives the request, its only concern is the destination port, which must be port 80. So what the source port number is makes no difference when servicing the request. The server simply sends the packet back to the requesting IP address, even though it is of a NAT-enabled router and not the actual workstation making the request. When the packet arrives at the NAT-enabled router, it examines the packet and finds that the destination port address correlates to a workstation on its private LAN in its NAT translation table. It modifies the packet with a new destination IP and port address, recalculates a new checksum, and then transmits it on to the private LAN. Therefore, knowing those temporary port addresses are available can come in handy when you’re using NAT. 5. At which layer of the TCP/IP model can the physical component of a network node be found? Layer 1 6. What determines the type of framing that is to be used on a particular network segment? The media being used for that network segment. 7. Which TCP/IP model Transport layer protocol is connection based? TCP 8. True or false: The TCP/IP model Internet layer IP protocol is a connectionless protocol. True 377 Page 377 Edwards 378 Part II ■ c08.tex V3 - 03/27/2009 The OSI Layers 9. What two ICMP applications can be used to verify the presence of an IP address on the Internet or local network? ping and/or traceroute 10. List what is required for a network card to have full-duplex capability. input frame control input frame buffer receive circuit output frame control output frame buffer transmit circuit 11. What is ARP used for? Address resolution 12. What is OSPF? A routing protocol 13. True or false: The maximum number of protocols the TCP/IP Application layer can have at any one time is two. False 14. True or false: The OSI model Presentation layer maps directly to the Transport layer of the TCP/IP model. True 15. At which layer of the TCP/IP model is the OSI Session layer found? Layer 5 11:22am Page 378 Edwards c09.tex V3 - 03/27/2009 11:23am CHAPTER 9 The Transport Layer Transport of the mails, transport of the human voice, transport of flickering pictures — in this century as in others our highest accomplishments still have the single aim of bringing men together. — Antoine de Saint-Exupéry The last chapter talked about the upper layers of the OSI reference model. You learned the specific purpose of each layer and how the layers interact with each other. This chapter covers the Transport layer, Layer 4 of the OSI reference model. The Transport layer is the highest layer of the lower layers of the OSI reference model. The Transport layer sits on top of the Network layer and below the Session layer. This layer is responsible for the end-to-end connection and datagram delivery, as well as congestion control and flow control. The two main protocols that operate at this layer are UDP and TCP, which were discussed in Chapter 5. The purpose of the Transport layer is to set up connections, maintain connections, shut down connections, and perform error checking.1 The protocols that operate at this layer are considered either connection-oriented (i.e., TCP) or connectionless (i.e., UDP). Remember that connection-oriented means that the connection must be set up before data can be transmitted, and connectionless means that data can flow without the connection being established first. 1 Error checking and other transport reliability attributes can be handled at this layer, if they are not already performed at the lower layers. 379 Page 379 Edwards 380 Part II ■ RANDOM BONUS DEFINITION 1000BASE-SX — A baseband Ethernet system operating at 1000 Mbps over two multimode optical fibers using shortwave laser optics. The Terms and Conditions of Chapter 9 Much like many other chapters in this book, there are some terms you need to have an understanding of, but not necessarily in-depth knowledge. Therefore, we start this chapter off with a few basic Transport layer functions and terms relating to these. 9.1.1 V3 - 03/27/2009 The OSI Layers So far this book has explained what the Transport layer is and the services and protocols it provides. This chapter takes a little deeper look into some of the functions that operate at this layer. 9.1 c09.tex RANDOM BONUS DEFINITION root port — In the Spanning Tree Protocol, the port through which a designated bridge forwards traffic in the direction of the root bridge. End-to-End Delivery The Transport layer provides logical communication between upper layer processes2 running on different nodes on a network (see Figure 9-1). Notice in the figure that the lower layer processes are transparent to the Transport layer. POP QUIZ The sending node takes the True or false: UDP is an example of a upper layer data and breaks it connectionless protocol. into smaller segments that are then passed to the lower layers to be encapsulated and transported to a receiving node. The receiving node will cache the data, put the segments back into the message, and pass it to the upper layers to be delivered to the Application layer. 2 Notice this says processes and not nodes. The Network layer provides the logical connection between nodes. 11:23am Page 380 Edwards c09.tex Chapter 9 V3 - 03/27/2009 ■ 11:23am The Transport Layer Application Presentation Session Transport Network Data Link Physical Application Presentation Session Transport Network Data Link Physical Figure 9-1 Logical Transport layer communications 9.1.2 Standards Before getting too much further into this chapter, there are a couple of standards that need to be mentioned that deal with the services and operations of functions at the Transport layer. The first of these is the ISO/IEC 8072 standard (Information technology – Open Systems Interconnection – Transport service definition), and the other is the ISO/IEC 8073 standard (Information technology – Open Systems Interconnection – Protocol for providing the connection-mode transport service). Following is a quick summary of these two standards. The remainder of the chapter covers the information that is defined in the standards. 9.1.2.1 ISO/IEC 8072 The ISO/IEC 8072 standard defines the recommended services provided by the OSI Transport layer while working with the Network layer to serve the 381 Page 381 Edwards 382 Part II ■ c09.tex V3 - 03/27/2009 The OSI Layers needs of protocols used at the Session layer. These are only recommendations or guidelines, and strict adherence is not upheld.3 Defined in this standard are recommendations for the implementation for the following functions: Connection-oriented mode services Connectionless-mode services The main thing to remember about this standard is that it defines the way the Transport layer interoperates with the other OSI layers it works with. 9.1.2.2 ACRONYM ALERT BGP — Border Gateway Protocol ISO/IEC 8073 The ISO/IEC 8073 standard sets the recommendations to be followed by nodes (entities) within a network that are utilizing the services of the OSI Transport layer. This standard is also available to future node deployments within an open systems environment. Defined in this standard are recommendations for the following functions: The recommendation and scope for classes of procedures that should be taken into account by the nodes when transporting data How peer nodes exchange data How the nodes exchange information with the transport service The manner in which the nodes exchange information with a service provider 9.1.3 This, That, and the Other This section takes a look at a few other ‘‘items of interest’’ regarding the Transport layer. 9.1.3.1 Types of Transport Service RANDOM BONUS DEFINITION aggregated link — A set of two or more physical links that appear to higher layer entities as though they were a single, higher capacity link. This is an easy one.4 There are two types of transport service: connection-oriented and connectionless. 3 Keep in mind that all of the functions at each of the layers in the reference model are only recommendations and guidelines that can be followed for conformity sake. 4 At least we hope it is. 11:23am Page 382 Edwards c09.tex Chapter 9 9.1.3.2 V3 - 03/27/2009 ■ 11:23am The Transport Layer Data Units The following two data units operate at the Transport layer: Transport protocol data unit (TPDU) Transport service data unit (TSDU) So, what is the difference between the two types of data units? The TSDU is the data that is transmitted to the various layers on both ends of a connection. The TPDU is the data that is sent from a protocol on one end to the peer protocol at the other end. 9.1.3.3 POP QUIZ TCP is a connectionprotocol. Classes of Transport Service The Transport layer defines the functions of service performed by it within five difference classes of transport service, as shown in Table 9-1. ACRONYM ALERT IETF — Internet Engineering Task Force Table 9-1 Classes of Transport Service CLASS NAME CLASS FUNCTION Class 0 Simple class Class 1 Basic error recovery class Class 2 Multiplexing class Class 3 Error recovery and multiplexing class Class 4 Error detection and recovery class 9.1.3.4 Types of Network Service The Transport layer takes into consideration the current error rate status of the connection being used. There are three types of network service used to classify the connection status. The data units are classified into one of the three types based on signal quality: Type A — A network connection with an acceptable residual error rate as well as an acceptable rate of signal failures. 383 Page 383 Edwards 384 Part II ■ c09.tex V3 - 03/27/2009 The OSI Layers Type B — A network connection with acceptable residual error rate but an unacceptable rate of signal failures. Type C — A network RANDOM BONUS DEFINITION connection with an unacceptable residual error promiscuous mode — A mode of operation rate for the user of the of a network interface in which it receives (or attempts to receive) all traffic, regardless transport service. of the destination address. 9.1.3.5 Multiplexing Multiplexing is the act of grouping several signals into a shared single signal. Multiplexing at the Transport layer is performed between the Transport layer and its adjoining layers. Multiple upper layer users can be multiplexed to share the services of a single Transport layer protocol. The signals are separated by what are known as transport service access points (TSAP). An example of this is shown in Figure 9-2. Upper Layers Transport Layer Protocol Transport Layer Figure 9-2 An example of multiplexing Network service multiplexing is also supported at the Transport layer. Multiplexing can occur in both an upward (multiple Transport layer signals to a single network signal) and a downward (multiple network signals to a single transport signal) fashion. The use of upward multiplexing (see Figure 9-3) is a cost-saving measure that allows multiple Transport layer signals to share a single network signal (a signal purchased from the network provider). 11:23am Page 384 Edwards c09.tex Chapter 9 V3 - 03/27/2009 ■ 11:23am The Transport Layer Transport Layer Network Layer Figure 9-3 Upward multiplexing Downward multiplexing (see Figure 9-4) is useful when bandwidth and throughput of data are priorities. RANDOM BONUS DEFINITION best-effort service — A service provided by an entity where frames or packets are delivered with high probability but with no absolute guarantee. Transport Layer Network Layer Figure 9-4 Downward multiplexing AN UNRELATED MOMENT OF PAUSE: WEB ACRONYMS It isn’t just the networking world that uses acronyms. Millions of users are typing away with acronyms that a few years ago didn’t exist. A lot of kids out there have added their own, such as POS (parent over shoulder). Jim has already prepared to start running a sniffer if he sees one of his kids using that one — having to warn a pal that the parent is looking in deserves a quick look-see. Enough rambling. Here is a list of some common web acronyms that you may come across at some point. (continued) 385 Page 385 Edwards 386 Part II ■ c09.tex V3 - 03/27/2009 The OSI Layers AN UNRELATED MOMENT OF PAUSE: WEB ACRONYMS (continued) 2L8 Too late AFK Away from keyboard AFN [That’s] all for now AISB As I said before B4 Before B4N Bye for now BAK Back at keyboard BBL Be back later BCNU Be seeing you BRB Be right back BTW By the way CU See you CYA See ya DL Download EZ Easy F2F Face to face FWIW For what it’s worth G2G Gotta go GMTA Great minds think alike HAND Have a nice day IC I see IDK I don’t know IK I know IKWUM I know what you mean IMAO In my arrogant opinion IMHO In my humble opinion IMO In my opinion IYKWIM If you know what I mean IYO In your opinion IYSWIM If you see what I mean JK Just kidding (continued) 11:23am Page 386 Edwards c09.tex Chapter 9 V3 - 03/27/2009 ■ 11:23am The Transport Layer AN UNRELATED MOMENT OF PAUSE: WEB ACRONYMS (continued) KISS Keep it simple, stupid LOL Laughing out loud ME2 Me too NP No problem ROTFL Rolling on the floor laughing TC Take care 9.2 Transport Layer Operations The purpose of the Transport layer is to provide end-to-end POP QUIZ delivery of data from one application to another. The TransWhich standard defines the way the port layer can deliver data in Transport layer interoperates with the other OSI layers it works with? a reliable or an unreliable fashion. Data flow can be regulated and each end can communicate lost datagram data with the other end. Protocols can operate in a connection-oriented manner as well as a connectionless manner. In the connection-oriented approach, a logical connection between nodes must be established before any data is transmitted. The connectionless approach does not require connection establishment; data is sent as it is received. In this section, we take a deeper look into the operations for both the connectionoriented as well as the connectionless ACRONYM ALERT protocols that are available within the ROM — Read-only memory Transport layer. 9.2.1 Connection-Oriented Operations Connection-oriented protocols require that a logical connection between two nodes is established before any data can be sent. To do this, rules are established that lay out how a connection is set up, maintained, and terminated. 387 Page 387 Edwards 388 Part II ■ 9.2.1.1 c09.tex V3 - 03/27/2009 The OSI Layers Setting Up the Connection If a node needs to pass data in a connection-oriented environment, a series of messages is passed between the node and the destination node it wants to send the data to. The series of messages is known as the three-way handshake, and it works like this: 1. The originating node will send a request known as a SYN5 to the destination node. 2. The destination node will let the originating node know that it has received the SYN request by sending back a SYN-ACK6 message. 3. The originating node will respond to the SYN-ACK by sending back an ACK message. Figure 9-5 shows an example of this. ACRONYM ALERT MSTI — Multiple spanning tree instance Step 1 SYN Step 2 SYN-ACK Step 3 ACK Figure 9-5 An example of a three-way handshake Don’t be fooled into believing this is all that’s going on in the connection setup phase. A number of variables are being negotiated during this phase. User node quality of service is matched to any available services that are provided by the Network layer. Some of the services negotiated include Which network services best match requirements set by the user for the connection 5 SYN 6 ACK stands for synchronize. stands for acknowledgment. 11:23am Page 388 Edwards c09.tex Chapter 9 V3 - 03/27/2009 ■ 11:23am The Transport Layer Whether multiplexing can (or should) be used Datagram size Address mapping Ability to separate multiple connections Inactivity timer information 9.2.1.2 RANDOM BONUS DEFINITION optical fiber — A communications medium capable of carrying and directing light signals. Normally extruded or drawn from transparent glass or plastic material. Maintaining the Connection Maintaining the connection is nothing more than ensuring the connection remains stable during the transfer of data between the endpoint nodes. The following activities occur during this phase: Segmentation of data Reassembly of data Splitting data over multiple connections Flow control Setting the identification parameters for a particular connection between endpoint nodes Attending to prioritized datagrams TSDU delimiting 9.2.1.3 Terminating the Connection Just like with the connection setup phase, there has to be a way to terminate the connection when the endpoint nodes are finished exchanging data. This phase operates much like the connection establishment phase. Any node that has an active connection can initiate a connection termination by sending RANDOM BONUS DEFINITION out a FIN 7 packet (or by setcollision detection — The act of detecting ting a flag in a datagram). The when packets collide during transmission. other node can continue receiving data until it sends out a FIN-ACK, acknowledging the request to terminate the session. 7 FIN stands for finished. 389 Page 389 Edwards 390 Part II 9.2.2 ■ c09.tex V3 - 03/27/2009 The OSI Layers Connectionless Operations Connectionless protocols do not require a connection; a transmitting device simply sends data as soon as it has data that is ready to be sent. Protocols that operate in a connectionless manner have a space available in the datagram to identify the source and destination addresses for the endpoint nodes. Connectionless protocols do need an available route to the destination in order to work. This means there must be some type of medium, a data link protocol, and a networking protocol to transmit the data. Other than these, there really is no other requirement. Protocols that use the connectionless method of transport will often provide error ACRONYM ALERT checking and recovery methods, which are lacking in the connectionless environment. SNA — Systems network architecture Some of these include: Hop count verification Verification of the reassembly of fragmented data Datagram priority information and verification POP QUIZ How many types of transport service are there? Datagram size verification TIME FOR SOMETHING NICE TO KNOW Following are some helpful MS-DOS commands that are available with most Windows OS platforms. ◆ To determine whether a remote node is reachable and its connection quality, use the ping command. C:\>ping Usage: ping [-t] [-a] [-n count] [-l size] [-f] [-i TTL] [-v TOS] [-r count] [-s count] [[-j host-list] | [-k host-list]] [-w timeout] destination-list Options: -t Ping the specified host until stopped. To see statistics and continue - type Control-Break; To stop - type Control-C. -a Resolve addresses to hostnames. -n count Number of echo requests to send. -l size Send buffer size. (continued) 11:23am Page 390 Edwards c09.tex Chapter 9 V3 - 03/27/2009 ■ 11:23am The Transport Layer TIME FOR SOMETHING NICE TO KNOW (continued) -f Set Don’t Fragment flag in packet. -i TTL Time To Live. -v TOS Type Of Service. -r count Record route for count hops. -s count Timestamp for count hops. -j host-list Loose source route along host-list. -k host-list Strict source route along host-list. -w timeout Timeout in milliseconds to wait for each reply. ◆ To follow the path that is taken by a datagram to a remote node, use the tracert8 command. C:\>tracert Usage: tracert [-d] [-h maximum hops] [-j host-list] [-w timeout] target name Options: -d Do not resolve addresses to hostnames. -h maximum hops Maximum number of hops to search for target. -j host-list Loose source route along host-list. -w timeout Wait timeout milliseconds for each reply. ◆ To view and manage the local routing table, use the route command. C:\>route ROUTE [-f] [-p] [command [destination] [MASK netmask] [gateway] [METRIC metric] [IF interface] -f Clears the routing tables of all gateway entries. If this is used in conjunction with one of the commands, the tables are cleared prior to running the command. -p When used with the ADD command, makes a route persistent across boots of the system. By default, routes are not preserved when the system is restarted. Ignored for all other commands, which always affect the appropriate persistent routes. This option is not supported in Windows 95. command One of these: PRINT Prints a route (continued ) 8 Tracert stands for trace route. 391 Page 391 Edwards 392 Part II ■ c09.tex V3 - 03/27/2009 The OSI Layers TIME FOR SOMETHING NICE TO KNOW (continued) ADD Adds a route DELETE Deletes a route CHANGE Modifies an existing route destination Specifies the host. MASK Specifies that the next parameter is the ‘netmask’ value. netmask Specifies a subnet mask value for this route entry. If not specified, it defaults to 255.255.255.255. gateway Specifies gateway. interface the interface number for the specified route. METRIC specifies the metric, ie. cost for the destination. ◆ To view and manage the ARP table, use the arp command. C:\>arp Displays and modifies the IP-to-Physical address translation tables used by address resolution protocol (ARP). ARP -s inet addr eth addr [if addr] ARP -d inet addr [if addr] ARP -a [inet addr] [-N if addr] -a Displays current ARP entries by interrogating the current protocol data. If inet addr is specified, the IP and Physical addresses for only the specified computer are displayed. If more than one network interface uses ARP, entries for each ARP table are displayed. -g Same as -a. inet addr Specifies an internet address. -N if addr Displays the ARP entries for the network interface specified by if addr. -d Deletes the host specified by inet addr. inet addr may be wildcarded with * to delete all hosts. -s Adds the host and associates the Internet address inet addr with the Physical address eth addr. The Physical address is given as 6 hexadecimal bytes separated by hyphens. The entry is permanent. Specifies a physical address. eth addr If present, this specifies the Internet if addr address of the interface whose address translation table should be modified. If not present, the first applicable interface will be used. 11:23am Page 392 Edwards c09.tex Chapter 9 9.3 V3 - 03/27/2009 ■ 11:23am The Transport Layer Transport Layer Protocols Chapter 5 provided some information on the TCP and UDP Transport layer protocols. Although these are the most popular and most commonly used, the following Transport layer protocols are also in use in some networks today: AppleTalk Transaction Protocol (ATP) Datagram Congestion Control Protocol (DCCP) NetBIOS Extended User Interface (NetBEUI) Real-time Transport Protocol (RTP) These are mentioned only to provide you with the names of a few more Transport layer protocols that you may come across. For the purposes of this book, TCP and UDP are the Transport layer protocols that we will stick with. 9.3.1 ACRONYM ALERT PPP — Point-to-Point Protocol A Few More Words about TCP TCP is a connection-oriented protocol. An originating node will contact a destination node to make sure it is available to get the message. Once confirmation is received that it is okay to send data, the transmission begins. TCP is also considered a reliable protocol because it has functions built into it that provide for various checks and balances to ensure the integrity of the data being transmitted. TCP is able to break data down into segments so that smaller chunks of data are RANDOM BONUS DEFINITION lost if there are problems with jumbo frame — A frame longer than the the transmission. TCP supports maximum frame length allowed by a acknowledgments for received standard. datagrams, and timers are set for the receipt of an acknowledgment to ensure that data is received on the destination end. TCP utilizes a checksum to monitor data receipt integrity. TCP also supports datagram reassembly, ensuring that it is put back into the same order it was sent. Finally, TCP supports both congestion control and flow control, allowing a sending node to monitor bandwidth availability as well as whether the receiving node can receive any more data. TCP uses sequence numbers between nodes to ensure that reliable communication is taking place. Receiving nodes use sequence numbers to put the data 393 Page 393 Edwards 394 Part II ■ c09.tex V3 - 03/27/2009 The OSI Layers back in order when it is received. Sequence numbers are also used to identify problems (lost packets, duplicate packets, etc.) that may occur with a specific packet that had been transmitted. Each end of the connection maintains its own sequence numbers, so data transmission can operate in a full-duplex manner. TCP is known as a byte-oriented sequencing protocol because every byte9 that is being transmitted is assigned a sequence number. The TCP packet is assigned the sequence number of the first byte of the packet. The following packet will get assigned the sequence number of its first byte, and so on. Figure 9-6 provides an example of sequencing. 1 2 3 4 5 6 7 8 9 10 Data Flow Figure 9-6 TCP sequencing In the figure, you can see that data is flowing from one node to another. The receiving node RANDOM BONUS DEFINITION recognizes that it is receiving a D-compliant — A bridge or switch that packet with a sequence numcomplies with IEEE 802.1D. ber of 1. As the node receives the packet, the number of bytes in the packet is counted. This will tell the node what packet sequence number is expected next. As you can see, there were 5 bytes10 in the first packet, so the next packet should start with a sequence number of 6. And that, my friend, is TCP byte sequencing. TCP also uses acknowledgment numbers that work hand POP QUIZ in hand with the sequence numbers. Acknowledgment numThe is the data that is bers are simply the sequence transmitted to the various layers on both numbers in reverse. They are ends of a connection. the reply from the destination node that sequence number such-and-such has been received. Figure 9-7 provides an example of how this works. 9 As opposed to some protocols that assign a sequence number to a whole datagram. TCP segments? Now, that’s funny. This number was picked at random for use in the example. TCP segments normally have 512 bytes. 10 Five-byte 11:23am Page 394 Edwards c09.tex Chapter 9 Sequenc V3 - 03/27/2009 ■ 11:23am The Transport Layer e #1, AC K #71 K #6 e #71, AC Sequenc Node A Node B Sequenc e #6, AC K #76 Figure 9-7 Sequencing and acknowledgement The figure represents communication between a pair of nodes. The originating node sends a packet that is assigned sequence number 1 (because the first byte ACRONYM ALERT sequence number is 1) and then sends an VLAN — Virtual local area network acknowledgment of a received datagram. The acknowledgment number is actually the sequence number that the node is expecting next. In the figure, Node A sends a packet to Node B. The packet has a sequence number of 1, and an acknowledgment number of 71. This means that Node ACRONYM ALERT A is telling Node B that it has received SRAM — Static random-access memory a packet and the next one it is expecting is sequence number 71. Node B sends a packet with sequence number 71 and the acknowledgment that packet sequence number 1 was received and the node is ready to receive sequence number 6. Node A then sends the next packet and acknowledges receipt of a previous packet. This process continues until data transmission is no longer required. 9.3.2 The TCP Header Format The TCP header and the upper layer data are joined to form a TCP segment. The TCP header is where the sequencing number and acknowledgment number 395 Page 395 Edwards 396 Part II ■ c09.tex V3 - 03/27/2009 The OSI Layers are maintained, as well as many other factors needed for proper data delivery. Figure 9-8 shows the format of the TCP header. Destination Port Source Port Sequence Number Acknowledgment Number Offset Rsvd U A P R S F Checksum Window Urgent Pointer Options (if used)–Variable Length Padding Data Figure 9-8 The format of the TCP header Source Port — A 16-bit number that identifies the application that sent the TCP segment. Destination Port — A 16-bit number that identifies the application the TCP segment is destined for. Sequence Number — A 32-bit number that identifies the first data byte in the segment. Acknowledgment Number — A 32-bit number that identifies the next data byte the node expects to receive. Offset — A field that identifies the length of the TCP header. Rsvd — An unused field reserved for potential future use. U/A/P/R/S/F — This field grouping contains the control fields: U — Urgent. If this field is set, the destination (receiving) node knows there is urgent data waiting to be sent. A — Acknowledgment. This is set when the packet has an acknowledgment for a received datagram. P — Push. When this field is set, the receiver needs to deliver the segment to the receiving application ASAP.11 11 As soon as possible. 