OSPF and IS-IS: Choosing an IGP for Large-Scale Networks / Edition 1

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Overview

The practical guide to large-scale networking with OSPF and IS-IS

This is the definitive guide to using OSPF and IS-IS protocols in large-scale IP enterprise, carrier, and service provider networks. Well-known network designer Jeff Doyle draws on his consulting experience, offering realistic advice and straight answers on every aspect of working with link-state protocols—from scalability, reliability, and security to area design and database synchronization.

This book is organized to help network engineers and architects compare OSPF and IS-IS. One feature at a time, Doyle first demonstrates how a topic or feature is implemented in OSPF, and then walks through a similar implementation using IS-IS. Professionals who are relatively new to large-scale networking will welcome his practical introduction to the concepts, goals, and history of link state protocols. Coverage includes

  • Understanding message types, encapsulation, architecture, LSAs, and LSPs
  • Optimizing addressing, neighbor discovery, adjacencies, and router designation
  • Improving scalability: controlling the scope of flooding, link state database size, SPF calculation efficiency, and much more
  • Designing and operating large-scale networks for maximum security and reliability
  • Hardening networks to thwart attacks against routing protocols
  • Comparing OSPF and IS-IS extensibility