11:23am Page 396 Edwards c09.tex Chapter 9 V3 - 03/27/2009 ■ 11:23am The Transport Layer R — Reset. When this is set, it tells the receiving node that the originator is terminating the connection. S — Synchronize. This field is set at startup when setting sequence numbers. F — Finished. There will be no more data coming. Window — A 16-bit number used by TCP for flow control. It indicates the number of available buffers the sending node has. Checksum — A 16-bit number used for error detection. Urgent Pointer — This is a 16-bit field. When the Urgent bit is set, there will be a number that points to the sequence number of the data that follows urgent data. This identifies to the destination node that the last byte of urgent data was received. Options — TCP support options that can be set for the data. This is a variable length field, depending on the option data. Padding — Padding fills the remainder of the 32-bit field. This is necessary due to the optional and variable length Options field. POP QUIZ How many different classes of transport service are there? Data — The application data: the payload! 9.3.3 A Little More on UDP UDP is a connectionless protocol. It does not guarantee that RANDOM BONUS DEFINITION data is going to be delivered to a destination. UDP simply E1 — A T-carrier technology commonly used in Europe, capable of multiplexing 32 transmits data when it has data DS-0 (64 Kbps) channels for a total that is ready to be transmitted. data-carrying capacity of 2.048 Mbps. Remember that UDP is usually used to send short bursts of datagrams between nodes where reliability is not a big concern. UDP can get data to a destination quicker, as it avoids the overhead required by all the checks and balances in TCP. Also, because UDP is connectionless, it can support broadcasting (sending messages to all nodes within a broadcast domain) and multicasting (sending messages to all nodes that are subscribed to the catenet). UDP accepts data (the payload) from the Application layer. It adds a UDP header and passes the header and the payload to the Internet layer. There 397 Page 397 Edwards 398 Part II ■ c09.tex V3 - 03/27/2009 The OSI Layers it is encapsulated into an IP packet and passed on to the Network Interface layer, then is passed over the transmission medium to the destination, where it makes its way up to the Application layer on the destination end of the connection. UDP segments can be lost along the way. They can also POP QUIZ be received out of sequence. This is why UDP is known as a True or false: FIN stands for finished. best-effort protocol. UDP is beneficial when you need to transmit a lot of data. There is no delay with UDP, as there is no need to set up a connection prior to the distribution of the data. If an application needs a method of recovering from errors, the application will handle this task itself. UDP also uses a checksum, which is a method for detecting transmission errors. 9.3.4 The UDP Header Format The UDP header and the upper layer data are joined to form a UDP segment. The UDP header is simpler than the TCP header due to the overhead required for the connection-orientation used by TCP. Figure 9-9 shows the format of the UDP header. Source Port Destination Port Message Length Checksum Data Figure 9-9 The format of the UDP header Source Port — A 16-bit number that identifies the application that sent the UDP segment. Destination Port — A 16-bit number that identifies the application the UDP segment is destined for. Message Length — A field that identifies the length of the UDP header. 11:23am Page 398 Edwards c09.tex Chapter 9 Checksum — A 16-bit number used for error detection. Data — The application data: the payload! 9.4 V3 - 03/27/2009 ■ 11:23am The Transport Layer POP QUIZ True or false: The AppleTalk Translucent Protocol is a transport layer protocol. The Meaning of Control In a connection-oriented environment, control of data transmission is important to ensure data delivery. Congestion control and flow control are two mechanisms used. Congestion control is used to avoid congestion on a link by avoiding the oversubscription of the rate that is supported by the link and reducing the rate of datagram transmission when congestion is on the link. Flow control is a mechanism that an originating node uses to POP QUIZ ensure that a destination node can handle the amount of data What is a TCP source port? being transferred. 9.5 Chapter Exercises 1. What are the two ISO/IEC standards that define recommendations for the transport layer? 2. What are the two types of transport service? 3. From the following list, fill in the class function in the table below. Multiplexing class Error detection and recovery class Simple class Error recovery and multiplexing class 399 Page 399 Edwards 400 Part II ■ c09.tex V3 - 03/27/2009 The OSI Layers Basic error recovery class Class Name Class Function Class 0 Class 1 Class 2 Class 3 Class 4 4. Match the type with the correct description: Type Network connections that maintain an unacceptable rate of residual errors Type Network connections that maintain both an acceptable rate of signaled errors and residual errors Type Network connections that maintain an acceptable rate of residual errors and an unacceptable rate of signaled errors 5. Define upward multiplexing. 6. Define downward multiplexing. 7. Explain how a three-way handshake works. 8. List four Transport layer protocols. 9.6 Pop Quiz Answers 1. True or false: UDP is an example of a connectionless protocol. True 2. TCP is a connection-oriented protocol. 3. Which standard defines the way the Transport layer interoperates with the other OSI layers it works with? ISO/IEC 8072 11:23am Page 400 Edwards c09.tex Chapter 9 V3 - 03/27/2009 ■ 11:23am The Transport Layer 4. How many types of transport service are there? Two 5. The TSDU is the data that is transmitted to the various layers on both ends of a connection. 6. How many different classes of transport service are there? Five 7. True or false: FIN stands for finished. True 8. True or false: The AppleTalk Translucent Protocol is a Transport layer protocol. False. It is the AppleTalk Transaction Protocol. (Gotcha!) 9. What is a TCP source port? The TCP source port is part of the TCP header. It is the 16-bit number that identifies the application that sent the TCP segment. 401 Page 401 Edwards c09.tex V3 - 03/27/2009 11:23am Page 402 Edwards c10.tex V3 - 03/27/2009 11:24am CHAPTER 10 The Network Layer It’s not what you know but who you know that makes the difference. — Anonymous There is not much difference between human networking and computer networking. You can be the most gifted human or the highest powered computer, but lacking the ability to share those resources, you can do nothing as far as the progression of humankind is concerned. The power of information is in its capacity to be shared. Since the evolution of wireless networking, information can be shared not only globally but beyond this world into outer space.1 The number of connected computers on the World Wide Web is staggering. Two computers are able to share information between them without concern about how that information is to navigate over the Internet. This is the ‘‘who you know that makes the difference’’ portion of what networking is about. Networking is about being able to route information to a particular computer and receive requested information from that computer without a need to know the path it travels over the Internet. Think of the Internet as a giant matrix with routing devices at every crossing point to aid in the movement of a packet of information along the cables connecting to the next crossing point. The route a packet of information travels can be different each time another packet of information is sent. The routing device’s responsibility is to make sure that the packet will arrive at the destination it is intended for. 1 Amateur Radio on the International Space Station (ARISS) has been experimenting with packet mail from amateur radio operators from around the world to the International Space Station. Although this is not conventional wireless networking, it may be a precursor of things to come when there is a manned base on the moon. 403 Page 403 Edwards 404 Part II ■ c10.tex V3 - 03/27/2009 The OSI Layers A computer is concerned only with its locally connected default gateway. A default gateway is where network traffic is sent when a computer wants to send information to a computer that it knows does not reside on its local network. Every computer and network-connected device has a default gateway set within their network configuration parameters. When information comes in via the Internet, it is accepted by the default gateway and routed on to the local network, directed toward the computer the received data is intended for. Routing or network traffic forwarding devices need not know every other device that is connected to the Internet. They just need to have a good working relationship with their immediate peers. It is dependent upon networking through these other peer routing devices to know other devices that they also have a working relationship with. It is essential that networks know the right entities to network to. The Network layer occupies Layer 3 of the OSI model. It RANDOM BONUS DEFINITION receives network requests from the Transport layer and, in mirror port — A switch port configured to reflect the traffic appearing on another of turn, issues network requests the switch’s ports. to the Data Link layer. It is the layer that is responsible for end-to-end information transfer. The delivery of information is within a datagram, also known as a frame or packet. The Network layer loosely maps to the Internet layer of the TCP/IP model, but the Internet layer deals only with the Internet Protocol (IP), whereas the OSI Network layer encompasses a broader range of both connection-oriented and connectionless network services. 10.1 Network Connection Types What does a connection-oriented service versus a connectionless network service really mean? All network-enabled devices2 are connected to a network, right? So they must be connected, right? Well, in the physical sense that is true. However, as far as a network service is concerned, it does make a difference how information is delivered between network nodes. The easy way to differentiate between the two types of network services is that a connected-oriented network service is one where the endpoint network nodes know who a session was established with, whereas in a connectionless network service, the two network nodes do not need to establish a direct connection in order to share information 2 A ‘‘network-enabled’’ device is simply any computer or packet-forwarding device with the right network interface for the network medium connecting the device, along with the appropriate network software. 11:24am Page 404 Edwards c10.tex Chapter 10 V3 - 03/27/2009 ■ 11:24am The Network Layer 10.1.1 Connectionless Network Services How can two network nodes exchange information if they do not have a connection established between them? This is where connectionless network services come into play. A great example of a connectionless network service is e-mail. E-mail is addressed to a particular user residing in a particular domain. It has no relation to a particular computer or geographical location. The following is an example of a typical e-mail address: john.doe@hishome.com The recipient of this e-mail is john.doe who resides in the network domain of hishome.com. This brings in the concept ACRONYM ALERT once again of domain names and their XNS — Xerox Network System relationship to network services. There is a hierarchy to network addressing, and the domain name is the highest level. Figure 10-1 illustrates the network addressing hierarchy. Domain Sub Domain 1 User 1 User 2 Sub Domain 2 User 3 User 1 User 2 Sub Domain 3 User 3 User 1 User 2 User 3 Figure 10-1 The network addressing hierarchy As shown in Figure 10-1, the top level of addressing is the domain.3 A domain can contain subdomains that have a varying number of users assigned. For example, the Widget Company has various departments with varying groups of users assigned to those departments. Figure 10-2 could be a method the Widget Company uses to set up their domain. The Widget Company is a family-owned business founded in the mid-1800s. It prides itself on being wholly American owned and its operations being located only within the geographic boundaries of the United States. Although their products are shipped globally, they support sales and customer service from within the good old USA. Even though they face fierce price cutting from 3 Domains are named by the organization that wants to create a domain for its network infrastructure. Domain names are usually classified with either a company name or some other meaningful words or acronyms for the easy identification of domain ownership. 405 Page 405 Edwards 406 Part II ■ c10.tex V3 - 03/27/2009 The OSI Layers manufacturers that off-shored their operations, the Widget family of products have maintained their competitive edge due to superior product reliability and what is considered to be best-in-class customer service. Users .marketing .widget.com .sales Users .manufacturing Users Domain Sub Domain Users Figure 10-2 The Widget Company’s domain hierarchy The Widget Company wants to create three subdomains for its marketing, sales, and manufacturing departments. These departments have control of various servers that service the users of each department. These users may be either internal or external users over the Internet. The Widget Company domain does not require that all the entities of the domain be located within a single building, city, state, or country. Components that are not only for the overall domain but also for the subdomains may be located in geographically distant locations.4 However, the network nodes that are part of the domain can still be reached using domain names without the need for absolute address locations. Figure 10-3 illustrates what the overall network topology of Widget Company might look like on a top level network map. The top level drawing of the Widget Company network RANDOM BONUS DEFINITION shows locations that are solely contained within the United link aggregation — The process of States in various distantly locacombining multiple physical links into a ted sites. The various sites single logical link for use by higher layer link clients. are interconnected using the 4 Geographically distant locations can be in the building next door, down the street, in the next town, in the next state, or in the next country. If they are not on the same local network, they are considered to be distant and require special handling to ensure information is transmitted reliably. 11:24am Page 406 Edwards c10.tex Chapter 10 V3 - 03/27/2009 ■ 11:24am The Network Layer Internet as a transportation medium for the domain’s network infrastructure. Because these sites are connected over the Internet, they utilize IP for the transmission from site to site. Corporate Headquarters Milwaukee Manufacturing Canton, Ohio WEB Marketing New York City MAIL Sales Chicago Corporate Users Network Figure 10-3 The Widget Company’s top level network diagram It was already mentioned that the TCP/IP model’s Internet layer is a subset of the OSI model’s Network layer. There will be places in this chapter where we ACRONYM ALERT discuss the aspects of TCP/IP where it Telnet — Teletype Network is relevant within the OSI Network layer. The domain aspect can be used for either connection-oriented or connectionless network services. However, the world of TCP/IP uses IP to move information along the world’s information highway. To bridge between domain names and IP protocol addresses requires domain name resolution, commonly referred to as DNS (Domain Name System). Further discussion of DNS can be found in Section 10.1.3. 407 Page 407 Edwards 408 Part II ■ c10.tex V3 - 03/27/2009 The OSI Layers As you can see in Figure 10-3, the corporate offices located in Milwaukee have multiple networks, various computer systems, and a number of servers. This diagram is simplistic in its presentation for a large corporate network, which is far more complex. However, the base principles of network interoperability5 are fairly similar due to the scalability6 of networking technologies. The figure shows two servers: a mail server and a web server. The remote offices also have servers located at their sites that are able to pass information from other servers and users located either locally or over the Internet. Using domain names to reach various servers has the following format: Host name.domain name.sub domain name.domain name suffix7 The mail server named mail located at the corporate office would have a domain name that appears as follows: mail.widget.com If the marketing group located in New York City also has a mail server that gathers its mail from the corporate mail server, its name could be: mail.marketing.widget.com Mail shared between users is connectionless8 because the computer sending the mail does not need a connection directly to the mail server the recipient of the e-mail is connected to. There are differences in e-mail, and perhaps there is some confusion due to the type of e-mail service being used. A local mail program on a computer is capable of creating a mail program entirely independent from any other computer. When it is ready to send the e-mail message, it does so by forwarding the mail to a Simple Mail Transport Protocol (SMTP) server where the user has an account. The message is forwarded by the SMTP server without any further action by the user to aid in the delivery of the message. 5 ‘‘Interoperability’’ is just a fancy name for network node devices to play nice with all the other network node devices connected on the same network. 6 ‘‘Scalability’’ simply means that networks can start small and grow larger as needed. However, larger networks usually require higher capacity network devices able to handle the amount of information that is to traverse the network within a fixed period of time. 7 Domain names as illustrated in this example do not have spaces within the name. So, using the above example as a domain name would actually appear as hostname.domainname .subdomainname.domainnamesuffix. 8 A computer connected to its local mail server uses the POP or POP3 protocol to receive mail and SMTP to send mail. These protocols are connection-based because the PC has a direct session with its local mail server. However, mail user to mail user is connectionless because a user-to-user PC session is not needed to send or read mail. 11:24am Page 408 Edwards c10.tex Chapter 10 V3 - 03/27/2009 ■ 11:24am The Network Layer If a user is using web-based mail, the session established by RANDOM BONUS DEFINITION the browser to create the e-mail learning state — A transition state in the is a connection-oriented netSpanning Tree Protocol state machine where work service. In using web mail, a bridge port is learning address-to-port mappings to build its filtering database the user establishes a connection before entering the forwarding state. to the server serving his or her account to create and forward the message. However, the type of service is still connectionless since the user is not required to provide any further action to ensure delivery of the e-mail message. This illustrates that even connectionless processes may require some elements of a connection-oriented network service. SMTP mail servers deliver e-mail to the SMTP mail server servicing a particular domain. Although a user name is attached as part of the message, the SMTP server does not deliver the message to the user. A user must have an account on a mail server in order for the mail to be delivered to that user’s post office box. In the case of incorrect spelling of a user name or if a user never had an e-mail account or their account had been deleted, the SMTP server would return the original message with an error header9 stating the cause for the message not being delivered. The most common reason for return is ‘‘user unknown. E-mail for a user is held on the mail server for a period of time established by the administrator of that server. There are various parameters on most mail servers that allow for a mailbox’s size, usually in megabytes, length of time a message is held, and the maximum allowable size of a message. An error message may be returned to an e-mail sender if the recipient is not in compliance with any of the preset parameters. Depending on the mail service provided by the mail server, mail may be read while remaining on a mail server or it may have to be downloaded using the Post Office Protocol (POP or POP3) to the local workstation for reading and any other required action.10 To summarize, a connectionless network service has the capability to prepare informaPOP QUIZ tion for transmittal to another Mail is what type of network service? network node without the creation of a real-time connection to that network node in order to complete the transfer of the information being sent. 9 In computerese, the header is simply the top of the message. In other words, you do not need to read the whole message to see why it was bounced back. 10 The required action is usually reading the message and either filing it or discarding it. Unfortunately, just like your postal mailbox, your e-mail mailbox also gets a lot of junk mail. 409 Page 409 Edwards 410 Part II ■ c10.tex V3 - 03/27/2009 The OSI Layers 10.1.2 Connection-Oriented Network Services A connection-oriented network service is exactly what the name implies. A network connection11 is established between two computers to transfer information from one computer to the other over the Internet. Many client/server application programs are connection-oriented network services. A good example of this would be the interaction between an FTP client and an FTP server.12 Figure 10-4 illustrates a user residing on a local network at IP address 192.168.2.13 requesting information from a local FTP server whose IP address is 192.168.2.5. 192.168.2.5 192.168.2.13 FTP Server FTP Client Figure 10-4 An FTP client/server connection-oriented network server The following portion of the FTP server log illustrates the interaction of the client with the FTP server: Oct 11 20:28:35 Oct 11 20:28:35 Cerberus FTP Server started Local Host: Rbramant-2 Oct 11 20:28:35 Oct 11 20:28:35 Local Interface 0 located at 192.168.2.5 Listening on Port 21 Oct 11 20:34:39 1 Oct Oct Oct Oct 1 1 1 1 11 11 11 11 20:34:39 20:34:52 20:34:52 20:34:57 Incoming connection request on interface 192.168.2.5 Connection request accepted from 192.168.2.13 USER anonymous 331 User anonymous, password please PASS *********** 11 Although networks use electrical connections for signal transmission, a network connection is when two endpoint network node devices know each other and establish a session that is connected. 12 Many places within the text server and client are shown and discussed as totally separate network entities. In reality, a computer can be both a server and a client simultaneously for network services. 11:24am Page 410 Edwards c10.tex Chapter 10 Oct 11 20:34:57 Oct 11 20:34:57 1 1 Oct Oct Oct Oct Oct Oct Oct 1 1 1 1 1 1 1 11 11 11 11 11 11 11 20:35:00 20:35:00 20:35:00 20:35:00 20:35:00 20:35:08 20:35:08 V3 - 03/27/2009 ■ 11:24am The Network Layer 230 Password Ok, User logged in Anonymous user ‘‘anonymous’’ logged in with password ‘‘guest’’ PORT 192,168,2,13,19,137 200 Port command received LIST 150 Opening data connection 226 Transfer complete QUIT Connection terminated. You can see that the client initiated the connection to the server. The server forced the client to supply a user ID and a password. The client responded with a user ID and password combination, and is authenticated and allowed to maintain the session with the FTP server. The FTP client user requested a directory listing from the FTP server. After the ACRONYM ALERT listing was received, the user quit the session and thus caused the termination of STP — Spanning Tree Protocol the connection between the client and the server. A packet capture of this session was performed at the FTP server, as illustrated in Figure 10-5. Figure 10-5 A packet capture of an FTP session 411 Page 411 Edwards 412 Part II ■ c10.tex V3 - 03/27/2009 The OSI Layers The FTP session uses the TCP/IP protocol to establish the session and complete the transfer of information from the FTP server to the FTP client. Packet number 7 shows the client requesting a session with the FTP server. Packet 10 is the FTP server acknowledging the session request. Packets 12 through 21 are the packets showing the interaction between the FTP client and FTP server to authenticate the FTP client and establish the FTP session. Packets 22 through 36 are the directory listing request and the transfer of the directory contents information to the FTP client. Packets 37 through 44 are the packets showing that the FTP client is terminating the FTP session and thus terminating the network connection. An FTP13 session does involve layers above the Network layer, POP QUIZ but FTP helps illustrate the concept of a connection-oriented Name the ports used by an FTP client to network service. The two comrequest an FTP session with an FTP server. Which port is used for data transmission? puters establish a connection session and transfer information between them. The Network layer is responsible only for the end-to-end connections and is not involved with the hop-to-hop14 transfer of the packets over the network. WANT TO TRY SOMETHING? You are encouraged to reproduce the FTP session as illustrated in this section. It requires two computers and software that can be obtained by a free download from the Internet. The FTP session was accomplished by using FTP server software from www.cerberusftp.com and using the ftp command from the command prompt of a Windows XP PC. You can obtain packet capture software for free from www.wireshark.org. The computers can either be on the same network segment or on different segments with network routing devices between the network segments. 10.1.3 Domain Name Services Many of you are probably familiar with the term URL (uniform resource locator). A typical URL would appear as follows: http://www.mydomainname.com 13 The FTP protocol uses two ports for control and data transfer. Control is dedicated to port 21, and port 20 is dedicated to data transfer. An FTP server would listen on port 21 for FTP requests, and the FTP session is negotiated and controlled using this port. 14 A network hop is any network node a data packet needs to be forwarded through on its journey to the requested destination. 11:24am Page 412 Edwards c10.tex Chapter 10 V3 - 03/27/2009 ■ 11:24am The Network Layer The http indicates this is a request for port 80 on a computer with the host name www located in the domain mydomain.com. In the TCP/IP world, computer addresses take the following form: XXX.XXX.XXX.XXX where XXX can be a numeric decimal value between 0 and 255. We are preconditioned to think of URLs as being as follows: prefix.domainname.suffix We are accustomed to seeing .com, .org, .gov, .edu, or .net being used as a suffix, although many others are in use. Also, a country code may be used as the suffix to denote where the domain and host computer are found. So how does one get from a text-based URL name to an IP address? Someone has to take care of it, like the telephone company has with the use of area codes, exchange numbers, and the last four unique digits to reach a particular telephone. So in the case of finding an IP address for a particular computer by its host name, who would have the super-sized host name book that lists every computer connected to the Internet? Telephones are basically static devices. They are wired RANDOM BONUS DEFINITION into a particular telephone switch with a fixed number. jam — In Ethernet, the process of sending Computers can be moved or an additional 32 data bits following the exchanged with other computdetection of a collision to ensure that all parties to the collision properly recognize ers, and occasionally IP addrethe event as such. sses associated with a particular URL can also be changed. So, host-name-to-IP addresses can be pretty dynamic, and a dynamic system is required to maintain the capability to perform host name resolution. There needs to be some form of registration to enable this to occur. There are many companies that sell domain name registration for a fee. But what does that really mean? As with IP addresses, domain names also need to be unique. Domain names must be registered to ensure that they are not duplicated on the Internet. The Internet Assigned Numbers Authority (IANA) is an organization created to establish standard naming for what is called the top level domain (TLD), or root zone. The suffix portion of a URL is the root zone. It is used to parse a host name URL to establish which root zone the host name is a member of. The Internet Network Information Center (InterNIC) is maintained by the Internet Corporation for Assigned Names and Numbers (ICANN) and is responsible for the registration of domain names through registered domain name hosting companies. 413 Page 413 Edwards 414 Part II ■ c10.tex V3 - 03/27/2009 The OSI Layers When a domain name is registered, it is associated with an IP address and is mainACRONYM ALERT tained on a DNS server. Each DNS server needs to know what the designated authorSD — Start delimiter itative name server is in order to receive DNS updates. Although the service is fairly dynamic, caching15 is used to save time querying the root name servers each time a request is made for a particular host name. Figure 10-6 illustrates a typical DNS server scenario. ISP DNS Server Local DNS Server Authoritative DNS Servers Local Computers Figure 10-6 A typical DNS server scenario DNS is part of the TCP/IP protocol suite. The computers on the local network have configured the IP address of the local DNS server into their 15 Caching is the process of saving information for a predetermined amount of time. In DNS, caching can save time for address resolution. However, to ensure that a name resolution stays ‘‘fresh,’’ there is usually an expiration time associated with the entry. Old entries are aged out automatically. When a DNS request is made, if it is not in the cache, name resolution needs to be performed. Although under normal circumstances it is completed fairly rapidly, it does take more time than just pulling it up from the local cache storage. 11:24am Page 414 Edwards c10.tex Chapter 10 V3 - 03/27/2009 ■ 11:24am The Network Layer TCP/IP configuration settings. You can verify these settings by issuing an ipconfig /all command at the command window of a Windows-based PC. The response would be similar to the following: Ethernet adapter Local Area Connection: Connection-specific DNS Suffix . : Description . . . . . . . . . . . : Broadcom NetXtreme Gigabit Ethernet Physical Address. . . . . . . . . : 00-17-08-30-6A-01 DHCP Enabled. . . . . . . . . . . : Yes Autoconfiguration Enabled . . . . : Yes IP Address. . . . . . . . . . . . : 192.168.2.5 Subnet Mask . . . . . . . . . . . : 255.255.255.0 Default Gateway . . . . . . . . . : 192.168.2.1 DHCP Server . . . . . . . . . . . : 192.168.2.4 DNS Servers . . . . . . . . . . . : 192.168.2.1 Lease Obtained. . . . . . . . . . : Sunday, October 12, 2008 8:08:02 AM Lease Expires . . . . . . . . . . : Monday, October 13, 2008 8:08:02 AM In this example, there is only one DNS server, and it is the same as the device that is acting as the default gateway. In this particular setup, the router is capable of running a DNS service, and its DNS servers are the upstream servers at the ISP, as shown in Figure 10-6. Using the example of a browser attempting to reach a particular URL, if the computer does not have the resolved host name stored in its local DNS cache, it will request it from its assigned DNS server. Figure 10-7 shows a packet capture of a DNS request from a local PC to its local DNS server. The user is calling the URL www.imagesbybramante.com and, not having the host name cached, it places the request to its local DNS server. If the local DNS server does not have the host name cached, it makes a DNS request to its upstream server and would eventually work its way back to a root authoritative server until the name is resolved. If the name cannot be resolved, an error message is returned. When the name is resolved, it is passed back through the servers until it reaches the computer that made the original request. Figure 10-8 shows a successful host name lookup for the query used in this example. This has been a top-level discussion of DNS to give you a basic understanding of name POP QUIZ resolution in regard to IP. Name some top-level domain names. You are encouraged to explore literature dedicated solely to DNS concepts for additional, in-depth information. 415 Page 415 Edwards 416 Part II ■ The OSI Layers Figure 10-7 A packet capture of a DNS request Figure 10-8 A packet capture of a DNS response c10.tex V3 - 03/27/2009 11:24am Page 416 Edwards c10.tex Chapter 10 V3 - 03/27/2009 ■ 11:24am The Network Layer SOMETHING TO TRY We suggested earlier that you download a freeware version of Wireshark. It is a useful tool not only for troubleshooting but to give added insight to what is occurring on your computer in terms of network communications. It can be loaded on either a desktop or laptop with your other Windows-based applications. It may be launched prior to opening any application and allowed to capture the packets of that application. This will help build familiarity with the Wireshark application itself and aid in increasing your understanding of TCP/IP and the protocols supported within the TCP/IP protocol suite. 10.2 TCP/IP Network Layer Protocols The Network layer of the OSI model provides for both connecRANDOM BONUS DEFINITION tionless network services and connection-oriented services. It filtering — The process of inspecting frames encompasses the protocols of received on an input port of a switch and deciding whether to discard or forward the TCP/IP model’s Internet them. layer. However, the OSI model’s Network layer is broader in scope than TCP/IP’s Internet layer, and at times, it includes other TCP/IP protocols from its Link layer. Due to this difference, the two layers should not be considered mirror images of each other, although they do have some protocols in common. 10.2.1 Internet Protocol The Internet Protocol (IP) is primarily a method of moving packets of data across networks comprising various media, seamlessly delivering these packets solely based on the destination address. This is accomplished by encapsulating data from the upper layers into packets16 in preparation for delivery over the network. IP is a connectionless protocol since packets can be transmitted without the establishment of a circuit to the destination network node. Because IP is a best-effort delivery service, it makes no guarantee that a packet will be delivered. Therefore, data can become corrupted, packets can 16 The word ‘‘packet’’ is synonymous with ‘‘datagram’’ or ‘‘frame.’’ These three words are used interchangeably and refer to the structure containing all the pertinent information for the proper construct so that the data can be reliably transmitted and that it can be properly unencapsulated when received at the intended network node. 417 Page 417 Edwards 418 Part II ■ c10.tex V3 - 03/27/2009 The OSI Layers arrive out of order, duplicate packets can be received, and packets can be lost or discarded. The mainstay for many years has been Internet Protocol version 4 (IPv4), but due to its limitation of addressing, Internet Protocol version 6 (IPv6) is currently being deployed worldwide. To work around the limited address space of IPv4, the development of Network Address Translation (NAT) helped delay the need to deploy IPv6 any sooner. 10.2.1.1 Internet Protocol Version 4 Because IPv4 utilizes 4 bytes to express an address, it has only 32 bits that can be used for its address. This allows for a maximum combination of addresses that can be supported of 232 , or 4,294,967,296 unique addresses. Since some of the addresses are within reserved address space, the total space is not available as public Internet addresses. IPv4 addresses are mostly expressed in what is referred to as dot-decimal notation, for example: 192.168.15.85 Each dotted section is representative of the decimal value of the byte. So it would look as follows in binary: 11000000.10101000.00001111.01010101 There is a multitude of variations to express IP addresses, but the dot-decimal notation is the most widely used. REMEDIAL EXERCISE For those of you who are not proficient in manipulating numbers between various number systems, try to convert the above dotted binary number to a hexadecimal-dotted notation. Hint: The bits of a byte are equally divided to create two hexadecimal numbers for each dotted binary section. A hexadecimal number is usually represented by 0x. If you want the answer, wait until you give it an honest try, and then look at the footnote below.17 If the upper layers present the TCP/IP Internet layer with data that is too large to transmit within a single packet, the data will be fragmented and transmitted over the network in 17 xC0.0xA0.0x0F.0x55, POP QUIZ What is a maximum transmission unit? or in not-dotted notation, 0xC0A00F55 11:24am Page 418 Edwards c10.tex Chapter 10 V3 - 03/27/2009 ■ 11:24am The Network Layer multiple packets. IP performs the fragmentation since it is host dependent, not machine dependent. The maximum transmission unit (MTU) is the number of bytes of data that a particular network medium is capable of handling. It is determined by the largest packet the medium is capable of handling, minus any number of bytes required as a header to transmit the packet over the medium. In the case of Ethernet, which has a maximum packet size of 1500 bytes, the MTU is the maximum packet size minus the number of bytes required for the header. Ethernet normally requires 20 bytes for a header, which provides for an MTU of 1480 bytes. The data is fragmented into the number of packets needed, with each packet tagged indicating it contains a fragment. The receiving network nodes unencapsulate the received fragmented packets and reassemble the data before passing it up to the layer above it. 10.2.1.1.1 Network Address Translation It was mentioned that Network Address Translation (NAT) was developed to provide a method of using addresses designated as private address space behind a network device, such as a router, that is able to perform the translation from a nonroutable IP address to a publicly known IP address. Table 10-1 shows the reserved addresses for private networking.18 Table 10-1 Private Networking Reserved Addresses ADDRESS RANGE CIDR NETWORK CLASS ADDRESSES 10.0.0.0 to 10.255.255.255 10.0.0.0/8 Single Class A 16,777,216 172.16.0.0 to 172.31.255.255 172.16.0.0/12 16 Contiguous Class B 1,048,576 192.168.0.0 to 192.168.255.255 192.168.0.0/16 Single Class B 65,536 USEFUL NOTE Generally speaking, the first address of a subnet range ending in 0 is used to designate the network address, and the last address ending in 255 is used to designate the broadcast address of the subnet. It depends on the subnet mask being used whether 0 or 255 is assigned to a host. For example, if the whole (continued) 18 Although these address ranges are shown to be contiguous, they may be subdivided (subnetted) into smaller subnet ranges within a private network space. Often, these private network classes can be found using class C 24-bit subnet masks of 255.255.255.0 to form smaller network ranges. 419 Page 419 Edwards 420 Part II ■ c10.tex V3 - 03/27/2009 The OSI Layers USEFUL NOTE (continued) class A subnet of 10.0.0.0 with a subnet mask of 255.0.0.0 is used for a network, then 10.0.0.0 would be the network address and 10.255.255.255 would be the broadcast address for that subnet. This would permit 10.0.1.0, 10.0.1.255, 10.0.2.0, 10.0.2.255, etc., to be used for host addresses. It is important to know the subnet mask that is assigned to a particular IP address range. USEFUL NOTE #2 The CIDR is the number that indicates the number of mask bits being assigned to a subnet. So, /8 would have a subnet mask of 255.0.0.0, /12 would have a subnet mask of 255.240.0.0, and /16 would have a subnet mask of 255.255.0.0. Have you guessed what the CIDR represents yet? The CIDR indicates the number of bits for the subnet mask starting at the highest significant position and working its way down to the least significant position.19 USEFUL NOTE #3 When you see /32 or a subnet mask of 255.255.255.255, it is called a host route. This means there is only one network node connected to that address; there is no network, just one device — and that is it. Host routes are used more frequently than you would think, but be aware of this when someone talks of a ‘‘slash 32’’ or ‘‘32-bit route.’’ Figure 10-9 illustrates three separate networks all performing NAT on the ACRONYM ALERT 192.168.0.0/16 network address space. You will notice that the IP addresses PHY — Physical layer interface assigned to the public interface20 are addresses that are assigned by an ISP (Internet service provider). These addresses can be either statically or dynamically assigned IP addresses. The type of installation usually dictates how IP address assignment is handled. DSL (digital subscriber line), PPPoE (Point-to-Point Protocol over Ethernet), and dialup network circuits are usually configured to have a dynamically assigned IP address. Dynamically assigned addresses 19 Okay, for the readers who fell asleep during math class: The higher the power of the number, the more significance it has. In our number system, the number to the left of another number has a higher power, thus more significance. The leftmost number is always the most significant number. 20 A public interface is one that has a publicly routable IP address assigned to it. Private IP addresses cannot be routed over the Internet. 11:24am Page 420 Edwards c10.tex Chapter 10 V3 - 03/27/2009 ■ 11:24am The Network Layer change each time a connection is made. However, the ISP can usually assign a static IP address, if requested to do so. Internet 72.34.21.168 69.54.112.145 168.34.78.99 192.168.0.0/16 Network 192.168.0.0/16 Network 192.168.0.0/16 Network Figure 10-9 A NAT example Notice that the private IP addresses for all three are in the 192.168.0.0/16 network and that the NAT-enabled router will translate ACRONYM ALERT those addresses to its public IP address. CSMA/CA — Carrier Sense, Multiple Access with Collision Avoidance The receiving node over the Internet will see the public IP address in the packet’s source address. The sending NAT-enabled router keeps a translation table in order to recall which sending workstation on its private IP address space has initiated the session. When the receiving node sends a reply back to the NAT-enabled router, it removes its address from the destination address field of the packet, replaces the IP address from its translation table of the workstation that started the session, and passes the packet into the private network. Since workstations are on a private IP address space, they are not reachable from the Internet unless a policy to allow this is embedded within the NAT-enabled router. Such policies are called port forwarding policies. Usually servers offering web services, e-mail services, or FTP services are located on private networks behind a NAT-enabled router. Figure 10-10 illustrates 421 Page 421 Edwards 422 Part II ■ c10.tex V3 - 03/27/2009 The OSI Layers services being offered to users over the Internet while being located behind a NAT-enabled router on a private IP address space. 192.168.0.0/16 Private IP Address 192.168.0.1 FTP Server Internet NAT Enabled Router 192.168.0.2 Web Server 192.168.0.3 Mail Server Offering both SMTP & POP Mail Services Figure 10-10 Servers behind a NAT-enabled router Table 10-2 is representative of a NAT port forwarding table a NAT-enabled router would have to accept service requests on its public IP address interface. When a packet arrives at the public interface with the destination address set to its public IP address and a port service request that matches a port address in the NAT port forwarding table, the packet is modified and passed on to the network, directed toward the server that supplies that service. An example of this is a web page request that arrives at the public IP address of the NAT-enabled server. The NAT-enabled router sees the requested port is port 80, so it replaces its address in the destination field with the IP address of the web server that is at 192.168.0.2, recalculates the checksum for the packet, and passes it on to the private network. 11:24am Page 422 Edwards c10.tex Chapter 10 V3 - 03/27/2009 ■ 11:24am The Network Layer Table 10-2 NAT Port Forwarding Table SERVICE PORT SERVER ADDRESS SMTP Mail 25 192.168.0.3 POP Mail 109 192.168.0.3 POP3 Mail 110 192.168.0.3 FTP Control 21 192.168.0.1 FTP Data 20 192.168.0.1 HTTP 80 192.168.0.2 NAT does offer some firewall protection since the addresses POP QUIZ used on the private IP address are not routed over the Internet. What is the type of address translation that Unsolicited connection requests is used to keep track of sessions initiated by a computer on a private network to a are dropped by the NATservice on the Internet? enabled router since there is no entry in its NAT translation table. However, when port forwarding policies are enabled within the NAT-enabled router, there is an possibility that one of the servers may be hacked and compromised. A prudent measure would be to have a DMZ (demilitarized zone) by using a router that has multiple private IP address interfaces. Place the servers on one interface isolated by different network addresses and policies. This will prevent the servers from initiating connections into the private network where other users and devices are protected behind the NAT-enabled router. Due to the development of NAT-enabled devices and the capability to use private network IP address space, the stress of coming up with a new standard to replace IPv4 was lessened. This has allowed the life span of IPv4 to be extended and a gradual transition made to the newer IP address standard IPv6. Although current devices are IPv6-capable, they are still able to be installed and used within the IPv4 environment. 10.2.1.2 Internet Protocol Version 6 The real thrust of moving to IPv6 is the larger address space that it provides, with 128 bits dedicated to address space. The number is so large that it exceeds the national debt, which is pretty hard to do these days. Our scientific calculator claims it is 3.4028 e38 , give or take a few addresses. It is so great that each person alive on the face of the earth can have multiple devices using 423 Page 423 Edwards 424 Part II ■ c10.tex V3 - 03/27/2009 The OSI Layers IP addresses and there would still be addresses left over. Although these numbers are staggering, the real intent of IPv6 is to increase the efficiency of network management and routing. There is a high probability that only a small percentage of the address space will actually be used. Figure 10-11 illustrates the IPv6 header, which is 40 bytes in total length. BIT 0 0 4 Version 8 12 16 20 Traffic Class 24 28 32 Flow Label 32 Payload Length Next Header Hop Limit 64 Source Address 192 Destination Address 320 Figure 10-11 The IPv6 header The first 4 bits are the Version field for IPv6. The next 8 bits are the Traffic Class field, which RANDOM BONUS DEFINITION adds in the control options. The encapsulating bridge — A bridge that next 20 bits are the Flow Label encapsulates LAN frames. field, allocated for QoS (quality of service). The Payload Length field indicates the packet length in bytes. When this field is set to 0, the packet contains a jumbo-sized payload. The Next Header field indicates which encapsulated protocol follows. These protocol values are compatible with the IPv4 protocol field values. The Hop Limit field replaces the TTL (time to live) field of IPv4. Both the Source Address and Destination Address fields contain 128 bits of address data. The standard sized payload can be 65,536 bytes, and if the option is set, the payload can be jumbo-sized.21 All data fragmentation is controlled by the sending network node since routers will never fragment a packet. However, IPv6 sending network nodes are expected to use a technique known as path 21 Techno-geeks like to use jargon and you may hear a variety of words used to describe an entity. A ‘‘jumbo-sized’’ payload merely refers to a payload that is very large. 11:24am Page 424 Edwards c10.tex Chapter 10 V3 - 03/27/2009 ■ 11:24am The Network Layer MTU discovery (PMTUD) to determine the MTU that can be used to send packets over the network. The address notation used in IPv6 is quite different from that of IPv4. IPv6 addresses consist of eight groups of four hexadecimal numbers, where each field is separated by a colon. For example: 113A:00AB:8900:0000:0000:7EA3:0034:3347 A shorthand notation would be to reduce the fields conPOP QUIZ taining zeros to just a pair of colons (::). IPv6 address notaWhat is the difference between how IPv4 tion has a variety of rules that IP addresses are denoted and how IPv6 IP addresses are denoted? allow for various methods of displaying the same address. As IPv6 begins to be deployed more widely, there is sure to be a particular notation format that will become the more widely used and accepted format. 10.2.2 Internet Control Message Protocol Internet Control Message Protocol (ICMP) is an essential part of the TCP/IP protocol suite. It provides a means of messaging when a sent datagram is unable to be received by the intended network node. The ping and traceroute networking tools are also part of this protocol. ICMP error messages are generated in response to detected errors in the IP datagram, routing, or diagnostics. The ICMP protocol suite is part of IPv4, but there is an equivalent to ICMP that is a protocol within IPv6, referred to as ICMPv6. For the most part, computer users are unaware of network problems until a network error message is triggered by ICMP. When a user suspects that a network problem may exist, he or she can use the ping and traceroute22 commands to aid in troubleshooting the problem. 10.2.2.1 Ping The ping command within the Microsoft Windows operating system has the following syntax: Microsoft Windows XP [Version 5.1.2600] (C) Copyright 1985-2001 Microsoft Corp. 22 traceroute is the normal command for many various operating systems. However, in the Microsoft Windows-based world, the actual command for traceroute is truncated to tracert. 425 Page 425 Edwards 426 Part II c10.tex V3 - 03/27/2009 The OSI Layers ■ C:\>ping Usage: ping [-t] [-a] [-n count] [-l size] [-f] [-i TTL] [-v TOS] [-r count] [-s count] [[-j host-list] | [-k host-list]] [-w timeout] target name Options: -t -a -n -l -f -i -v -r -s -j -k -w count size TTL TOS count count host-list host-list timeout Ping the specified host until stopped. To see statistics and continue - type Control-Break; To stop - type Control-C. Resolve addresses to hostnames. Number of echo requests to send. Send buffer size. Set Don’t Fragment flag in packet. Time To Live. Type Of Service. Record route for count hops. Timestamp for count hops. Loose source route along host-list. Strict source route along host-list. Timeout in milliseconds to wait for each reply. The most common use of ping is to verify that a particular network is available over the network. In the following example, a computer has issued a ping command for its default gateway. ACRONYM ALERT NIC — Network interface card or network interface controller C:\>ping 192.168.2.1 Pinging 192.168.2.1 with 32 bytes of data: Reply Reply Reply Reply from from from from 192.168.2.1: 192.168.2.1: 192.168.2.1: 192.168.2.1: bytes=32 bytes=32 bytes=32 bytes=32 time<1ms time<1ms time<1ms time<1ms TTL=64 TTL=64 TTL=64 TTL=64 Ping statistics for 192.168.2.1: Packets: Sent = 4, Received = 4, Lost = 0 (0% loss), Approximate round trip times in milli-seconds: Minimum = 0ms, Maximum = 0ms, Average = 0ms There are many ways to use the ping command for troubleshooting various network issues. Chapter 16, ‘‘Troubleshooting,’’ will go into greater detail about how this POP QUIZ What option would be used to modify the size of a ping packet? 11:24am Page 426 Edwards c10.tex Chapter 10 V3 - 03/27/2009 ■ 11:24am The Network Layer tool can be used as an aid in determining what is causing certain network issues. 10.2.2.2 Traceroute The traceroute command is used to trace the path from the sending network node to the receiving network node on a hop-to-hop basis. It reports back on each hop as it is traversed over the network on the path to the destination network node. The following is the syntax for the traceroute command for the Microsoft Windows operating system: C:\>tracert Usage: tracert [-d] [-h maximum hops] [-j host-list] [-w timeout] target name Options: -d -h maximum hops -j host-list -w timeout Do not resolve addresses to hostnames. Maximum number of hops to search for target. Loose source route along host-list. Wait timeout milliseconds for each reply. The options are selfexplanatory. The -d option is often used to save the time that is required to resolve IP addresses to host names for each hop along the path. The following is an example of a successful completion of a traceroute command: RANDOM BONUS DEFINITION common and internal spanning tree — A collection of the internal spanning trees in a multiple spanning tree region, combined with the common spanning tree that connects MST regions to form a single spanning tree that ensures all LANs in the bridge network are fully connected and loop-free. C:\>tracert www.richardbramante.com Tracing route to www.richardbramante.com [68.180.151.74] over a maximum of 30 hops: 1 2 3 <1 ms <1 ms 4ms <1 ms <1 ms 4ms <1 ms <1 ms 4ms 4 3ms 4ms 4ms 5 26ms 27ms 27ms 192.168.2.1 192.168.0.1 L100.VFTTP-12.BSTNMA.verizongni.net [72.74.235.1] P4-1.LCR-04.BSTNMA.verizongni.net [130.81.60.226] so-7-0-0-0.ASH-PEER- 427 Page 427 Edwards 428 Part II ■ c10.tex V3 - 03/27/2009 The OSI Layers 6 7 28 ms 65 ms 27 ms 64 ms 27 ms 64 ms 8 91 ms 92 ms 92 ms 9 92 ms 92 ms 92 ms 10 100 ms 92 ms 92 ms 11 92 ms 94 ms 92 ms RTR2.verizon-gni.net [130.81.17.179] 130.81.14.98 so-2-0-0.pat2.dax.yahoo.com [216.115.96.21] as1.pat2.pao.yahoo.com [216.115.101.130] ae1-p151.msr2.sp1.yahoo.com [216.115.107.79] ge-1-41.bas-b2.sp1.yahoo.com [209.131.32.33] www.richardbramante.com [68.180.151.74] Trace complete. If a target node does not allow ICMP, a traceroute would not end normally and would appear as follows: C:\>tracert www.wiley.com Tracing route to www.wiley.com [208.215.179.146] over a maximum of 30 hops: 1 2 3 3 ms <1 ms <1 ms 4ms 4 5 ms 4ms 4ms 5 6 8 ms 4 ms 7ms 4 ms 7ms 7 15ms 14ms 14ms 8 13ms 14ms 14ms 9 10 16 ms 18 ms 17 ms 17 ms 11 16 ms 17 ms 12 13 14 15 16 18 ms * * * 17 ms * * * ˆC <1 ms <1 ms 4ms <1 ms 192.168.2.1 <1 ms 192.168.0.1 L100.VFTTP-12.BSTNMA.verizongni.net [72.74.235.1] P4-1.LCR-04.BSTNMA.verizongni.net [130.81.60.226] 4 ms 130.81.29.170 0.so-1-0-0.XL2.BOS4.ALTER.NET [152.63.16.141] 0.so-7-0-0.XL4.NYC4.ALTER.NET [152.63.17.97] 0.ge-5-1-0.BR3.NYC4.ALTER.NET [152.63.3.118] 17 ms 192.205.34.49 17 ms tbr1.n54ny.ip.att.net [12.122.105.14] 17 ms gar3.nw2nj.ip.att.net [12.122.105.49] 19 ms 12.88.61.178 * Request timed out. * Request timed out. * Request timed out. 11:24am Page 428 Edwards c10.tex Chapter 10 V3 - 03/27/2009 ■ 11:24am The Network Layer The traceroute command RANDOM BONUS DEFINITION was truncated with the Ctrl+C key combination to shorten the 10BASE-T — A baseband Ethernet system number of hops, as the comoperating at 10 Mbps over two pairs of mand would have continued Category 3 UTP cable. with ‘‘Request timed out’’ for the default number of 30 hops. For further testing, the ping command was then issued with the following results: C:\>ping www.wiley.com Pinging www.wiley.com [208.215.179.146] with 32 bytes of data: Request Request Request Request timed timed timed timed out. out. out. out. Ping statistics for 208.215.179.146: Packets: Sent = 4, Received = 0, Lost = 4 (100% loss), So, it first appears that the target network node is not available, but we know that is not true since our browser displays the following image for that network node address, as shown in Figure 10-12. Although the ping and traceroute commands are very useful tools, they are not 100 perPOP QUIZ cent accurate in predicting what What is the default maximum hop count for is going on in the network. From the traceroute command? the above indications it appears that the Wiley website is dropping ICMP packets. 10.2.3 Internet Group Management Protocol Internet Group Management Protocol (IGMP) is a protocol for handling multicast group memberships, which are required in situations with streaming video or multiplayer games. The protocol is used by the client computer to establish a connection to a local multicast23 router. With the use of local and 23 Multicast is when you have many users connected to a single service simultaneously. It is analogous to the broadcast of a radio or TV program. 429 Page 429 Edwards 430 Part II ■ c10.tex V3 - 03/27/2009 The OSI Layers multicast routers, these streaming applications are able to provide service to many multicast clients simultaneously. Figure 10-12 The Web page for www.wiley.com IGMP-enabled routers check on the users within the group to determine if they have an active session. As long as there is an active group member, the router will continue to forward the multicast to that subnet. If all members are inactive, the packets are simply dropped.24 A computer running an IGMP-based application issues an IGMP report packet to join a group. When the router serving ACRONYM ALERT that group determines it has one member DSAP — Destination service access point of the group, it would forward multicast packets to that subnet. Member computers need not inform the IGMP-enabled router when they leave the group. The IGMP-enabled router will perform member queries at fixed intervals to determine if there are any connected IGMP members. If there are, it continues to forward multicast packets to that subnet. The 24 ‘‘Dropped’’ is a techno-geek word for a packet not being forwarded. The router just discards the packet to work on the next packet that it receives. 11:24am Page 430 Edwards c10.tex Chapter 10 V3 - 03/27/2009 ■ 11:24am The Network Layer reason for this behavior by the IGMP router is to prevent flooding subnets with multicast packets when there are no connected users from that subnet. 10.2.4 Internet Protocol Security Internet Protocol Security (IPSec) uses authentication and RANDOM BONUS DEFINITION encryption to establish a secure connection between endpoint bit time — The length of time required to transmit 1 bit of information. network nodes. The terms VPN and tunneling25 are used along with IPSec; however, IPSec is the means that permits the use of these capabilities. Tunnels between endpoint VPN devices normally are point-to-point and use a preshared key (PSK) as part of the authentication process. Once authenticated, the endpoints are able to pass traffic between them that is encapsulated using strong encryption26 to prevent data from being compromised. Figure 10-13 illustrates the use of IPSec endpoint devices as well as IPSec client workstations establishing VPN network connections over the Internet. In Figure 10-13, Network A and Network B are connected to VPN-enabled routers. These routers use IPSec to establish a peer-to-peer tunnel to allow data ACRONYM ALERT to flow between the private internal netGARP — Generic Attribute Registration Protocol work of Network A and Network B. Peer-to-peer networks know each other’s statically assigned IP address, and that is part of the security mechanism. The major component of safe data transfer is the use of preshared keys with strong encryption. Depending on the policies established on the VPN routers, users from one network can connect to resources on the remote network the VPN tunnel was established with. Another aspect for consideration when conceptualizing a VPN is determining the permissions that will be allowed for network users. Some users might need access to services on the Internet, whereas users might not require this as part of their jobs. VPN routers act as firewalls and are policy-intensive devices. The normal default state for these 25 Tunneling is the term used to describe a virtual protected conduit between two endpoint network nodes. There is no way to actually ‘‘build’’ a real tunnel. The idea is that with strong encryption the packet is undecipherable; thus, it is as if the data stream is traveling within a protected tunnel, unseen to the rest of the Internet. In reality, packets can be snooped, but hacking the real information out of the packet is next to impossible. 26 Encryption depends on key length. There are two predominant key lengths used within the Data Encryption Standard (DES): 56-bit, referred to as simply DES or single DES, and 128-bit, referred to as triple DES or 3DES. 431 Page 431 Edwards 432 Part II ■ c10.tex V3 - 03/27/2009 The OSI Layers devices is to allow only tunnel traffic to pass through from one VPN tunnel endpoint to another. User B Network B Indicates VPN Tunnel Connection Using IPsec Network A User A Figure 10-13 VPN networking using IPSec Remote users using only a computer connected to an Internet access point require a client to be loaded on their PC to establish a VPN tunnel between the computer and a remote VPN router. Once a tunnel session is established, users can launch other applications, which will utilize the connection to gain access to the resources located on the private network protected by the VPN router. The VPN router is capable of setting user policies, either by user or group of users, to limit access to only some of the network’s resources. It is also capable of denying remote users the capability to access the private network and then access the remote private network through the established peer-to-peer tunnel. The use of IPSec to create VPNs using the Internet eliminates the need for direct POP QUIZ point-to-point telecommunicaName two components that help make tions between remote network VPNs safe and secure. nodes. This is a large cost savings over using directly connected dedicated lines between remote office locations. However, careful planning and thought needs to go 11:24am Page 432 Edwards c10.tex Chapter 10 V3 - 03/27/2009 ■ 11:24am The Network Layer into the design of the network, and policies may have to be developed to secure the network from a security breach or the compromise of information as it travels over the Internet. 10.3 Chapter Exercises 1. Name the type of network service being used for each of the following: HTTP FTP Mail Telnet 2. A client/server application is considered to be what type of network service? 3. What is a TLD and can you name a few? 4. How is the MTU size determined? 5. What does NAT accomplish? 6. Name two network tools that can troubleshoot a network problem? 10.4 Pop Quiz Answers 1. Mail is what type of network service? Connectionless 2. Name the ports used by an FTP client to request an FTP session with an FTP server. Which port is used for data transmission? Ports 20 and 21. Port 20 is used for data. 3. Name some top level domain names. .com, .gov, .edu, .net 4. What is a maximum transmission unit? The maximum payload size that can be transmitted without the use of fragmentation. 433 Page 433 Edwards 434 Part II ■ c10.tex V3 - 03/27/2009 The OSI Layers 5. What is the type of address translation that is used to keep track of sessions initiated by a computer on a private network to a service on the Internet? Port mapping 6. What is the difference between how IPv4 IP addresses are denoted and how IPv6 IP addresses are denoted? Dot-decimal notation versus hexadecimal numbers separated by colons. 7. What option would be used to modify the size of a ping packet? The -l option 8. What is the default maximum hop count for the traceroute command? 30 hops 9. Name two components that help make VPNs safe and secure. Authentication and encryption 11:24am Page 434 Edwards c11.tex V3 - 03/27/2009 3:33pm CHAPTER 11 The Data Link Layer Power consists in one’s capacity to link his will with the purpose of others, to lead by reason and a gift of cooperation. — Woodrow Wilson The Data Link layer is Layer 2 of the OSI reference model. This layer allows for direct communication between nodes over the physical channel provided at the lower layer. The communication can be point-to-point (one-to-one communication between two nodes) or point-to-multipoint (one-to-many communication, from one node to many nodes), depending on the nature and configuration of the network. LAN technology exists primarily at the Data Link and Physical layers of the architecture. The functions performed by a network bridge or switch occur mainly at the Data Link layer. Network switches are able to tremendously enhance the capabilities provided by the Data Link layer. This is true to the point where you have to be careful that the implementation of the features doesn’t affect the operations of some protocols within the upper layers. The generic operation performed at the Data Link layer is the movement of data between nodes within a network over a physical connection. Once the Data Link layer has ensured that a connection is set up, the layer divides data into frames and transmits them to other nodes within a network. The receiving node sends acknowledgments and ensures that the data is received by keeping track of bit patterns in the received frames. 435 Page 435 Edwards c11.tex 436 Part II ■ V3 - 03/27/2009 The OSI Layers In this chapter, we discuss the Data Link layer. We cover conRANDOM BONUS DEFINITION cerns that are experienced in a Layer 2 switch — Synonymous with bridge. LAN, as well as some of the mechanisms that are in place to recover from problems. In addition to the operations of this layer, we will discuss the use of switches and bridges in a LAN. 11.1 Concerns of the LAN Most typical network users do not care about all the protocols and mechanisms that are in use to get their data; they just care that they get it. Because you are not the typical network user, however, you should care how this data gets there. Networks of all sizes produce conditions that are less than optimal, so actions have to be taken to address these needs. If there were no way to control the flow of data, the networking world would be a mess. If you worked in an organization that only used several thousand 10/100 Mbps Ethernet hubs, you would find that the end users would be less than satisfied (especially if you consider the types of data that are flowing in a normal LAN). What should you concern yourself with in relation to operations at the Data Link layer POP QUIZ within a network? This question The Data Link layer is what layer of the OSI is the reason this section is in reference model? the book. There are a lot of considerations to be aware of if you want to have an understanding of good old Layer 2. In general, the Data Link layer must provide mechanisms for framing, addressing, and detecting errors in data that is being sent to and fro over the physical link. The framing mechanisms provide a way for the frames to be delimited. Node addressing identifies the source and destination for communication on the LAN. Error detection ensures that only good data is received at the destination and then delivered to the upper layers. In some cases, the Data Link layer discards any errors it discovers, or it may employ a recovery mechanism — it all depends on which action it was designed to do. 11.1.1 It Just Is It’s really hard to compare one LAN to another. There really isn’t anything typical about any LAN. There are similarities in both design and functionality, but each LAN is unique. This LAN is your LAN, if you will. The concerns 3:33pm Page 436 Edwards c11.tex Chapter 11 ■ V3 - 03/27/2009 3:33pm The Data Link Layer of the LAN are fairly typical. There are commonalities that exist in any LAN.1 The purpose and the expected outcome are the same in any LAN. The purpose of the LAN is to provide the avenue needed for communication of data. The expected outcome is for the LAN to live up to its configured expectations. No matter what you do, there are some things about a LAN that are a fact.2 These have remained (and probably always will remain) a constant throughout the lifetime of LANs. A LAN consists of multiple nodes that are attached to a single shared medium. There are geographical distance limitations. Every LAN will have a ceiling on the number of nodes that it supports. A LAN cannot survive without error detection, correction, and recovery. A LAN needs to support broadcasting and multicasting. Like nodes are peers to one another. A LAN is administered locally and is not subject to the same rules that are maintained by networks outside of the LAN. ACRONYM ALERT DA — Destination address These are only a few of the things that most LANs have in common. This is an important list for this chapter because these are what the Data Link layer is all about. 11.1.2 Highs and Lows Another concern for any LAN is to ensure that the highs and lows are met. What do we mean by this? The LAN is there to provide the best possible methods to deliver data over a shared link. This means that the LAN should meet the following expectations: Highs — This is the portion of data communication that you want more of. High throughput3 — The data throughput is simply the rate of error-free delivery of messages within a network. This includes data 1 Believe it or not, there are some network administrators who still do not understand that point. An important thing to remember is that there is technology coming out all the time that not only pushes the limits of the facts of the LAN, but also gives reason for upgrades. 3 There are other terms that mean the same thing, but some of those have multiple definitions. For instance, when we were determining exactly which term to use, we had originally considered using the term ‘‘data rate.’’ Although this would have been perfectly appropriate, it may have been a bit confusing. Data rate is a term that is used to define signaling rate, bit rate, transfer rate, etc. Throughout, we determined, is more specific in this case. 2 437 Page 437 Edwards c11.tex 438 Part II ■ V3 - 03/27/2009 The OSI Layers that is transmitted over a physical or wireless channel, switched through a node, or passed through the portals on both sides of the link. The expectation of the LAN is that the data throughput stays at a level as close as possible to the maximum allowable throughput. This is determined by the configuration and design of the network. High total bandwidth4 — Bandwidth is the available capacity of the physical or wireless channel, and network nodes provide for the delivery of data messages in the LAN. Lows — Things that you know will happen, but you don’t want to happen. Low delays — No delays is optimal, but unlikely. There will be peaks and there will be lulls. You can take action to try to stagger network chores (for instance, you can transfer large amounts of data at night so that it does not affect the times when users are all at work). Delays will occur, but the goal is to have as few as possible. Low error rate — The number of errors in POP QUIZ the network needs to Multiple nodes attached to a single shared stay as low as posmedium can define what? sible. You can take actions to detect and recover from errors, but you want to be as proactive as possible to prevent them from occurring in the first place. AN UNRELATED MOMENT OF PAUSE: FUN TECHNICAL TRIVIA! 1. The Macintosh computer was launched by Apple Computer in 1984, with an ad that played during the Super Bowl. (The Raiders beat the Redskins, 38 to 9.) 2. How many approximate lines of code did the following Microsoft OS original releases have? ■ Windows 3.1 had over 3 million lines of code. (continued) 4 Consider bandwidth as the amount available, and throughput as the actual amount of successful data messages that are transmitted. The throughput normally does not match the bandwidth, as there is other chatter that consumes some of the capacity of the communication channel (Hellos, Acks, etc.). For instance, if the link is a 10 GB Ethernet link, the bandwidth is going to be 10,000 Mbps. The throughput would be the rate of successful messages sent over the link. Of course, this is based on the performance of the network and is variable. 3:33pm Page 438 Edwards c11.tex Chapter 11 ■ V3 - 03/27/2009 3:33pm The Data Link Layer AN UNRELATED MOMENT OF PAUSE: FUN TECHNICAL TRIVIA! (continued) ■ Windows 95 had over 15 million lines of code. ■ Windows 98 had over 18 million lines of code. ■ Windows 2000 had over 35 million lines of code. 3. The computer mouse was invented in 1963 by Dr. Douglas C. Engelbart. 4. The term ‘‘computer’’ was first used to describe a mechanical calculating device in 1897. 5. We all know that 8 bits is called a byte, but did you know that 4 bits is called a nibble? 6. The type of keyboard that we are all familiar with is known as the QWERTY keyboard. This name is derived from the first six letters on the top line. 7. Netscape was the most popular Internet browser until Microsoft released Internet Explorer 4. 11.2 Accessing the Medium We all know that the LAN is made up of nodes connected to one another over a shared POP QUIZ medium. We also know that Define throughput. it is called ‘‘shared medium’’ because everyone shares it for transmitting data. It’s important to cover a few of the rules that must be upheld in a LAN as far as actually connecting to the network. Sure, we have discussed some of this before, but now is a good time for a refresher! 11.2.1 Rules of Accessing the Medium The previous section talked about some of the facts of a typical5 LAN. When dealing with the shared medium, there are some facts as well. Within a shared medium, only one node can successfully transmit data at any given time. 5 Typical is used loosely. 439 Page 439 Edwards c11.tex 440 Part II ■ V3 - 03/27/2009 The OSI Layers Bandwidth is allocated to support the nodes that are sharing the medium so that each node gets a fair amount of bandwidth, with little to none left over.6 The shared medium should support as much throughput as it is intended to handle. The network administrator should ensure that RANDOM BONUS DEFINITION delays are kept to a minGigabit Ethernet — 1000 Mbps Ethernet. imum for data that is transported over a shared medium. A reasonable amount of waste, overhead, and delay should be taken into account when setting up and maintaining the network, and network monitoring will help you ensure that you meet the goals that you set. 11.2.2 From Tokens to Contention So, how exactly do you go about ensuring the bandwidth is distributed fairly to the nodes using the shared medium? There are two methods that can be used in a LAN: tokens and contention. When using the token method, a token is passed from node to node. The nodes then pass data among one another in a round-robin fashion. When the contention method is used, the nodes transmit data when they want to. Therefore, it is entirely possible that two stations send data at the same time, causing a collision) to occur (see Figure 11-1). Collision Figure 11-1 A collision A collision causes datagrams to be dropped, but it doesn’t necessarily mean that the data can’t be recovered in some way. There are mechanisms that can 6 When POP QUIZ Name two methods of ensuring bandwidth is distributed fairly to the nodes that share connectivity within a LAN. allocating bandwidth, it is important to use as much as possible. Some will be used by other processes, so a small amount of waste is possible. 3:33pm Page 440 Edwards c11.tex Chapter 11 ■ V3 - 03/27/2009 3:33pm The Data Link Layer be configured to recover from data loss and even prevent conditions that may cause it. Even so, there is a potential for data loss, so you can consider the token method a guarantee, whereas the contention method is more of a probability. 11.2.2.1 Using the Token Method In Chapter 1 you learned that the token-passing topology consists of a single frame, known as a token, that is passed from one station to the next. When a node wants to pass data, it must wait until it receives an empty token. The node can then add its data to the token and pass it along the way. IEEE 802.5 is the official standard for Token Ring, which is the most common LAN token method in use. A Token Ring topology can be set up physically in either a token ring (Figure 11-2) or a token bus (Figure 11-3) configuration. There is no logical difference between the two methods, as both operate in a token-passing manner. Figure 11-2 A token ring In a token bus configuration, there is a central node called a media access unit (MAU) or a multistation access unit (MSAU). This device is similar to an Ethernet hub, but it has a computer chip that provides the logical ring that the end nodes are concerned with. The benefit of the token bus is that when a node goes down, the ring can be adjusted so that the other nodes will continue to operate on the network. In a physical ring, if a node goes down, the communication for all nodes goes down as well. 441 Page 441 Edwards 442 Part II ■ c11.tex V3 - 03/27/2009 The OSI Layers Figure 11-3 A token bus 11.2.2.2 Using the Contention Method Nodes that use the contention method transmit their data at any time. The first node to get data on the line gets served first. When two nodes transmit at the same time, a collision occurs and the data will be resent. If the network is experiencing a high rate of data at any particular time, there will most likely be a lot of collisions, which will continue until bandwidth availability is restored. Fortunately, some enterprising individuals came up with a way to sense when there is data being transmitted, thus reducing the number of collisions that can occur. Following are the protocols that are used to ensure that data flow in a contention method environment passes as smoothly as possible: Carrier Sense Multiple Access (CSMA) — Allows multiple nodes to be attached to a shared network. Prior to transmission, the nodes listen to see if the shared channel is busy and will transmit when they sense that the channel is not busy. ‘‘Carrier sense’’ simply means that a node is listening to see if it can detect an unused channel. If the node senses that there is a busy channel, it will defer transmission of its data until the channel is idle. ‘‘Multiple access’’ defines the fact that there are multiple nodes accessing the shared medium to transmit data. 3:33pm Page 442 Edwards c11.tex Chapter 11 ■ V3 - 03/27/2009 3:33pm The Data Link Layer Carrier Sense Multiple Access with Collision Avoidance (CSMA/CA) — This is an enhanced version of the CSMA protocol in that it adds collision avoidance as a function. In this type of network, collisions are avoided because the station will not transmit data when it senses the channel is busy. The node will listen to the channel for a defined amount of time, and when the node is ready to send data, it will send a jam signal,7 which lets all the other nodes know that the node is ready to transmit data. Carrier Sense Multiple Access with Collision Detection (CSMA/CD) — This is an enhanced version of the CSMA protocol in that it adds collision detection as a function. This function allows the transmitting node to monitor the channel for other transmissions. If while transmitting a frame, the node detects a signal coming from another node, it will terminate the transmission, send out a jam signal, and then try to send the frame again.8 There are different ways for collisions to be detected, depending on the shared medium that is being used. The most popular and most often used CSMA/CD protocol is Ethernet. 11.3 Meet the Sublayers In order to handle service requests from the network, the Data Link layer is broken into two sublayers: the Logical Link Control (LLC) sublayer and the Media Access Control (MAC) sublayer (see Figure 11-4). ACRONYM ALERT DEC — Digital Equipment Corporation LLC Data Link MAC Figure 11-4 The Data Link layer’s sublayers LLC is the upper sublayer and is responsible for flow control, error control, and multiplexing and demultiplexing data transmitted over the MAC sublayer. The LLC sublayer is the sublayer that serves the higher layer client. LLC does 7A jam signal in CSMA/CD is a message to all other nodes that a collision has occurred and that they should stop transmitting. 8 A random time interval is set that will determine when a station will try to transmit a frame again. 443 Page 443 Edwards 444 Part II ■ c11.tex V3 - 03/27/2009 The OSI Layers not have to worry about the design and functions of the LAN, which allows it to buffer these functions so that the higher layer protocols need not worry about the details but can focus on the tasks at hand. The MAC sublayer is responsible for framing formats and determining which frame is going to be the next to access the shared medium. 11.3.1 Logical Link Control LLC is a protocol developed by the IEEE 802.2 working group and provides three different types of service: LLC Type 1 (LLC-1) — Used for connectionless services. LLC Type 2 (LLC-2) — Used for connection-oriented services. LLC Type 3 (LLC-3) — Used for acknowledgments in conjunction with connectionless services. LLC-1 is used for connectionless services. It is a best-effort POP QUIZ delivery, providing none of the bells and whistles (for instance, The most popular and most often used flow control). LLC-1 provides . CSMA/CD protocol is multiplexing services to the upper layers. LLC-29 is used for connection-oriented services. Because it serves the connection-oriented operations, it does support the bells and whistles (for example, flow control, error control and recovery, call setup, call management, and call termination). LLC-3, which is seldom used, acknowledges frame delivery in a connectionless environment.10 11.3.1.1 LLC Framing LAN source and destination addresses are determined by the MAC sublayer and will be in the MAC header portion of the frame. The LLC PDU11 contains the following fields: Destination Service Access Point (DSAP) — This is used to identify the LLC that is supposed to receive the PDU. 9 Note that nodes that support LLC-2 must also support LLC-1. This is because LLC-2 connections are established from an LLC-1 connectionless session. is used over LLC-1. This provides you with a bit of reliability without having the overhead of LLC-2. In most LANs you will usually only see LLC-1 and LLC-2. This is because many upper-layer protocols provide for recovery and don’t need more than best-effort delivery. 11 In case you forgot, PDU stands for protocol data unit. The PDU is the entity that all information is transferred in within a network. 10 LLC-3 3:33pm Page 444 Edwards c11.tex Chapter 11 ■ V3 - 03/27/2009 3:33pm The Data Link Layer Source Service Access Point (SSAP) — This is used to identify the LLC that is supposed to send the PDU. Control — The Control field provides sequencing data, command information, and responses to requests. Note that any or all of these can be used in any combination. Figure 11-5 shows the format of the LLC header. The LLC header is either 3 bytes or 4 bytes in length, depending on the version of LLC service type that you are using. The DSAP and SSAP are always 8 bytes in length each, which leaves the control field as the variable length field in the PDU. 7 bits Destination address 8 bits 8 bits 8 or 16 bits DSAP SSAP CONTROL 7 bits I 1 bit G Source Address Individual DSAP = 0 Group DSAP = 1 C 1 bit R Command = 0 Response = 1 Figure 11-5 An LLC PDU (LLC header) The DSAP field and the SSAP field are pretty straightforward. The DSAP field is an 8-bit (or 1-byte) field that contains 7 bits for the destination address portion of the field; the additional bit identifies whether it is destined for an individual or group DSAP. The SSAP field is an 8-bit (or 1-byte) field that contains 7 bits for the source address portion of the field; the additional bit identifies whether it is a request or a response to a request. The Control field is a variable length, depending on what type of LLC you are using, and the type of the frame. The three frame formats you will see are: Informational frame (I-frame) — Used with LLC-2 only. This type uses a 2-byte (16-bit) field. Its purpose is to send numbered information transfer in LLC-2. Figure 11-6 shows an example of the format of the I-frame format. 8 bits 8 bits 8 or 16 bits DSAP SSAP CONTROL 0 SSN Figure 11-6 The format of the I-frame P F RSN 445 Page 445 Edwards c11.tex 446 Part II ■ V3 - 03/27/2009 The OSI Layers SSN — Sender sequence number RSN — Receiver sequence number PF — Poll on command frames or Final on response frames Supervisory frame (S-frame) — Used with LLC-2 only. This type uses a 2-byte (16-bit) field. It is responsible for handling acknowledgments, retransmitting requests, and terminating requests of the I-frames in LLC-2. Figure 11-7 is an example of the format of the I-frame format. S — Supervisory function bits PF — Poll on command frames or Final on response frames RSN — Receiver sequence number 8 bits 8 bits 16 bits DSAP SSAP CONTROL 1 0 S S 0 0 0 0 P F RSN Figure 11-7 The format of the S-frame Unnumbered frame (U-frame) — Can be used with all LLC types. This type uses a 1-byte (8-bit) field. It is responsible for unsequenced data transfer and may handle some control functions as well (see Figure 11-8). M — Modifier bits RANDOM BONUS DEFINITION PF — Poll on command frames or Final on response frames frame — The Data Link layer encapsulation of transmitted or received information. 8 bits 8 bits 8 bits DSAP SSAP CONTROL 1 1 M M Figure 11-8 The format of the U-frame P M M M F 3:33pm Page 446 Edwards c11.tex Chapter 11 11.3.1.2 ■ V3 - 03/27/2009 3:33pm The Data Link Layer Subnetwork Access Protocol The Subnetwork12 Access Protocol (SNAP) is used in conjuncPOP QUIZ tion with LLC-1 for the purpose What does DSAP stand for? of upward multiplexing to more upper-layer protocols than what is available with the standard LLC 8-bit SAP fields. When SNAP is not in use, the LLC DSAP’s 8-bit field provides support of multiplexing to a maximum of 256 clients. Because the DSAP field reserves half its space for group SAPs, you actually can only multiplex to 128 clients. As far as the PDU goes, the SNAP header is placed directly behind the LLC header in the PDU. If SNAP encapsulation is being used, the DSAP and SSAP fields will be set to 0xAA, which indicates that SNAP is being used and that there is a SNAP header in the PDU. See Figure 11-9 for an example of SNAP encapsulation. 8 bits 8 bits 8 bits 24 bits 16 bits DSAP SSAP CONTROL SNAP OUI SNAP PID LLC Header SNAP Header Figure 11-9 SNAP encapsulation The fields in the SNAP header are as follows: SNAP OUI — This is a 24-bit field that contains the organizationally unique identifier (OUI). The OUI identifies the organization that the PID is assigned to. SNAP PID — This is a 16-bit field that contains the protocol identifier (PID), which identifies the upper-layer protocol that the PDU is destined for. ACRONYM ALERT EIA — Electronic Industries Association Here’s the clincher: SNAP encapsulation allows you13 to have up to 65,536 upper-layer protocol identifiers.14 12 It’s important to note that the term ‘‘subnetwork,’’ in the SNAP sense, does not have anything to do with a subnetwork in a TCP/IP sense. This is one of those acronyms that may have actually come before the term. It’s nothing more than a way to make SNAP have that fancy ring that we ‘‘catenet’’ lovers like. 13 In saying you, we are referencing the applicable organization. 14 This simply blows the 256 (if you are lucky) identifiers out of the water. 447 Page 447 Edwards c11.tex 448 Part II ■ V3 - 03/27/2009 The OSI Layers A LITTLE MORE ABOUT THE OUI The OUI is a 24-bit number that can be purchased from the IEEE. The number is unique to an organization (vendor, company, etc.) and serves several purposes. Many protocols reference the OUI (SNAP, for instance). Some even append a few bits to increase the functionality of the OUI. There are a lot of other terms that are used for the OUI. It is also known as a MAC address (more on this in the following section), vendor ID, NIC address, and many more. Here is a list of a few OUIs that are assigned today. Note that these are globally assigned, which is why they are unique for that particular organization. Also note that often the same company can be assigned multiple OUIs, regardless of the location they are registered to (as in the case of Nortel Networks). 00-00-C0 (hex) Western Digital Corporation 0000C0 (base 16) Western Digital Corporation 8105 Irvine Center Drive Irvine, CA 92718 United States 00-0C-41 (hex) Cisco-Linksys 000C41 (base 16) Cisco-Linksys 121 Theory Drive Irvine, CA 92612 United States 00-0D-54 (hex) 3Com Ltd. 000D54 (base 16) 3Com Ltd. Peoplebuilding 2 Peoplebuilding Estate Maylands Avenue Hemel Hempstead Hertfordshire HP2 4NW United Kingdom 00-0D-56 (hex) 000D56 (base 16) Dell PCBA Test Dell PCBA Test One Dell Way RR5 MS-8545 (continued) 3:33pm Page 448 Edwards c11.tex Chapter 11 ■ V3 - 03/27/2009 3:33pm The Data Link Layer A LITTLE MORE ABOUT THE OUI (continued) Round Rock, TX 78682 United States 00-0E-40 (hex) Nortel Networks 000E40 (base 16) Nortel Networks 8200 Dixie Road Suite 100 Brampton, Ontario L6T 5P6 Canada 00-1F-9A (hex) Nortel Networks 001F9A (base 16) Nortel Networks 2221 Lakeside Boulevard Richardson, TX 75082-4399 United States 00-23-0D (hex) Nortel Networks 00230D (base 16) Nortel Networks 2221 Lakeside Boulevard Richardson, TX 75082-4399 United States This list is an example and is only a short list compared to all the OUIs that are registered. If you want to see a complete list, you can view it on the IEEE website (http://standards.ieee.org). 11.3.2 The MAC Sublayer The MAC sublayer is responsible for interfacing between the LLC sublayer and Layer 1, the Physical layer. The MAC sublayer provides access control as well as addressing for the PDU. This sublayer is what makes multipoint communication with a LAN/WAN a reality. This sublayer is also able to operate as a full-duplex logical channel in a LAN. This logical channel supports unicast (point-to-point) services, multicast services (point-to-multipoint), and broadcast (point-to-multipoint) services. All these services are discussed in Section 11.4. 449 Page 449 Edwards c11.tex 450 Part II ■ V3 - 03/27/2009 The OSI Layers The MAC sublayer uses a POP QUIZ MAC address,15 the address assigned to the node’s network What does SNAP stand for? adaptor (commonly, a NIC). For channel access, the MAC sublayer employs some control functions that allow multiple nodes to use the same physical medium. We discuss both the MAC address and the channel access control functions next. 11.3.2.1 The MAC Address The IEEE 802 MAC address is a 48-bit address that is used to identify the network adaptor for a particular node or interface in the network. The MAC address was originally designed as a permanent address that is unique to the adaptor it is assigned to. Most hardware today allows a MAC address manipulation method known as MAC spoofing. That tidbit of trivia is informational, but we won’t be going into the details, as it is beyond the scope of this book. The format of the IEEE 802 MAC address is set up to make it as easy as possible to understand. It consists of six groups of two hexadecimal digits. The groups are separated by either a colon (:) or a hyphen (-). Following is an example of each method: 01:00:23:00:bf:00 01-00-23-00-bf-00 In these examples, the OUI would be 01:00:23, with the remainder being the NIC-specific identifier. Combined, they make up the MAC address. MAC addresses can be administered both universally and RANDOM BONUS DEFINITION locally. When the address is administered universally, the error control — A procedure used to MAC address is assigned to the recover from detected errors. interface by the device’s manufacturer. Locally administered addresses are manipulated by a network administrator for purposes that serve the needs of the LAN. 11.3.2.2 Access Control for the Channel The MAC sublayer is responsible for ensuring that multiple nodes are able to connect to and share the same physical medium. The groups of protocols that operate and perform this function are known as multiple access protocols (MAP). 15 The MAC address is also referred to as a physical address. 3:33pm Page 450 Edwards c11.tex Chapter 11 ■ V3 - 03/27/2009 3:33pm The Data Link Layer These protocols detect and avoid collisions in contention environments and ensure that there are enough resources to set up a logical connection when needed. Remember, earlier in this chapter we said that the most popular contention method used is Ethernet. 11.4 The ‘‘ings’’ — Casting, Detecting, and Addressing LAN traffic flow is a fairly simple process. There are a lot of standards and configuration options in a LAN that provide a lot of freedom to configure and maintain in an attempt to reach the maximum highs and minimum lows that we discussed in Section 11.1.2. None of this would really mean anything if we didn’t have a way of getting the data from the upper layers and making sure it reaches the appropriate process on the other end of the link. Well, we do have a way to do all of this in one very helpful and handy layer — Layer 2! It is important to keep in mind that some of the ‘‘ing’’ operations occur at other layers of the OSI reference model (upward/downward multiplexing, data link multicasting vs. IP multicasting, etc.). Unless otherwise stated, this section pertains only to the processes at the Data Link layer. This section covers MAC addressing and end-to-end delivery of data to a single node as well as multiple nodes. 11.4.1 Data Link Addressing Main Entry: ad.dress16 Function: noun, verb, -dresses or –drest, -dress.ing. 1. a direction as to the intended recipient, written on or attached to a piece of mail. 2. the place or the name of the place where a person, organization, or the like is located or may be reached: What is your address when you’re in Des Moines? 3. to direct (data) to a specified location in an electronic computer. Directing data is what addressing is all about. At the Data Link layer, this is done by pointing PDUs to the destination MAC address for delivery of a frame within a LAN. The MAC address is the number that is assigned by the manufacturer of a NIC or a network interface. In Figure 11-10, you 16 Dictionary.com ACRONYM ALERT IC — Integrated circuit Unabridged (v 1.1). Random House, Inc. April 18, 2008. 451 Page 451 Edwards c11.tex 452 Part II ■ V3 - 03/27/2009 The OSI Layers can see a group of individuals sharing a physical medium. If Bob needs to send anything to Larry, he simply enters the MAC address (01:bb:04:af:00:1f) that is assigned to the NIC card on Larry’s PC in the frame and sends it toward Larry’s PC. Bob George Lilly 01:bb:04:af:00:1f Sue Larry Figure 11-10 Data Link layer frame delivery That sounds simple, doesn’t it? But what we haven’t really discussed is how Bob’s PC learned the MAC address of Larry’s PC. We also need to cover how Larry’s PC knows how to get back in touch with Bob’s. It wasn’t until the early 1980s’ PC boom that there was really a need to formulate addressPOP QUIZ ing that could be learned in What does SSAP stand for? a dynamic fashion and could support several hundred nodes. Prior to the PC boom, there were not more than a few nodes in a network, and addressing was locally assigned and administered. In a network of only a few nodes, it was easy to maintain networks in this manner. Now, however, with hundreds of nodes communicating with hundreds of networks with hundreds of nodes, there is a real need to have a way to bridge traffic that is easy to administer. For now, and way into the future, LANs will continue to evolve and expand geographically as well as in technical achievements that are, and will probably remain, a constant. 11.4.1.1 The MAC Address Format The MAC header of a frame contains the destination and source MAC addresses for the interfaces involved in the communication stream. Figure 11-11 shows the 48 bits that make up the MAC address. 3:33pm Page 452 Edwards c11.tex Chapter 11 ■ V3 - 03/27/2009 3:33pm The Data Link Layer Organizationally assigned identifyer OUI I G GL Individual/Group bit (I/G bit) Globally/Locally administered bit (G/L bit) Figure 11-11 The MAC address format Regardless of whether it is the source or destination address, the format is the same for all but the first bit. When referring to the destination address field, the first bit (the I/G bit) identifies whether the destination target is an individual (unicast) or a group (multicast). The source address field only uses the first bit when using Token Ring or FDDI. When used, it identifies if there is any source routing data in the frame. The second bit in the source and destination address field indicates whether the address is globally or locally unique. This bit is called the G/L bit and it identifies whether the organizational assigned identifier17 is globally unique (G/L bit set to 0) or locally unique (G/L bit set to 1). If it is a locally unique identifier, then the address is unique only to the LAN. In any given LAN, there can be a mix of both globally and RANDOM BONUS DEFINITION locally unique addresses. The nodes within that LAN do not edge switch — A switch that is have to worry about whether an implemented at the boundary of a VLAN-unaware segment and a identifier matches theirs that is VLAN-aware segment of a LAN. in another LAN. This is because LAN-to-LAN communication is handled at the Network layer and the IP addressing scheme negates this concern. Nodes within a LAN cannot directly communicate at the Data Link layer with nodes in other LANs. Therefore, it is possible to have a duplicate locally assigned MAC, but they will not be aware of one another. 11.4.1.2 Unicast Addressing A unicast address is simply the address of a particular node’s interface within the LAN. The unicast address is the MAC address that is assigned to a device 17 The last 24 bits of the MAC address. 453 Page 453 Edwards c11.tex 454 Part II ■ V3 - 03/27/2009 The OSI Layers or an interface within the LAN. Unicasting is the act of sending a frame from one source node to a single destination node. Figure 11-12 shows an example of unicasting. 00:01:af:21:ab:11 00:01:af:21:ab:00 Figure 11-12 Unicasting The figure shows the server farm sending data to a single node on the LAN. The unicast address of the source is 00:01:af:21:ab:11, which is the MAC address of the interface on the source side of the transmission. The destination unicast address is the MAC address of the interface used by the destination node — in this case, 00:01:af:21:ab:00. All transmitted frames during the session will use the same destination and source unicast addresses. 11.4.1.3 Multicast Addressing Multicasting18 is the act of sending a message to multiple nodes. Multicasting can be handled at the Layer 3 level (IP multicasting) or at Layer 2 (Ethernet multicasting). This section will focus on Layer 2 multicasting; the Layer 3 multicasting was discussed in Chapter 10. 18 A type of multicast that you might come across in a LAN is the broadcast, which is destined for everyone in the network. Often called the ‘‘all F’s’’ MAC address, the broadcast address is always ff:ff:ff:ff:ff:ff. Table 11.2 shows how this address maps to various protocols over Ethernet. 3:33pm Page 454 Edwards c11.tex Chapter 11 ■ V3 - 03/27/2009 3:33pm The Data Link Layer Nodes that participate in a multicast group will be related in some logical fashion.19 Multicast addresses are group addresses of nodes within a shared internetwork. Multicasting provides the ability for multiple nodes to receive data sent from a single transmission. Figure 11-13 shows an example of multicasting. 00:01:af:21:ab:11 Figure 11-13 Multicasting Notice that in the figure not all nodes are receiving the transmission that is being sourced from the server farm. This is because not all of the nodes are in the same multicast group. Also notice that the source address for the originator will be a unicast address.20 When a node decides to join a multicast group, it needs to determine if a received ACRONYM ALERT frame is a unicast or a multicast frame. NIC cards are configured to recognize when a LACP — Link Aggregation Control Protocol frame is unicast and when it is not. How is this done? Remember the I/G bit that we discussed in Section 11.4.1.2? This is the bit that identifies if the frame is a unicast (I/G bit set to 0) or multicast (I/G bit set to 1). 19 The multicast will be sent only to those stations that share the function that requires them to receive the message. The stations that are not applicable won’t be bothered. 20 This is because there is only one source node involved. 455 Page 455 Edwards c11.tex 456 Part II ■ V3 - 03/27/2009 The OSI Layers Table 11-1 shows some well-known multicast MAC addresses that are used by Ethernet. Table 11-1 Ethernet Multicast MAC Addresses ADDRESS TYPE FUNCTION 01:80:C2:00:00:00 Length field Spanning tree BPDU 09:00:07:FF:FF:FF Length field AppleTalk Multicast 09:00:07:00:00:FC Length field AppleTalk Zone Multicast 09:00:2B:00:00:03 8038 DEC LanBridge Hello packet 09:00:2B:00:00:0F 6004 DEC LAT 09:00:2B:00:00:00 8038 DEC LanBridge copy packet 09:00:2B:00:00:01 8038 DEC LanBridge Hello packet 09:00:4EL00:00:02 8137 Novell IPX AB:00:04:04:00:00 6003 DECnet Phase IV router Hello packets AB:00:00:03:00:00 6003 DECnet Phase IV end node Hello packets CF:00:00:00:00:00 0900 Ethernet configuration test Broadcasting is really nothing more than multicasting to everyone in the LAN. Table 11-2 shows some of the various types and functions performed in the broadcast message. POP QUIZ A address is simply the address of a particular node’s interface within the LAN. Table 11-2 Ethernet Broadcast MAC Addresses ADDRESS TYPE FUNCTION FF:FF:FF:FF:FF:FF 0600 XNS hello packets FF:FF:FF:FF:FF:FF 0800 IP FF:FF:FF:FF:FF:FF 0806 ARP FF:FF:FF:FF:FF:FF 8035 Reverse ARP FF:FF:FF:FF:FF:FF 809B Ethertalk 3:33pm Page 456 Edwards c11.tex Chapter 11 ■ V3 - 03/27/2009 3:33pm The Data Link Layer 11.4.2 Error Detection Frames are either fixed-length PDUs (ATM uses a fixed-length PDU) or bit-oriented, which is more common and is what we discuss in this book. Regardless of the frame type, errors can occur in the LAN, and frames can disappear, duplicate, and even become corrupted on their way to a destination. An error in a length-type frame can cause the frame to terminate and skew the beginning of a new frame. Likewise, a bit can be set incorrectly in a bit-oriented type frame, which can cause duplication and even deletion of the frame. Errors can be caused by numerous reasons, environmental as well as traffic-related. Electrical interference can cause noise on the physical medium, which can corrupt the bits in the frame. Other causes of transmission errors include: Signal distortion Synchronization issues Crosstalk Errors will occur and there are acceptable error rates that are figured into any ACRONYM ALERT LAN design. Excessive errors are not good. Depending on the protocols in use, errors TB — Transparent bridge can cause transmission delays, and if not handled correctly, the problem can propagate itself, causing sluggishness and possible outages in the LAN. This is why you need a way to detect and possibly correct errors at the Data Link layer level (as well as some protocols within other layers). There are two methods of error detection used at Layer 2, parity check and cyclic redundancy check (CRC): Parity check — The simplest of the error-checking methods. This method adds a bit to a string of bits to ensure that the total number of 1s in the string is equal to an even or an odd number. For example: Odd parity — 01010101 + 1 parity bit = 010101011. Notice that the total number of 1s is an odd number. An POP QUIZ What is the Ethernet standard broadcast MAC address? 457 Page 457 Edwards 458 Part II ■ c11.tex V3 - 03/27/2009 The OSI Layers odd parity bit is always set to 1 if the total number of 1s in the string (before the parity bit is considered) is an even number. By adding 1 to the even number, it ensures that the number is odd, which matches the type of parity in use in this case. Figure 11-14 shows an example of data transmission using odd parity. Node A 0011 0 0 1 1 1 Node B 0 0 1 1 1 0+0+1+1+1 Figure 11-14 Odd parity In Figure 11-14, node A wants to send the data stream 0011 to node B. Node A computes the value of the data stream (0+0+1+1)21 and because odd parity checking is being used, node A turns the parity bit on to 1 before it transmits the data. Node B then receives the data and computes the overall value (0+0+1+1+1), which is an odd value. Odd Parity is in use, so node B reports a good frame received. Even parity — 01010100 + 1 parity bit = 010101001. Notice that the total number of 1s is an even number. An even parity bit is always set to 1 if the total number of 1s in the string (before the parity bit is considered) is an odd number. By adding 1 to the odd number, it ensures that the number is even, which matches the type of parity in use in this case. Figure 11-15 shows an example of data transmission using even parity. 21 When does 1 + 1 = 1? When dealing with binary, when you are either on (1) or off (1). 3:33pm Page 458 Edwards c11.tex Chapter 11 ■ V3 - 03/27/2009 3:33pm The Data Link Layer Node A 0011 0 0 1 1 0 Node B 0 0 1 1 0 0+0+1+1 Figure 11-15 Even parity In Figure 11-15, node A wants to send the data stream 0011 to node B. Node A computes the value of the data stream (0+0+1+1) and because even parity checking is being used, node A does not turn on the parity bit before it transmits the data. Node B then receives the data and computes the overall value (0+0+1+1+0), which is an even value. Even parity is in use, so node B reports a good frame received. Finally, let’s take a look at the parity check when an error has occurred. Figure 11-16 shows an example of a data stream that is being sent using even parity. Notice that an error occurs before the stream reached the destination. When node B receives the data, it counts the number of 1s and notices that there is an odd number, therefore realizing that an error has occurred. Cyclic redundancy check (CRC) — Also known as the frame check sequence (FCS). The CRC is a function used to detect common errors that may occur during data transmission. CRC is a much more complex method of error checking than the parity check method, but it isn’t necessarily complicated. The way the CRC method works is that the node that is transmitting the frame adds a value, known as a checksum, to the message that is being transmitted, The receiver uses the CRC method to calculate the checksum on its end and compares it with the checksum that was added by the transmitting node to determine if there was any corruption along the way. Figure 11-17 shows an example of a simple checksum. 459 Page 459 Edwards c11.tex 460 Part II ■ V3 - 03/27/2009 The OSI Layers Node A 0011 0 0 1 1 0 —***Error***— Node B 0 0 1 1 1 0+0+1+1+1 Figure 11-16 A parity error Node A 61 120 4 Checksum matches on both ends 61 120 4 185 Node B Checksum 61 + 120 + 4 = 185 Figure 11-17 A simple checksum For simplicity sake, this example uses decimal notation. Each decimal number represents a byte of data in a message. This means that there are 256 possibilities in each byte. The checksum algorithm in use simply adds ACRONYM ALERT POP — Point of presence or Post Office Protocol 3:33pm Page 460 Edwards c11.tex Chapter 11 ■ V3 - 03/27/2009 3:33pm The Data Link Layer the value of all the bytes and uses the combined value as the checksum. In Figure 11-17, node A is sending the message 61----120----4. Node A adds the total number of bytes and appends the checksum to the message (61 + 120 + 4 = the checksum of 185). Node B receives the message and adds the value of the bytes in the message (61 + 120 + 4). By adding the total numerical value, node B determines that the checksum should be 185. Once node B determines the value that it thinks it should be, it compares its checksum with the checksum of node A. If there is a match, it knows the message was received intact. Next, let’s take a look at how the simple checksum works when an error occurs. Figure 11-18 shows such an example. Node A 61 120 4 Checksum error 61 120 4 185 —Error— Node B 61 + 120 + 1 = 182 Figure 11-18 Checksum failure In this example, node A RANDOM BONUS DEFINITION is sending the message 61----120----4. Node access domain — The collection of nodes A adds the total number that share a network segment among which MAC arbitration can occur. of bytes and appends the checksum to the message (61 + 120 + 4 = the checksum of 185). Notice that somewhere between node A and node B, there is an error that causes the last digit of the message to change from a 4 to a 1. 461 Page 461 Edwards c11.tex 462 Part II ■ V3 - 03/27/2009 The OSI Layers Node B receives the message and adds the value of the bytes in the message (61 + 120 + 1). By adding the total numerical value, node B determines that the checksum should be 182. Once node B determines the value that it thinks it should be, it compares its checksum with the checksum of node A. In this example, node B recognizes that the message was corrupted (185 does not equal 182), so an error occurs. The simple checksum used in the example above would not be that reliable. There are too many possibilities of errors occurring with the checksum still intact at the opposite end. For instance: 61 + 120 + 4 = 185 51 + 130 + 4 = 185 60 + 120 + 5 = 185 CRC computes the checksum by using an algoPOP QUIZ rithm that is basically What is the simplest of all error-checking long division for binary. methods? Additionally, the CRC uses the first 16 bits in the calculation, creating 65,536 possibilities. (The chances of an erroneous calculation is far less than with the simple example above.) Taking it a step further, the remainder (not the quotient) is what is used as the checksum. Let’s assume that an originator wants to send the first 2 bytes of data used in the example above (61 and 120). Also assume that the CRC divider will be a constant 1-byte divider whose value would equal the decimal number 7. So, now we want to convert these numbers to binary: 61 = 00111101 120 = 01111000 7 = 00000111 Take a look at Figure 11-19, which shows that node A is sending a message to node B. The message is binary 00111101 01111000. The CRC constant divisor is set to binary 00000111. You want to take the message and divide it by the divider. The remainder is your checksum value.22 22 Calculated out (we cheated and used a scientific calculator), the remainder in this case would be binary 00000110 and that is what will be used as the checksum value. 3:33pm Page 462 Edwards c11.tex Chapter 11 Node A ■ V3 - 03/27/2009 3:33pm The Data Link Layer Node B 00111101 01111000 Figure 11-19 The CRC function CRC algorithms more RANDOM BONUS DEFINITION commonly use a method that closely resembles disabled state — A state used in the polynomial arithmetic. Spanning Tree Protocol that identifies a Instead of a simple divisor, bridge port that has been set to not receive message value, quotient, or transmit any frames. and remainder, as you saw in the last example, these integers are actually seen as polynomials with a binary coefficient. There are many ways to take this even further but are beyond the scope of this book. Really, we could fill pages with the different algorithms that can be used. Therefore, we bring this section to a close. What you really need to understand is that the CRC uses a checksum that can be complex and is used to validate data integrity in a LAN. Now, all this may sound complicated, but it is fairly simple. To show how simple it is, we want you to take a moment away from the book and relax. Just clear your thoughts and take a break from all of this technical mumbo jumbo and relax. And what better way of relaxing than eating a pizza? AN UNRELATED MOMENT OF PAUSE — DENISE’S PESTO CHICKEN PIZZA If you are a fan of interesting foods, you are bound to love this pizza recipe. It is a super-easy meal to make and well worth the time it takes to make it. Just be careful if you have an allergy to nuts, as pesto contains pine nuts and you may have a reaction. Note: Delivery is also an option, but they won’t have this recipe. Ingredients: ■ Refrigerated pizza dough ■ Pesto sauce ■ Cooked chicken, cubed or shredded (continued) 463 Page 463 Edwards c11.tex 464 Part II ■ V3 - 03/27/2009 The OSI Layers AN UNRELATED MOMENT OF PAUSE — DENISE’S PESTO CHICKEN PIZZA (continued) ■ Fresh mozzarella (that doesn’t mean shredded!) ■ Sun-dried tomatoes in oil (rinsed) ■ Rosemary ■ Olive oil Directions: 1. Preheat oven to 425◦ . 2. Roll out the pizza dough, brush on olive oil, and sprinkle with rosemary. 3. Bake for about 5 minutes. 4. Remove the pizza and spread on the pesto, chicken, and sun-dried tomatoes. Top with mozzarella. 5. Bake 10 to 15 minutes, or until the cheese is melted. 11.4.3 Control of the Flow Flow control is used to prevent the sender of data from sending more data than the receiver can handle. Without flow control, the sender would not be aware that the receiver can’t accept any more data and would continue to send the data, only to have to send it again once they are aware there is a problem. There are different methods of flow control that can be used. Sometimes it is medium dependant, but there are options that work with higher-layer protocols. The receiving node does not necessarily have to provide feedback when it can or cannot accept more data. Ethernet uses what are known as PAUSE frames for flow control. A PAUSE frame is a message sent by a receiver to the sender, letting the sender know that the receiving node can no longer receive data and that the transmission needs to be paused for a specified period of time. The PAUSE function only works within full-duplex environments.23 The PAUSE function has a reserved multicast MAC address of 01-80-C2-00-00-01. This is a MAC address that was set up by the IEEE and is used for the MAC PAUSE frame function. 23 Because this is the most common standard in today’s networks, we decided to focus on the PAUSE function in our discussion of flow control. Understand that, from a data link perspective, flow control is a function that prevents a sender from overloading a receiver. 3:33pm Page 464 Edwards c11.tex Chapter 11 11.5 ■ V3 - 03/27/2009 3:33pm The Data Link Layer ‘‘Knode’’ the LAN We assume that you are all thinking, ‘‘What in the heck is knode?’’ A knode is fictional, simply a term that we created as a combination of ‘‘know’’ and ‘‘node.’’ This may be a bit silly, but it is also good food for thought. Knowing your LAN is every bit as important as having the nodes you need to do what you want in the LAN. In Chapter 3, ‘‘Network Hardware and Transmission Media,’’ we introduced bridges and switches, two types of hardware that operate at the Data Link layer. We are going to finish this chapter by talking about bridge/switch deployment within the LANs. Although only an overview, this section should be a great lead-in to Part III of the book, which deals with network design and implementation. If you are interested in getting a good reference book, Jim’s last book,24 The All-New Switch Book: The Complete Guide to LAN Switching Technology, is a comprehensive reference to everything network switching. So what is different between a bridge and a Layer 2 switch? The answer to this question may surprise you. There is no functional difference between a bridge and a switch. That’s right! None! Nada! Zero! Zip! Switch is nothing more than a marketing term that came out in the 1990s. The change was brought about due to the ever-growing LANs. Original bridges could not offer wire speed transmission rates on more than two ports within the bridge. Bridges that could handle the higher rates were still not that reliable and carried a very high price tag. This all changed when the application-specific integrated RANDOM BONUS DEFINITION circuit (ASIC)25 was developed. Along with improvements in chassis switch — A switch that is designed in a modular fashion. This type of switch system memory and higher proconsists of a chassis and multiple plug-in cessor speeds, the ASIC allowed modules. the bridge to be developed, supporting a lot of ports that were capable of concurrent wire speed transmissions. The best part was that the cost was less than traditional bridges with the same number of ports per area, but not transmission speeds. 24 This is more than just a shameless plug – it’s a really good book. ‘‘a sick.’’ 25 Pronounced 465 Page 465 Edwards c11.tex 466 Part II ■ V3 - 03/27/2009 The OSI Layers These new devices were introduced to the world and the salespeople of the world decided to call them ‘‘switches,’’ and that was where the switch was born.26 For simplicity sake, we will use the term ‘‘bridge’’ for the remainder of this section. Feel free to substitute the word ‘‘switch’’ if you are so inclined. Two methods27 of address-to-port mapping are used: Source route bridging — This type of bridging is used in a source-routed internetwork. The path to a destination is determined by the end nodes, not by the bridge itself. An example of an environment that uses source route bridging is Token Ring. Transparent bridging — This is the type of bridging that is used ACRONYM ALERT in Ethernet (and others).28 In a RISC — Reduced instruction-set computer transparent bridging environment, the bridge makes the path determinations and the end nodes are not aware of decisions that are being made. They simply throw the data to the bridge and leave the decision making up to it. Let’s take a moment to look at a bridge in a network segment and how the bridge learns and gets the data to and from a set of endpoint nodes. Don’t be disappointed if there is not enough meat and potatoes in this section; we will discuss node implementation in further depth in the upcoming parts of this book. As a matter of fact, network design and implementation are up next. 11.5.1 Diary of a Network Bridge A bridge is a device that operates much like a repeater or a hub, but it makes data forwarding decisions that bridges traffic from one network segment to another. A network segment is simply a group of nodes that are connected to one another via a shared medium (see Figure 11-20). The only limitations to the number of network segments the bridge can connect would be the number of ports the bridge physically has. In order for a bridge to operate correctly, each node that connects to a segment that is connected to the bridge must have a globally unique MAC address.29 The bridge will have at least one interface that connects to the network segment 26 The term ‘‘switch’’ is often used when a new product is marketed. Some examples of a node that is called a switch but is nothing like a bridge include Layer 3 switching, routing switch, and application switch. 27 You may need to run the two types together if you are running a mixed environment. This is supported (thank goodness). 28 This is also the type that we will focus on (in keeping in line with our focus on Ethernet). 29 You wouldn’t want to introduce confusion when you have the same locally assigned MAC address in a node in two different network segments. 3:33pm Page 466 Edwards c11.tex Chapter 11 ■ V3 - 03/27/2009 3:33pm The Data Link Layer that it knows about, and it will build a table that maps the globally assigned MAC addresses to the port or interface that the bridge has determined the MAC can be reached through. Network Bridge Figure 11-20 A bridge connecting three network segments The bridge operates in promiscuous mode, which means it will take each frame that it receives (regardless of the destination MAC address). The switch then will use information in the frame to make a decision on which segment a MAC address belongs to. In Ethernet, the source and destination MAC address is the information the bridge uses. Figure 11-21 shows an example of two network segments that are able to communicate via an Ethernet bridge. Notice that the bridge has learned the MAC address of each of the nodes and has logged it in the MAC address table, along with the port that it knows it has to go through to reach the MAC. 11.5.1.1 Unicast Operation Earlier in this chapter, we said that unicasting is the act of sending a frame from one source node to a single destination node. Now let’s take a deeper look into what happens in multicast operations. The bridge receives frames on any active interface. The bridge then reviews the frame, looking at the destination and the source MAC addresses. It checks the MAC table to see 467 Page 467 Edwards c11.tex 468 Part II ■ V3 - 03/27/2009 The OSI Layers if it knows the destination and forwards the frame to the destination.30 The bridge follows the rules of the network protocols that are in use (for instance, the rules of CSMA/CD, flow control, congestion control, waiting for the token, etc.). Another thing that a bridge does when forwarding the frame is to use the source node’s MAC address as the outbound interface address, instead of its own. This keeps the bridge transparent to the end nodes and reduces the computations necessary when receiving and retransmitting a frame. Figure 11-22 shows an example of frame forwarding. Node address 08:00:58:6C:00:1B 08:00:58:6C:00:C5 08:00:58:6C:00:09 08:00:58:6C:00:C2 08:00:58:6C:00:41 08:00:58:6C:00:95 08:00:58:6C:00:AB 08:00:58:6C:00:01 Port 1 1 1 1 2 2 2 2 08:00:58:6C:00:41 08:00:58:6C:00:09 08:00:58:6C:00:C5 08:00:58:6C:00:95 Network Bridge 08:00:58:6C:00:02 08:00:58:6C:00:AB 08:00:58:6C:00:C2 08:00:58:6C:00:1B Figure 11-21 The operation of a bridge — mapping the addresses to the interface they belong on You can see that the source node contains a MAC address of 08:00:58: 6C:00:09, and it is sending a frame to MAC address 08:00:58:6C:00:AB. The bridge receives the frame, noting that the destination MAC address is 08:00:58:6C:00:AB. Looking at the address table, the bridge knows that MAC address is reachable via port 3, and the bridge forwards the frame on, leaving itself transparent by identifying itself with the source MAC of the destination. 30 When the bridge sees a source MAC address that it does not currently have in its address table, it will add the information at that point. 3:33pm Page 468 Edwards c11.tex Chapter 11 Node address 08:00:58:6C:00:1B 08:00:58:6C:00:09 08:00:58:6C:00:09 ■ V3 - 03/27/2009 3:33pm The Data Link Layer Port 08:00:58:6C:00:1B 1 08:00:58:6C:00:C5 2 08:00:58:6C:00:09 1 08:00:58:6C:00:C2 2 08:00:58:6C:00:41 3 08:00:58:6C:00:95 2 08:00:58:6C:00:AB 3 08:00:58:6C:00:01 2 08:00:58:6C:00:41 08:00:58:6C:00:AB 08:00:58:6C:00:AB Network Bridge 08:00:58:6C:00:C2 08:00:58:6C:00:C5 08:00:58:6C:00:95 08:00:58:6C:00:02 Figure 11-22 Unicast frame forwarding 11.5.1.2 Multicast Operation When a bridge receives a frame that is destined for a multicast address, the bridge forwards the frame to all the ports except the port on which it is received.31 In Figure 11-23, the source node that has a MAC address of 08:00:58:6C:00:09 sends a frame to all members of the multicast group. The bridge recognizes that this is a multicast frame and forwards it out to all ports except the port that it received the frame on (in this case, port 1). 11.5.1.3 When the Bridge Just Does Not Know Sometimes a bridge receives a frame that is destined for a node it does not know about. The bridge is limited in what it can do in these cases. It can 31 This is known as flooding. A couple of things that can be done to cut down on the amount of ports that are flooded to are multicast pruning and virtual LANs (VLANS). These allow the ports in the switch to be separated into groups so that not all are affected when a frame is flooded. This ensures that the multicast traffic only goes out the ports that are part of that multicast group. 469 Page 469 Edwards c11.tex 470 Part II ■ The OSI Layers Node address 08:00:58:6C:00:1B 08:00:58:6C:00:1B 08:00:58:6C:00:09 V3 - 03/27/2009 08:00:58:6C:00:09 Port 1 08:00:58:6C:00:C5 2 08:00:58:6C:00:09 1 08:00:58:6C:00:C2 2 08:00:58:6C:00:41 3 08:00:58:6C:00:95 2 08:00:58:6C:00:AB 3 08:00:58:6C:00:01 2 08:00:58:6C:00:41 08:00:58:6C:00:44 08:00:58:6C:00:AB 08:00:58:6C:00:C2 08:00:58:6C:00:C5 08:00:58:6C:00:95 08:00:58:6C:00:02 Figure 11-23 Multicast frame forwarding forward the frame to all ports (except the one it received the frame on), or it can discard the frame. Figure 11-24 shows node 08:00:58:6C:00:09 sending a RANDOM BONUS DEFINITION frame to node 08:00:58:6C: 00:44. The bridge can’t find ARP cache — A data structure that provides that MAC address in its table, the current mapping of 32-bit IP addresses to 48-bit MAC addresses. so it floods the frame out and eventually the frame will arrive at the node via port 3. 11.5.2 The Address Table The bridge would be nothing more than a bulkier rendition of a network hub if it were not for its ability to direct traffic to a proper port for data delivery. The address table is the backbone for the proper operation of a bridge. The 3:33pm Page 470 Edwards c11.tex Chapter 11 Node address 08:00:58:6C:00:1B 08:00:58:6C:00:09 08:00:58:6C:00:09 ■ V3 - 03/27/2009 3:33pm The Data Link Layer Port 08:00:58:6C:00:1B 1 08:00:58:6C:00:C5 2 08:00:58:6C:00:09 1 08:00:58:6C:00:C2 2 08:00:58:6C:00:41 3 08:00:58:6C:00:95 2 08:00:58:6C:00:AB 3 08:00:58:6C:00:01 2 08:00:58:6C:00:44 08:00:58:6C:00:41 08:00:58:6C:00:44 08:00:58:6C:00:AB 08:00:58:6C:00:C2 08:00:58:6C:00:C5 08:00:58:6C:00:95 08:00:58:6C:00:02 Figure 11-24 Unknown destination frame forwarding address table is built based on the source address of received frames. As we discussed previously, one of the functions of the bridge is to forward and flood frames based on the information in the address table. Another important function is to see if a source address contained in the frame is in the address table, and if not, to add it. If it is, then the port mapping is updated so that the latest destination information is synchronized. Eventually, the bridge will know about every bridge32 that connects to a shared segment that it interfaces with. Another important process that needs to occur pertaining to the address table is that the address entries must expire after a period of time. Imagine how big an address table would become if entries were never removed. Additionally, the performance of the bridge could suffer, as the list could become cumbersome to review if too large. When the bridge receives a frame, it checks the address table to see if the source MAC is present. If it is, it flags 32 And have updated and accurate forwarding information. 471 Page 471 Edwards 472 Part II ■ c11.tex V3 - 03/27/2009 The OSI Layers the address so that it realizes that the MAC is still active and the information needs to be retained until the address finally does expire. 11.6 Chapter Exercises 1. How is a jam signal used in a CSMA/CD environment? 2. How is a jam signal used in a CSMA/CA environment? 3. An unnumbered frame type is used with which type of LLC? 4. Find the MAC address of your PC’s NIC card. Once you have found it, take the OUI and look it up on the IEEE website. What is the information that is listed for that particular OUI? 5. What are the three fields in an LLC PDU, and what do they do? 6. How many bits are in an IEEE 802 MAC address? 7. What are the two error-checking methods used at the Data Link layer? 8. What does full-duplex Ethernet use for flow control? 9. What is the functional difference between a bridge and a Layer 2 switch? 11.7 Pop Quiz Answers 1. The Data Link layer is what layer of the OSI reference model? Layer 2 2. Multiple nodes attached to a single shared medium can define what? A LAN 3. Define throughput. Throughput is the average rate of successful messages transmitted over a channel. 4. Name two methods of ensuring bandwidth is distributed fairly to the nodes that share connectivity within a LAN. Token Contention 5. The most popular and most often used CSMA/CD protocol is Ethernet. 6. What does DSAP stand for? Destination Service Access Point 3:33pm Page 472 Edwards c11.tex Chapter 11 ■ V3 - 03/27/2009 The Data Link Layer 7. What does SNAP stand for? Subnetwork Access Protocol 8. What does SSAP stand for? Source Service Access Point 9. A unicast address is simply the address of a particular node’s interface within the LAN. 10. What is the Ethernet standard broadcast MAC address? FF:FF:FF:FF:FF:FF 11. What is the simplest of all error-checking methods? Parity check 3:33pm 473 Page 473 Edwards c11.tex V3 - 03/27/2009 3:33pm Page 474 Edwards p03.tex V3 - 03/27/2009 2:58pm Part III Network Design and Implementation In This Part Chapter 12: Design Methodologies Chapter 13: Implementation Page 475 Edwards p03.tex V3 - 03/27/2009 2:58pm Page 476 Edwards c12.tex V3 - 03/27/2009 11:27am CHAPTER 12 Design Methodologies Take a method and try it. If it fails, admit it frankly, and try another. But by all means, try something. — Franklin D. Roosevelt Planning and designing a network can be a daunting task. In the early days of data networking, a network consisted of a handful of nodes. Any addressing schemes were normally manually assigned and maintained. This required human intervention any time a node was moved, removed, or changed in any way. This manual intervention was not that bad, however, due to the fact that there were not that many numbers to keep track of. In today’s LANs, this manual addressing would not work. Networks are changing, technology is changing, and LANs have grown to a size that was not foreseeable 20 years ago. In addition, other concerns exist that were not there 20 years ago, including security, the highs and lows1 of the LAN, and many others. LANs can be a simple as a handful of nodes in a remote office to as complex as thousands of nodes in a fully meshed routing environment supporting applications that require as much of the highs and lows as can be squeezed out of the LAN any time the application want to do so. LANs are responsible for supporting multiple protocols running over multiple nodes and multiple media types. Many of these node and media types are from different vendors, all of which can potentially be running some proprietary features that may or may not play nice with the nodes, media, and even protocols that are running the LAN. 1 These were discussed in Chapter 11 — high throughput, high total bandwidth, and low error rates and delays. 477 Page 477 Edwards 478 Part III ■ c12.tex V3 - 03/27/2009 Network Design and Implementation Sounds challenging, doesn’t it?2 And we haven’t even gotten to plans for future growth. What is the organization’s five-year plan? Are you installing gear that can be upgraded? How do you know how much gear to plan for without getting more than you need? These are just some of the questions you will need to ask yourself if you are going to design a network. Planning and designing a data network is complex enough from a LAN perspective, and that is the focus of this chapter. For comparison purposes, we may discuss commonalities between the LAN and networks of smaller and larger size. Proper planning and design of the network can be trial and error at times. Sometimes things just don’t do what you want them to.3 But don’t get discouraged. When you encounter problems with a design in the network, follow President Roosevelt’s advice. 12.1 Your Task Is to Design a Network By no means will you be a professional network designer after reading this chapter, but that isn’t our intention. As with most of the topics in this book, we are trying to teach you the fundamentals. It’s important to understand the difference between a network that was planned and designed carefully and one that was thrown together haphazardly, no matter what you end up doing in a networking career. Careful planning is essential to ensure that your network will support your organization’s needs. So what do you want to consider when you are designing a network? What are the needs of the business or organization? What should be considered in order to meet current and future needs? What are the cost considerations (short and long term)?4 These are all important questions to consider. You might want to design the most ACRONYM ALERT ultra-fantastic network with all the bells and the whistles, but the budget may not NFS — Network File System cover it. You might also want to build in some features to make life simpler for you, but that may be beyond the scope of the business model or plan. 2 If you like challenge, you would love a job in data networking. matter what the salesperson told you. 4 This would include costs factored in for network maintenance. 3 No 11:27am Page 478 Edwards c12.tex Chapter 12 ■ V3 - 03/27/2009 11:27am Design Methodologies 12.1.1 Types of Organizational LANs Following are examples of some of the various network LAN types that are in use. While reading through this list, consider the impact that might occur if these LANs were improperly designed. Hospital LANs — Life-critical data is delivered to various departments via the computer. Emergency logging is automated. Lives could be in danger if there are any network delays.5 Banking and financial corporate LANs — Can you imagine how much money can be lost during the middle of the trading day on Wall Street6 if the network has delays? What about the delays that could occur in the online trading world? Not to mention all the remote automatic teller machines. Manufacturer LANs — Production lines in all sorts of different manufacturing environments run with the use of robotics and automation. If there is a data hiccup, thousands of dollars can be lost. Retail LANs — Retail stores often have a LAN running within them, taking care of inventory and sales along the way. Periodically, the store will connect to the corporate LAN to exchange the data collected. Today, some retail sites run a remote connection into the LAN and are able to provide real-time updates. Imagine the impact if the store is not able to ring sales or communicate as needed with the corporate LAN. Government LANs — Consider the POP QUIZ amount of security Name five businesses or organizations that that has to be deployed are not listed above. What do you think the for government biggest concern would be pertaining to each LANs. Authenorganizational LAN type? tication methods and authorization are of the utmost importance. Consider what might occur if a hacker gains access to a government LAN.7 5 Getting off topic a bit, here is an interesting tidbit of information. There are hopes that one day a doctor can connect to the hospital from home and perform an operation over a video feed with the use of robotics. Won’t that be amazing if it ever happens? 6 Although based on the days that Wall Street had beginning in late September 2008, maybe the network should fail every time that stocks start to tumble. No network . . . no trading . . . problem solved. 7 And for any other LAN — security is very important. 479 Page 479 Edwards 480 Part III ■ c12.tex V3 - 03/27/2009 Network Design and Implementation And this is just a small list. Name a business type, and there will be some form of network for it. The network itself may not be optimized,8 but it will most likely be there. What the design of the network for these organizations looks like all depends on the needs of that organization. 12.1.2 Other Things to Consider Now that we have an idea of the needs of the business, we still have some work to do. The next thing is to ensure that you have a fair balance between what is available to be considered in the network environment (needs vs. wants) and whether the projected business needs limit what technically is available. You don’t want to provide more than is needed for the LAN, but you also don’t want unreasonable demands driving design decisions.9 Another important concern for network planners is to not be too cutting edge. You don’t want to deploy a brand new switch, new feature, or new code until it has had time to be field tested.10 Most products undergo a serious amount of testing, but environments are different and new products often introduce new problems that can take time to iron out. There are also several external factors: Consideration needs to be given to WAN interfacing, as well as interfacings with LANs that are within of your realm of control. Make sure you know about any government regulations and are in compliance with them. What are your competitors using/doing? What network type would you like to have, and who has done it right? What did they do? ACRONYM ALERT LSAP — Link service access point What is the potential technological growth, and will your proposed design be prepared to support it? 12.1.3 Building the Foundation Now that you have an idea of the things that need to be considered, you can move on to the planning stage. Before you do so, however, we are going to let you in on a secret. If you have been paying attention to what we have written thus far in the book, guess what? Without knowing it, you already have some of the fundamentals that are necessary to design a network. 8 This can be for several reasons. The network may be outdated, poorly designed, or simply not maintained properly. If you try modeling yourself based on a similar network type, make sure you model your LAN after one that has been operating a while and proved itself successful. 9 Just because someone wants something to work a certain way does not mean that it can be done. 10 Keep in mind that you may be able to make some kind of deal if you are willing to test any or all of these. 11:27am Page 480 Edwards c12.tex Chapter 12 ■ V3 - 03/27/2009 11:27am Design Methodologies You have an understanding of networking concepts. You have an understanding of the needs of the organization. You have an understanding of the hardware types that operate in a LAN. You have an understanding of LAN protocols. You know the different types of network topologies that are used in today’s LANs. You know about LAN protocols and MAC and IP addressing. You know the seven OSI layers and what functions at each layer. RANDOM BONUS DEFINITION aggregator — The entity that performs the operations required to make multiple physical links function as an aggregated link. You know how to make spaghetti and meatballs! Give yourself a pat on the back. You are ready to start planning the network. 12.2 Let’s Start Planning We just realized that there has not been anything really technical about this chapter so far.11 You will be surprised at how much nontechnical thought is put into the initial planning stages. Don’t worry — there is plenty of technical thought left in the upcoming pages.12 We already have established that you have been tasked with planning and designing a network. More than likely, you will be given a team to work with to get this project going.13 The first task you will want to attack is to develop an action plan and project scope. 12.2.1 Development of Scope Main Entry: scope (skop)14 Function: noun 1. The range of one’s perceptions, thoughts, or actions. 2. Breadth or opportunity to function. 3. The area covered by a given activity or subject. 11 Then again, when is cooking up some of Mama Bramante’s spaghetti a technical task? Jim is thinking that a nontechnical book might be fun to write, perhaps The Networker’s Guide to Homebrewing Beer. 13 At the very least you should insist on access to someone who knows something when and if you have any questions along the way. 14 The American Heritage® Dictionary of the English Language, Fourth Edition. Houghton Mifflin Company, 2004. 12 481 Page 481 Edwards 482 Part III ■ c12.tex V3 - 03/27/2009 Network Design and Implementation Developing a project scope is important in the early phases of network design. This is where you gather the information you will need in order to proceed with the project. One of the big considerations is the nature of the organization (the type of network). Do the users only communicate with other users on the network, or is there a need for access to networks outside of your own LAN? Do users need remote access? Do any vendors or customers need access? What applications need to be supported by the network? Finally, it is good to know the budget that is available for the project and the time frame for completion. The next thing that needs to be addressed is to determine RANDOM BONUS DEFINITION whether the wants and needs are even doable. Is there enough aging time — This is used in a spanning tree environment — the amount of time a node money available to meet the can be inactive before a dynamic filtering requests? Can the project be database will remove the node’s entry. completed in the proposed time frame? Will the project’s completion keep up with technological growth? Now that the scope has been discussed, and it was determined to probably work, the scope has to be refined even more. The specific services that are to be placed in the LAN need to be determined. Information such as: Will the network support voice communications? Will the network support data communications? Will the network support e-commerce? Will the network support video streaming? After identifying the services, you must determine what the potential traffic flow will be in such a network. What will be required in the future? Some of these can be answered if you are fortunate enough to have a network you can model yours after. Also, if you have a way to test (or get a vendor to help you out), you can possibly get some traffic analysis data that will give you a good idea of what to look for and expect. But the real test is when you go live. The secret is making sure you have enough, but not too much. Data traffic patterns are subject to variations and fluctuations. Sometimes this is due to a certain time of day or a particular day of the week. Even the weather can affect data flow. Usually the trends point to an event (Friday night backups, Monday morning end node boot-up, etc.), and you just won’t know about all of them until you get the network up and running. In the next chapter, we will be discussing ways to baseline. 11:27am Page 482 Edwards c12.tex Chapter 12 ■ V3 - 03/27/2009 11:27am Design Methodologies 12.2.2 You Are Not Alone The great thing about all this is that you are not alone. You can POP QUIZ send your scope out to some of Define scope. the many networking vendors and ask them what they have that will help you do what you want to do. This will get you a lot of information and maybe even some deals along the way. The request for information (RFI) is a standard process used in business to obtain just this type of information from vendors. Once you find what you like, the request for proposal (RFP) is used to seek the best deal. 12.3 A Hierarchical Design Model There is that word again — hierarchical. The hierarchical design model is the most commonly used model in most high-speed LANs today. This model allows for easy expansion. It also makes network management and troubleshooting easier. By breaking nodes in the LAN into three functions, the nodes are able to focus on specific tasks instead of each of them working to perform all tasks. Figure 12-1 shows an example of what we are talking about. The hierarchical model has the following three different layers, with nodes within each layer performing a specific function: Access layer Distribution layer Core layer Keep in mind that a model is a recommendation or a guideline more than it is a rule. Sometimes a single node can take care of all the layers itself, sometimes it can’t. It’s always easier to follow a model, and this one is tried and true. 12.3.1 Access Layer The access layer is the lowest layer. This is the layer that interfaces with the endpoint nodes. Types of nodes that are found at this layer are wireless access points, hubs, repeaters, bridges, Layer 3 switches, and routers. The access layer is what enables end users to connect to the network. This layer is also responsible for determining when nodes are not allowed access to certain portions of the network. 483 Page 483 Edwards 484 Part III ■ c12.tex V3 - 03/27/2009 Network Design and Implementation The Core Layer The Distribution Layer The Access Layer Figure 12-1 A hierarchical approach to LAN design The access layer can also be the gateway to the LAN for remote users (see Figure 12-2). For this to occur, some form of WAN technology must be used. Examples of WAN technologies that can be used to connect remote sites to the corporate LAN include: Frame Relay ISDN Leased lines The access layer can simply be thought of as the endpoint node access to the LAN. It manages the data between the endpoint nodes and the distribution layer. Switched bandwidth and MAC filtering are functions performed at this layer. 11:27am Page 484 Edwards c12.tex Chapter 12 The Access Layer ■ V3 - 03/27/2009 11:27am Design Methodologies Remote Office The Core Layer The Distribution Layer Figure 12-2 Remote relations to the access layer 12.3.2 Distribution Layer The distribution layer is the middleman between the access layer and the core layer. Data received from the access layer is sent to the core layer to be routed to the destination. Broadcast domains are separated at this layer with the implementation of virtual LANs (VLANs). Security is also a function that is implemented at this layer. Network access can be implemented at this layer when policy-based connectivity is rePOP QUIZ quired. High-performance Layer Name three WAN technologies that are 3 switches are implemented at used to connect to remote sites. this layer. Guarantees that are required at this layer are high performance, high reliability, high availability, and redundancy. Policy-based connectivity between the other layers is what you get from the distribution layer. Figure 12-3 provides an example of a method of connecting the three layers together. 485 Page 485 Edwards 486 Part III ■ c12.tex V3 - 03/27/2009 Network Design and Implementation The Access Layer The Distribution Layer The Core Layer Variable network–typically a WAN, MAN, or LAN (or a combination of) Remote Office Figure 12-3 Connecting the three layers Notice that the distribution layer has nodes that aggregate with other nodes in the access and code layers. Additionally, there is a remote connection that is coming from a remote office and accessing the network via the distribution layer. 12.3.3 Core Layer This is the big daddy layer of this model. The core layer is the backbone of the LAN and often provides connectivity to WANs as well as to Internet services. The core routers15 are highly available and support redundancy in the connections with the distribution layer nodes. These nodes need to be hefty, as they process data flowing throughout the whole LAN. They have to do that reliably and quickly. Refer to Figure 12-4 and answer this question: If the POP QUIZ drawing represents the physiWhat are the layers of the hierarchical cal layout of a network, is this design model? an example of a hierarchical design? 15 When we say routers, we are referring to any node that can provide network layer services. So, a router may be a router, a Layer 3 switch, etc. 11:27am Page 486 Edwards c12.tex Chapter 12 ■ V3 - 03/27/2009 11:27am Design Methodologies Figure 12-4 An example of a LAN physical layout The answer to that question is maybe. We do not know the logical layout of the network, so it is entirely possible that this figure represents a hierarchical design. Hierarchical in a logical manner, that is; the physical layout is pretty much a moot concern at this point. AN UNRELATED MOMENT OF PAUSE — BARBECUE CHICKEN NACHOS We know how easy it can be to get wrapped up in the reading of this book and time can get away from you. Before you know it, you don’t have time to make dinner and the last pizza delivery ran 30 minutes ago. This is a super-easy recipe and a really excellent quick fix when you need to fill the void left from skipping dinner. The layers of the nacho model are as follows: ◆ The Determination layer ◆ The Preparation layer ◆ The Application layer ◆ The Thermal layer ◆ The Devour layer (continued) 487 Page 487 Edwards 488 Part III ■ c12.tex V3 - 03/27/2009 Network Design and Implementation AN UNRELATED MOMENT OF PAUSE — BARBECUE CHICKEN NACHOS (continued) The Determination Layer This is the first layer, in which you decide on the toppings that you want on your nachos. Thanks to this handy reference model, you are not confined to the recipe listed here. Anything goes well on nachos, so if you like it, try it out. For this recipe, we have determined that we will be using the following ingredients: ◆ Cheese — Use whatever kind you like (cheddar and/or Monterey jack is good). ◆ Tortilla chips — Any kind will do. We normally use restaurant-style tortilla chips, which make good nacho chips. ◆ Chicken — Chicken breast is the best. ◆ BBQ sauce — At least one bottle of your favorite kind. The ones with the squirt top works well for presentation purposes. ◆ Bacon — One package. The Preparation Layer This layer is where you prepare everything that needs to be prepared. Here are the preparation steps: 1. In a bowl, place enough cheese to cover the amount of nachos you plan on eating. You can shred it yourself or buy it preshredded. 2. The chicken can be boiled and then shredded, or sliced and cooked in a pan; the choice is up to you. Don’t add a lot of seasoning to the chicken as it cooks, as you will be gaining flavor in your nachos. 3. Fry the bacon and then cut it into small pieces. The Application Layer This is where is you put everything together. Apply all your toppings to your nachos in any way you want — it’s hard to mess up nachos. This is what we did; it came out yummy and had a nice presentation. Use a microwave-safe plate or platter. Put a generous handful of tortilla chips (make sure to cover the plate completely). Now put a layer of cheese and about half of your bacon. Make sure to cover the chips completely with the cheese. Now put on another layer of chips. Next, put the chicken, enough cheese to (continued) 11:27am Page 488 Edwards c12.tex Chapter 12 ■ V3 - 03/27/2009 11:27am Design Methodologies AN UNRELATED MOMENT OF PAUSE — BARBECUE CHICKEN NACHOS (continued) cover the chips, and the rest of bacon. Make sure that you get some cheese on the chicken, but it does not have to bury the chicken. Finally, squirt the BBQ sauce over the whole thing — be creative. The nachos should be in a heap on the plate, but not flowing off of the plate. The Thermal Layer Stick all this in the microwave and cook it for about 30 seconds, then pause for about 5 seconds and then another 30 seconds. Keep your eye on it — when the cheese is melted, they are done. The Devour Layer Eat the nachos. 12.3.4 Why Hierarchical? Some of you probably wondering, ‘‘Why hierarchical?’’ Well, we have been putting a lot of effort into presenting you with a slew of networking information while making the book as enjoyable to read as possible. So when the recipe was written, we thought it might be funny to present it in a hierarchical model. It worked too. Wait — you were not thinking about the recipe at all, were you? What you are really wondering is what the benefits are of a hierarchical model. Here are just a few: Design replication — Once you have a working model, you can simply change the addressing schemes and design the next network expansion based on the way the original design was configured. Expandability — As the network grows, it is very simple to introduce additional nodes into the topology. Future growth planning is a breeze. Redundancy — Redundancy from the access layer to the core layer is very important in high-speed LANs. When a node fails, you have to have another node picking up the pieces until the node comes back on line. Better performance — Nodes that operate in the hierarchical model are able to maintain close to wire speed transmissions to all of the nodes it supports. 489 Page 489 Edwards 490 Part III ■ c12.tex V3 - 03/27/2009 Network Design and Implementation Security — Access control security is provided at the access layer. The distribution layer can support advanced security that meets the security needs of the LAN. Easy to manage and maintain — Because of the scalabilACRONYM ALERT ity of the hierarchical design model, the network is easy to manage and NetBEUI — NetBIOS Extended User Interface maintain. A layered approach to troubleshooting helps you find the source of a network connectivity issue. Additional nodes can be installed fairly simply, and configurations can be built from existing configurations, saving you time and money.16 Over time, the hierarchical model will pay for itself in money saved due to the ease of maintaining and managing the LAN. And there you have the hierarchical design model in a nutshell. Now let’s take a look at a design model that is used in planning Ethernet segments. The next section covers the 5-4-3-2-1 design reference model.17 It is a nice model to follow when you are planning a network, as it pulls together many tasks that are needed for basic design principles. THINGS YOU JUST HAVE TO KNOW Before we move ahead, here are a couple of terms you need to know. ◆ Collision domain — A group of nodes, sharing a communication channel, that are all in a group where a collision can occur. These nodes are connected to the same shared medium and are part of the same collision domain. These nodes are not concerned with other collision domains, as they do not have to negotiate for a communication channel bandwidth with them. Collision domains are normally separated by a bridge. ◆ Broadcast domain — A group of nodes that are all within the same broadcast area. The broadcast domain comprises multiple collision domains. Broadcast domains are normally separated by a node that functions at Layer 3 or higher. ◆ Propagation delay — The amount of time it takes to transmit a set number of bytes from endpoint to endpoint in a LAN. ◆ Network segment — A physically related grouping of nodes. Similar in function to a subnet, which is a logical grouping of nodes. ◆ Repeater — A Layer 1 node that connects network segments. 16 When it works in one segment, it should work in another. is also known as the 5-4-3 rule. Either term is fine, as long as you understand the overall concept. 17 This 11:27am Page 490 Edwards c12.tex Chapter 12 12.4 ■ V3 - 03/27/2009 11:27am Design Methodologies 5-4-3-2-1, Speed Is Not the Big Concern The rule used for designing a collision domain is known as the 5-4-3-2-1 rule. This is more of a reference model than it is a rule, providing guidance as to the number of repeaters and network segments that can be on a shared access Ethernet backbone.18 The 5-4-3-2-1 rule says that between two communication nodes in a shared environment, the following are the maximums that are allowed: 5 — This is the total number of segments allowed. 4 — This is the number of repeaters used to join the segments together. 3 — This is the maximum number of segments that have nodes that are active. 2 — This is the maximum number of segments that are not active. 1 — This is the number of collision domains. The 5-4-3-2-1 rule is used in networks that use a tree topolRANDOM BONUS DEFINITION ogy (a combination of a bus and backbone — A network used primarily to a star topology). The tree topolinterconnect other networks. ogy used groups (segments) that attach to a linear backbone. Figure 12-5 shows an example of this rule. In a tree topology environment, there can be a maximum of five segments between two POP QUIZ communication nodes. AddiWhat is the purpose of a network’s access tionally, data can pass through layer? a maximum of four repeaters. Finally, there can be a maximum of three segments that are populated with active nodes. In Figure 12-5, you can see that there are five segments, four repeaters, and no more than three active segments between the source and destination endpoint nodes. By placing these limits on the collision domain, you are essentially ensuring that the propagation delay is decreased (fewer nodes to pass through). This greatly improves the reliability in the collision domain. 18 Note that this rule only is beneficial in a shared access domain. Switched backbones should consider other methods (most commonly, the hierarchical). 491 Page 491 Edwards Part III ■ V3 - 03/27/2009 Network Design and Implementation gm Se t en Repeater 1 Se gm en t 2 Repeater 2 3 1 Repeater 3 t4 Segmen Segment 492 c12.tex Segment 5 Repeater 4 Source Destination Figure 12-5 The 5-4-3-2-1 rule in action 12.5 Making Determinations Now that all the preliminary mumbo-jumbo has been taken care of, it’s time for you to determine what you will need out of your network. Some things will be contingent on others (for instance, if authentication is going to be used, what will you need to support it?). This is exactly why you will want to review and adjust your plan as you go along. You have determined the needs and wants of the users of the network. Now you start making determinations on what should be put into the network to support those needs. Be sure to consider potential future growth in your determinations. Some decisions you will make include: Which topology are you going to be using? Will you be using Ethernet or Token ring? How many ports will be needed at each level? What is the target transmission speed(s)? Which node types are you planning on deploying? 11:27am Page 492 Edwards c12.tex Chapter 12 ■ V3 - 03/27/2009 11:27am Design Methodologies Which end-user applications will be used? Which protocols will need to be supported? Will remote access be required? Which types of WAN protocol options will be used (if required)? ACRONYM ALERT DDP — Datagram Delivery Protocol What are the security concerns? In the next few sections, we will discuss some things you should consider when making these determinations. 12.5.1 Determining Which Topology to Use Deciding on the network topology really depends on the requirements at each level of the network. More than likely you will be using Ethernet (the most popular shared network protocol), so your biggest decision will be the speed and the actual physical layout of the building in which the network is being installed. Chapter 1 introduced the topology types in most LANs. The three most popular topology types are the bus, the star, and the ring. Let’s take a moment to review these. 12.5.1.1 Bus Network Topology The bus topology is the most often used topology in LANs. In this topology, the nodes connect to a common shared communication channel, referred to as a bus. Figure 12-6 shows an example of a network with a bus network topology. Figure 12-6 The bus topology So what makes the bus topology the most often implemented? 493 Page 493 Edwards 494 Part III ■ c12.tex V3 - 03/27/2009 Network Design and Implementation Advantages: It’s easy to install. It’s easy to extend. It’s less expensive to implement than other topologies. Disadvantages: There is a limitation to the distance a cable can go without a repeater. There is a limit to the number of nodes that can be supported. The LAN can experience sluggishness in performance when there are heavy traffic loads. Security risks exist because all stations can hear what the others are saying on the shared channel. The cost and ease of use are the biggest reasons for considering the bus network. However, if there are concerns about speed, performance, reliability, or number of supported nodes, another design might need to be considered. 12.5.1.2 RANDOM BONUS DEFINITION blocking state — A stable state in the Spanning Tree Protocol in which a bridge port will receive BPDUs but will neither receive nor transmit data frames. Star Network Topology The star topology can be divided into two categories. It can be a logical star topology or a physical star topology. Figure 12-7 shows an example of a physical star topology where a central bridge or a hub controls the communications to and from attached nodes. The advantages of a star topology include: It offers better performance. It’s easy to troubleshoot. It offers high scalability of the network through the central node. The disadvantages of a star topology include: There is too much dependency on the central node. It can be complex to manage. Wiring can become cumbersome. If neither the bus nor the star topology fit your specific needs, you might want to consider implementing a ring topology. 11:27am Page 494 Edwards c12.tex Chapter 12 ■ V3 - 03/27/2009 11:27am Design Methodologies Router Hub Figure 12-7 The star topology 12.5.1.3 Ring Network Topology The ring topology is used for Token Ring and FDDI LANs. In the ring topology, a frame is passed from node to node until it reaches its destination. Figure 12-8 shows an example of a network with a ring network topology. The advantages of a ring topology include: There is no need to have a mechanism to ensure collision-free datagram passing. It can be expanded to cover a greater number of nodes than some of the other topology types. It’s fairly simple to maintain. The disadvantages of a ring topology include: A failure with one node on the ring can cause an outage to all connected nodes. Any maintenance (e.g., adding a node, making a change to a node, removing a node) affects all the nodes that connect to the ring. Some of the hardware required to implement a ring is more expensive than Ethernet network cards and nodes. Under normal traffic load, a ring is much slower than other topologies. 495 Page 495 Edwards 496 Part III ■ c12.tex V3 - 03/27/2009 Network Design and Implementation Router MAU Figure 12-8 The ring topology Another determination is which nodes to deploy and where to deploy them. The following section discusses some things to consider. 12.5.2 Determining Which Nodes to Use Traditionally, packet-switched LANs have comprised four main network nodes: concentrators, repeaters, bridges, and routers. ACRONYM ALERT For the most part, these traditional nodes CST — Common spanning tree are still used and make up various levels of the network. Repeaters and bridges are used heavily in user workgroups, server farms, and in the access layer of hierarchical networks. The list of nodes has really grown in the past 20 years. The traditional nodes are still in use, but so many other nodes have been introduced in that time. In addition to the traditional nodes, many networks use Layer 3 switches, Layer 4–7 switches, VPN remote access solutions, etc. Chapter 3 discussed each of these node types extensively, but here is a quick overview, along with some examples of node deployment. 11:27am Page 496 Edwards c12.tex Chapter 12 12.5.2.1 ■ V3 - 03/27/2009 11:27am Design Methodologies Traditional Nodes The first types of nodes we want to cover are what we will call the traditional nodes. These node types are the most often deployed and are found in many home networks. In traditional node networks, each node serves a distinct and specific function. Repeaters and hubs pass data without using any logic at all. Bridges (also known as Layer 2 switches) connect like networks to one another and are able to make correct forwarding decisions. Routers are able to connect different network types to one another and can also make correct forwarding decisions. Get it? Got it! Good! 12.5.2.1.1 Repeaters The repeater is a node that simply passes information on. It is used to extend the segment when medium-distance limitations have been reached. Figure 12-9 shows an example of a repeater separating two parts of a network. Repeater Figure 12-9 A repeater A repeater amplifies a signal, but that is not its only task. A repeater also filters out any distorted data it has received, and it will not pass that data along. Technically, you can say that the function of the repeater is to amplify good data. ACRONYM ALERT ADSP — AppleTalk Data Stream Protocol 497 Page 497 Edwards 498 Part III ■ c12.tex V3 - 03/27/2009 Network Design and Implementation 12.5.2.1.2 Concentrators The concentrator used within a LAN is either a hub or an MAU that allows the combination of data transmissions for a group of nodes. Figure 12-10 shows an example of a hub deployed in a LAN. Bridge Hub Figure 12-10 A hub A hub is used to connect segments in a LAN. Hubs have multiple ports, and when data is received on a port, the hub will send the datagram to all of the other ports, so all segments will see all datagrams that are passed through the hub. An MAU is a type of concentrator that is used to connect nodes within a Token Ring environment. The MAU connects the nodes in a physical star configuration, but the logical operations are Token Ring. The MAU allows the Token Ring to continue operating when a node on the ring breaks. This is much better than the alternative, where a node breaks on a physical ring and the whole ring goes down. 12.5.2.1.3 Bridges The bridge is a LAN node that operates at Layer 2 of the OSI reference model. The bridge is used to connect different networks to one another. Data received from one network can be forwarded through the bridge to get the data to the correct destination. Figure 12-11 shows an example of bridge deployment. Bridges are smart enough to know how to send datagrams to a specific port so that not all areas of the network have to receive the data as well. This frees up the other segments to pass data separately, without having to analyze all the datagrams. When a bridge gets the datagram, it passes the data based on the MAC address of the destination node. 11:27am Page 498 Edwards c12.tex Chapter 12 ■ V3 - 03/27/2009 11:27am Design Methodologies Bridge Hub Hub Hub Hub Figure 12-11 A bridge 12.5.2.1.4 Routers The final node in the traditional node family is the router. OperRANDOM BONUS DEFINITION ating at Layer 3, the router is the backbone of most LANs. coaxial cable — A communications medium Routers use IP addresses to used in 10BASE5 and 10BASE2 Ethernet systems. route datagrams in a network. Routers support multiple protocols of different types and are able to separate networks of different types because of this. Figure 12-12 is an example of the placement of a router in a LAN. Routers are still in use in LANs today, but Layer 3 switches are becoming increasingly more popular. The reason for this is simple. The advanced switches are able to function as a router at a much higher speed due to application-specific integrated circuit (ASIC) technology. Additionally, switching hardware is cheaper to replace than traditional router hardware. Routers are still used as boundaries between the LAN and the Internet.19 Figure 12-13 shows an example of this. Routers are also often used for remote connectivity for remote offices. 19 Or other networks that are not controlled by the organization. 499 Page 499 Edwards 500 Part III ■ c12.tex V3 - 03/27/2009 Network Design and Implementation Bridge Hub Router Hub Hub Bridge Bridge Hub Hub Hub Hub Hub Hub Figure 12-12 A router The Internet Router Organizational LAN Figure 12-13 Routers connecting a LAN to the Internet 11:27am Page 500 Edwards c12.tex Chapter 12 ■ V3 - 03/27/2009 11:27am Design Methodologies Routers are used to route data between different networks. Routers control the flow of data in and out of the LAN, often working with a firewall solution to limit and/or control data coming into the LAN, as well as data going out from the LAN to the Internet. 12.5.2.2 Node Evolution Main Entry: ev·o·lu·tion20 Function: noun. 1. A gradual process in which something changes into a different and usually more complex or better form. 2. The process of developing. 3. A movement that is part of a set of ordered movements. 4. Mathematics: The extraction of a root of a quantity. Networking never stops growing. As a new product is being introduced, there is another product just around the corner that will replace it. Software upgrades and new program implementations also see the same changes and growth. No longer is a modem the standard for accessing the LAN. No longer do we have to rely on filtering and VLAN techniques to authorize and authenticate. There are a lot of nodes out there that do the trick, and a lot of nodes out there that just plain do it better than traditional nodes. We already have discussed how the term Layer 2 switch replaced the term bridge, but it is RANDOM BONUS DEFINITION really just a marketing term. As congestion — The state where the offered a matter of fact, it was so well network load approaches or exceeds the received21 that almost anything locally available resources designed to networking is a switch now. In handle that load this section, we talk about some other switches that are in a lot of LANs. In addition to the switches, we discuss a bit about VPN and wireless nodes. 12.5.2.2.1 Layer 3 Switches Layer 3 switches perform the same task as routers and are deployed in high-speed LANs as well as in WANs. The Layer 3 switch is preferred over a router because routing decisions are hardware-based and thus are able to be performed much faster than traditional routers. Layer 3 switches are also able The American Heritage® Dictionary of the English Language, Fourth Edition. Houghton Mifflin Company, 2004. 21 Well received by the customers or the salespeople. We are not really sure which, but we all know how those sales guys are. 20 501 Page 501 Edwards 502 Part III ■ c12.tex V3 - 03/27/2009 Network Design and Implementation to perform as Layer 2 switches, giving the best of both worlds. They give you the control of data flow that is offered in a routed network and the speed that is offered in a switched environment. Figure 12-14 shows an example of Layer 3 switch deployment. Layer 2 switch Hub Layer 3 switch Hub Hub Layer 2 switch Hub Layer 2 switch Hub Hub Hub Hub Hub Figure 12-14 A Layer 3 switch deployment Traditional routers do a great job, but the logic decisions they make are software-based and therefore are a slower process than what is offered by the Layer 3 switches. Layer 3 switches can support the same protocols that are supported by a traditional router and generally cost less than traditional switches. So what is the hardware feature on the Layer 3 switch? It is the ASIC that makes the Layer 3 switch. A Layer 3 switch can have from one ASIC per chassis up to one ASIC per port.22 12.5.2.2.2 Layer 4–7 Switching Layer 4–7 switching is not traditional Layer 2 switching. Many vendors now market nodes that are able to perform Layer 4–7 functions. It’s important to note that even though a node may be labeled a Layer 4–7 switch, multiple 22 This depends on how badly the vendor wants to make sure that you can get wire speed throughput through the device. 11:27am Page 502 Edwards c12.tex Chapter 12 ■ V3 - 03/27/2009 11:27am Design Methodologies vendors use the definition loosely, so it may not really be exactly the same between vendors. Some terms you might come across to describe Layer 4–7 switching include: Web switch Application switch Content switch VPN switch Generally, Layer 4 switches are used to assist in balancing data destined for servers within a network. Layer 4 switches operate at the TCP/UDP level and will make decisions about where to send traffic based on information that is stored at the Transport layer. Not all Layer 4 switches actually do transfers based on that information. Web load balancers are often termed Layer 4 switches, as they are able to forward Layer 2 switches based on the MAC address, but are also able to send some MAC address data to multiple physical ports within the load-balancing switch. Some load balancers are able to monitor load on the server ports and can switch requests that are received to the data port that connects to the server with the lightest load. As we have said, some of these nodes can function at up to Layer 7 of the OSI model. These are used to load-balance traffic among groups of servers. These servers can provide applications such as HTTP, HTTPS, and many others that use TCP/IP to transport traffic via a specified port. Layer 4–7 switches use Network Address Translation (NAT), often at wire speed, to provide an avenue to allow multiple clients access to multiple host servers without having to know the exact physical server that is handling the request from the individual client. Some Layer 4–7 switches are also able to provide SSL encryption and decryption services so that the servers don’t have to, as well as being able to manage digital certificates. Layer 4–7 switches provide an excellent service — the almost instant, endless, and secure flow of data to end users. This is cerACRONYM ALERT tainly an improvement for many users who MAC — Media Access Control are beginning to expect instant gratification when connecting to a website. The Layer 4–7 switch may not be for everyone, but it does come in handy when a network needs to have it. 12.5.2.2.3 Virtual Private Networks VPN solutions have changed the way that organizations connect to remote sites. Traditionally, site-to-site connectivity was done over leased lines (such as ISDN, dial-up, etc.). As the Internet grew, and technology grew, so did the way that we connect to remote offices. VPN technology allows remote 503 Page 503 Edwards 504 Part III ■ c12.tex V3 - 03/27/2009 Network Design and Implementation connectivity over a secure tunnel to the organizational LAN, so it will appear as if the remote user or office is actually geographically located within the LAN. Figure 12-15 provides an example of three uses of the VPN solution. Organizational LAN User Tunnel Home Office The Internet Remote Office Customer LAN Branch Office Tunnel Figure 12-15 Typical VPN deployments In the figure, you can see that there are three different tunnels going into the corporate LAN. One of these tunnels is a remote user connecting from home through a user tunnel.23 The other two tunnels connect remote LANs and are known as branch office tunnels. One of the branch office tunnels goes to a remote office for workers in the VPN’s organization.24 The other branch office tunnel is used by customers to connect to the corporate network.25 12.5.2.2.4 Wireless Networks The last topic we will talk about in this node evolution section is wireless networks. Wireless seems to be where networking is really growing. Almost everyone has at least one cell phone, but it does not stop there. You have Bluetooth, infrared, wireless PC connections, etc., almost everywhere you go. We can’t tell you the last time we were out and about when there wasn’t 23 This type of VPN is also known as a remote access VPN. type of VPN is also known as an intranet VPN. 25 This type of VPN is also known as an extranet VPN. 24 This 11:27am Page 504 Edwards c12.tex Chapter 12 ■ V3 - 03/27/2009 11:27am Design Methodologies at least one person using some form of wireless device. Thanks to wireless networks, this is all possible. IEEE 802.11 is the standard that outlines wireless LAN RANDOM BONUS DEFINITION standards. Another standard, called wireless IP, allows mobile dedicated bandwidth — A configuration in devices to remain connected, which the communications channel attached to a network interface is dedicated even when they move into an for use by a single station and does not have wireless area that has a differto be shared. ent IP scheme than the user has. Basically, this standard allows roaming without losing connectivity. Security is a big concern in wireless networks, so encryption and authorization options need to be considered. 12.5.3 LAN Switching Technology Layer 2 switches changed what we can do in a network. These LAN switches broke up the transitional shared network and converted it into a switched network. This greatly improved the performance of the LAN as a whole. Figure 12-16 shows an example of a small switched network consisting of the switch (of course) and six end users. If this were a shared network connected by a central hub, all the nodes would have to read all the data transmitted, and the end-user nodes would have to negotiate in order to transmit. Mike Derrick Zung Walter Steve Figure 12-16 A switched network Tuyett 505 Page 505 Edwards 506 Part III ■ c12.tex V3 - 03/27/2009 Network Design and Implementation As you can see, there are a total of six end users. Each user is exchanging data with one other node, but each node is communicating with only one node at a time. Notice that there are three simultaneous active connections (Mike to Tuyett, Zung to Steve, and Derrick to Walter) in the example. Try to do that in a shared network! 12.5.3.1 Switch Types The way that a switch handles the data it receives depends on whether the switch is a cut-through type or a store and forward type. Cut through — In cut-through operations, the switch reads the header of the datagram as it is received on a port. Once the switch determines the port that reaches the destination, the datagram is sent to the port and on to its destination. There is no storing of data in a cut-through environment. There are also no options for error checking or control because the cut-through switch only reads the header for an address and sends the datagram on. Store and forward — In store and forward operations, the switch stores the data and does error checking on the datagram before it sends the datagram off toward its destination port. Although this makes the transfer of datagrams slower than with a cut-through switch, the data is delivered reliably. 12.5.3.2 By All Means, Be Redundant A well-designed network will be built with plenty of redundancy throughout. The last thing a network administrator needs is a single point of failure anywhere in the network. A single point of failure is a location within the network that does not have a backup link of some sort. In other words, if the link fails, the network that is relying on that link will not be able to reach some or all of the LAN. Figure 12-17 shows an example of a single switch that is used to connect LAN 1 to LAN 2. Consider what would happen in this example if the switch failed, or if one of the links between a LAN and the switch failed. ACRONYM ALERT If there were a failure, LAN-to-LAN comBPDU — Bridge protocol data unit munication would not happen. Depending on the problem, it could be several hours before service is restored, and in most businesses, there is a financial impact that could be detrimental. 11:27am Page 506 Edwards c12.tex Chapter 12 ■ V3 - 03/27/2009 11:27am Design Methodologies LAN switch LAN 1 LAN 2 Figure 12-17 A switched network without redundancy This is why you absolutely want to place redundancy throughout the network. Always have some sort of backup so there is a network convergence to a separate parallel link between endpoints (an example would be the LAN-to-LAN connection in the example we used above). Not only does this improve the reliability of data delivery, but a redundant network in a network diagram also really adds something to the overall picture. Figure 12-18 is an example of a network with redundancy. So, the problem is resolved, right? Well, technically, yes, the problem of LAN-to-LAN communication being lost when a link goes down is now resolved. But like many solutions in the data world,26 in resolving one issue, a new issue was introduced. The new issue is a loop, and we discuss it in the next section. 12.5.3.3 I’m Loopy! We have established that there is a requirement for redundancy if we want our LAN to be reliable. However, in introducing that reliability by adding a second link, we have now created an environment where a loop may occur (see Figure 12-19). We’ll now take a different look at the switched network. 26 This includes both public solutions and proprietary solutions. 507 Page 507 Edwards 508 Part III ■ c12.tex V3 - 03/27/2009 Network Design and Implementation LAN switch A LAN switch B LAN 1 LAN 2 Figure 12-18 A switched network with redundancy Layer 2 Switch A Port 1 Port 8 L1-1 L2-1 Port 1 Port 8 L1-2 L2-2 Layer 2 Switch B LAN 1 Figure 12-19 A switched network that is vulnerable to a Layer 2 loop LAN 2 11:27am Page 508 Edwards c12.tex Chapter 12 ■ V3 - 03/27/2009 11:27am Design Methodologies Assume that both switches are aware of all the endpoints. If node L1-1 sends data to node L2-2, that data will be received on port 1 on both switches and will exit out of port 8 on both switches. The data will be sent to the appropriate node, but that node will get two copies of every datagram it receives. Duplication of effort is never desirable27 and is a big no-no in a LAN. So, if the redundant solution caused that kind of problem with the unicast traffic, what will happen with multicast traffic?28 Believe it or not, this is a much bigger problem. Let’s assume that node L1-2 sends out a frame to a multiRANDOM BONUS DEFINITION cast address. The frame will be directed to port 1 on both error detection — A procedure used to switches. Once received, the detect whether received information contains errors. frame will be forwarded to port 8 on both switches. This is where it gets fun. We learned that our LAN switch will receive all data received and will forward the multicasts to all other ports except the one it was received on. This means that both switches will receive the frame on port 8 and will forward it to port 1 on both switches. Port 1 will receive the frame, forward it to port 8, and so on. This multicast frame continues in that same loop indefinitely. Pretty bad, isn’t it? Now assume something is plugged into every interface on the switch we used in the example, and that every one of them has multiple loops going on. It does not take long for this condition to saturate the bandwidth and overwhelm the resources of the switches involved. 12.5.3.3.1 Darn that Redundancy Anyway Now that we have determined that a loop is created when a redundant switch is added to the network, we can let you in on a little secret. Redundant switches are not the only thing that might cause a loop within your switched LAN. Here are a few other things that may be the root cause of a loop on the LAN: A configuration error within the LAN Introducing a duplicate route Introducing an additional node into the LAN LANs can actually become quite complex (if they were not complex from the outset). The more complex a LAN becomes, the more it can create confusion, 27 Remember, your job in designing the network is to ensure performance and reliability. Imagine the extra processing that will occur by not only all of the nodes in the domain, but also the upper-layer tasks that are used. 28 See if you can answer this yourself before continuing on. 509 Page 509 Edwards 510 Part III ■ c12.tex V3 - 03/27/2009 Network Design and Implementation especially if there is incomplete, inaccurate, or missing network documentation. Poor documentation and lack of preparation are leading causes of configuration errors that can lead to a loop in the LAN and can cause disruption in traffic flow, as well as bring portions of the network completely down. Lack of experience and training is also a problem in many LANs. Sometimes a configuration mistake is made by someone who is not really sure what they are doing.29 Incorrect provisioning can cause duplicate routes, and duplicate routes can cause loops. 12.5.3.3.2 Loop Resolution The good news is that there are ways to resolve loops in the LAN and even to prevent them. You can prevent a loop by ACRONYM ALERT not making the LAN vulnerable to a loop. DECnet — Digital Equipment Corporation Network In other words, don’t do anything that can cause a loop. Although this is the optimal choice, it really isn’t practical in today’s LANs.30 The second option is to implement vendor-specific design solutions that manage and eliminate loops. The problem with this choice is that vendor-specific means vendor-specific.31 The final option is to implement a protocol designed to control loops in LANs. The Spanning Tree Protocol (STP) is just that protocol! 12.5.3.3.3 Spanning Tree The Spanning Tree Protocol (STP) is based32 on a protocol that was developed by Digital Equipment Corporation (DEC). Many of DEC’s bridges were equipped with their version of the protocol, eradicating loops in the DEC environment. As bridge technology grew, the IEEE eventually set up a task force to develop a public version of this protocol, and STP was born. STP is covered in IEEE 802.1D. 29 When this happens, you can only hope the individual can either fix the issue quickly or be honest about what they did when you are trying to find the solution. Rich and Jim both have network support backgrounds and they can tell stories of troubleshooting issues that would have been resolved a lot faster had the mistake been pointed out early on. 30 However, it is an open question for some networks where the reliability of data is not the biggest concern. For instance, a mom and pop shop might have a LAN, but really would not suffer if the LAN went down, so the price of redundancy is not worth it. 31 Although many vendors have proprietary protocols that can interact with another vendor’s proprietary protocol, that does not mean they are 100 percent functional in a mixed environment. By implementing a vendor-specific solution, you are effectively tying yourself to that vendor for a while (or will have to wait until the protocol becomes an open standard). 32 ‘‘Based’’ is the operative word. While the protocols share many similarities, they are not fully interoperable. Therefore, if you are using DEC’s version of the protocol, you cannot use the public version in the same network. 11:27am Page 510 Edwards c12.tex Chapter 12 ■ V3 - 03/27/2009 11:27am Design Methodologies Spanning tree33 uses an algorithm, known as the spanning tree algorithm, to make calculations that are used to prevent the dreaded loop. It does this by determining where there are multiple paths to a segment, and then making a calculation that will determine the best bridge to use. Once it determines the best bridge, it will elect that bridge as the root bridge. All other bridges in the group will be assigned the title of designated bridge when they are participating in forwarding data that the root is sending to a destination. In other words, the designated bridge is the one that is responsible for sending data over the best path. Any bridge that is not the root bridge can be a designated bridge.34 At the designated-bridge level, there are different port types: the designated port, the root port, and the inactive port. The inactive port can be either a disabled port (a port that is not used) or a port that has been set into a blocking status. Figure 12-20 shows an example of a network containing physical loops (segments A, C, E and segments B, E, D). SN-1 SN-2 Segment A Segment C tE en gm Se Segment B Segment D SN-3 SN-4 Figure 12-20 A physically looped network Spanning tree will determine which bridge in the group will be elected to function as the root bridge. The root bridge is always the logical center of the network. The way the root bridge is elected is a process that relies on a data message known as a bridge protocol data unit (BPDU). 33 RANDOM BONUS DEFINITION globally administered address — A node or interface identifier whose uniqueness is ensured through the use of an assigned organizationally unique identifier (OUI), typically by the manufacturer of the device or interface. A lot of times instead of calling the protocol STP, network professionals just say ‘‘spanning tree.’’ We like spanning tree; it flows better. 34 We should note that as time goes on and changes happen on the network (adding nodes, removing nodes, etc.), each bridge has the potential of being elected as the root bridge at some point in time. 511 Page 511 Edwards 512 Part III ■ c12.tex V3 - 03/27/2009 Network Design and Implementation The BPDUs are sent by all the bridges that want to participate35 in the spanning tree. One of the fields in the BPDU contains the bridge identifier of the sending bridge. Once all BPDUs have been compared, the one with the lowest value will be elected the root bridge. Once the root bridge is selected, designated bridges are used to forward data. The main rule the designated bridge needs to follow is that only one bridge can forward data from the root bridge to the destination nodes. This rule ensures that no loops can occur because only one designated bridge is sending data from the root. BPDUs are sent by spanning tree nodes to a well-known multicast address. This ensures that everyone in the group will receive the data. Spanning tree will decide which designated bridge it will use to forward a frame and will also decide which designated port to use to forward the data away from the root. Another field that is found in the BPDU is the root path cost. The root path cost is a configurable value that is used to set a priority on a preferred link. The port that is identified as having the lowest path cost will become the designated port and is the port that will be used to forward the frame to its destination. Once the spanning tree has determined the designated port, it will prevent traffic from flowing on other links to that destination by putting the ports on the other links into a blocking state. 12.5.3.3.4 Spanning Tree Port States Every port on a bridge that is participating in spanning tree will have one of five possible port states assigned to it. A port state is exactly what it sounds like: it identifies the current state of the port. Each port state is important as it will identify the function the port is performing. These port states are as follows: Disabled — A port in a disabled state is simply that, disabled. There are many reasons why a port may be disabled. It may be a Physical layer problem, a communication problem, may not be used, etc. Blocking — A port in a blocking state is an active port that is not being used. Any port that is not a designated port or a root port is going to be in a blocking state. A block port listens for BPDUs to determine if it should become active, but does not participate in frame passing when it is in this state. Listening — A port in a listening state is not forwarding frames, but is listening to, and sometimes sending, BPDUs. Learning — A port in a learning state is learning paths to destinations and preparing to forward the frame. This state is used on a port that has not built an address table.36 A learning port 35 By participate, we mean that the node will use the BPDU to find out about other nodes as well as to receive information that will be used to calculate the spanning tree. 36 Normally this is due to the port coming up. 11:27am Page 512 Edwards c12.tex Chapter 12 ■ V3 - 03/27/2009 11:27am Design Methodologies will wait a period of time before it starts forwarding frames.37 This gives it an opportunity to gather path information. Forwarding — This is the port state for the active port that is forwarding frames. 12.5.3.4 Link Aggregation Main Entry: ag·gre·ga·tion (ag-ri-gey-shun) Function: noun. 1. Several things grouped together or considered as a whole. 2. The act of gathering something together. As networks grow and the end application becomes more complex, there is a real need to increase the capacity of a given link. Link aggregation is a method of increasing the capacity of a channel by allowing multiple physical links to act together as a whole. The parallel links make the endpoint nodes think there is a better performing single channel. Figure 12-21 shows an example of two networks; one is using aggregation and one isn’t. 1 Gb/s Single Link 1 Gb/s x 3 Aggregated Link Figure 12-21 The benefits of link aggregation The standard that covers link aggregation is IEEE 802.1ad, the Link Aggregation Control Protocol (LACP). Most high-speed LANs can support larger data rates, so it makes sense to use link aggregation. In smaller, lower-speed LANs, aggregation may not make sense due to the restrictions of the environment.38 The benefits of link aggregation include: Increased link capacity High link availability Often can be done with existing hardware 37 This 38 is known as ‘‘forwarding delay.’’ Why would you want to aggregate to double your capacity if the network cannot support it? 513 Page 513 Edwards 514 Part III ■ c12.tex V3 - 03/27/2009 Network Design and Implementation A few disadvantages of link aggregation include: Requires additional interfaces on each end39 Higher potential of configuration errors POP QUIZ What is forwarding delay? May require device driver updates to ensure compatibility with link aggregation LACP was introduced in 1999 as a standardized way to aggregate multiple gigabit links in a high-speed LAN. As many LANs already supported some proprietary form of aggregation — for instance, Multi-Link Trunking (MLT) for Nortel and Inter-Switch Link (ISL) trunking for Cisco — for lower-speed networks, it was already well known that these were proprietary and did not work with other vendors’ equipment. LACP resolved this for the gigabit world, and things have been growing ever since. Link aggregation has been supported from switch-to-switch, router/server-to-router/server, and switch-to-router/server since it came out, but now many NICs support LACP, allowing aggregation to the end-user level. Although it isn’t used everywhere and a lot of LANs still use proprietary standards, we predict that it won’t be long for this to be the standard of choice. Of course, at the time of this writing, there are a few proprietary solutions that are under standards review, so who knows what tomorrow will bring? 12.5.3.5 Virtual LANs Early on in this book, we determined that a LAN is a data network that serves a small geographical area. Most of us think of a group of nodes connected to one another as forming a LAN (in other words, a broadcast domain). Larger organizations have an organizational LAN that is made up of several broadcast domains, the extent of the LAN being the area it covers or a distance-limiting factor. With the LAN, the limits remain for as long as the node exists in the LAN. What we mean by this is that within a LAN, the logical topology is limited to the physical topology as well. Figure 12-22 shows an example of this. You can only adjust those limits by having additional nodes to collect the broadcast domains that may be located within the same area. In addition, a router is required to ensure that broadcast domains are separated, reducing the effectiveness of the router. 39 You may have to buy more equipment, either now or in the future. Not only may you have to purchase more gear now to support this, this also means that you could be consuming empty slots on existing nodes. Although this is great for now, you may have to buy more in the future. 11:27am Page 514 Edwards c12.tex Chapter 12 ■ V3 - 03/27/2009 11:27am Design Methodologies Switch Collision Domain Collision Domain Broadcast Domains Router Switch Collision Domain Collision Domain Figure 12-22 A traditional LAN The virtual LAN (VLAN) was developed to give a LAN bridge the capability to separate these broadcast domains. This not only frees up the router to perform other important functions, but it also allows the network administrator to be flexible in domain configurations. Nodes no longer have to be in the same physical area to participate in a particular broadcast domain. Figure 12-23 shows an example of this. Notice that in each of the VLANs, there are members of each VLAN on each switch. This is a rough example, but the intention is to show that members of VLANs no longer have to be physically together to be in the same broadcast domain. 12.5.3.5.1 Benefits of VLANs There are a lot of benefits to having VLANs configured in your LAN. Some of these include: Better performance. Only VLAN members receive multicasts. Members of a group no longer have to physically be located close to the group. Administration is easier. Changes to any work area can be done with simple configuration change. 515 Page 515 Edwards 516 Part III ■ c12.tex V3 - 03/27/2009 Network Design and Implementation VLAN 3 VLAN 2 VLAN 1 Switch Switch VLAN 1 VLAN 2 VLAN 3 Figure 12-23 A VLAN Increased security. Only nodes within a VLAN have access to data. No need for a router in order to separate the broadcast domain. RANDOM BONUS DEFINITION individual port — A switch port that cannot form an aggregated link with any other port. 12.5.3.5.2 VLAN-Awareness A node that participates in a VLAN, whether it is a user node or a LAN switch, is known as a VLAN-aware node/switch. This simply means that the node is aware of the VLAN rules and is participating in such an environment. VLAN-awareness is the capability to understand that there is an underlying function that allows the mapping of frames to the correct and appropriate 11:27am Page 516 Edwards c12.tex Chapter 12 ■ V3 - 03/27/2009 11:27am Design Methodologies destination(s). VLAN-aware switches make forwarding decisions based on the destination address as well as the VLAN to which the frame belongs. 12.5.3.5.3 Tag! You’re It! To determine which VLAN a particular frame is a member of, the VLAN environment uses either implicit tagging or explicit tagging. When the switch receives a frame, it will ‘‘tag’’40 the frame with the VLAN identifier where the data came from. This process is known as explicit tagging (commonly referred to as VLAN tagging, or simply tagging). The other type, implicit tagging, is a method of mapping an untagged frame to its associated VLAN by inspecting the contents of the frame. Information that is contained within the explicit tag can be based on MAC address, port, and any other combination of information, but will always contain the VLAN identifier. VLAN tags can be set by a VLAN-aware node, or they can be assigned to a frame when received on a VLAN-aware switch. When a VLAN-aware switch receives an untagged frame, it applies the VLAN mapping rules and forwards the frame with the tagged bit set. The implicitly tagged frame is a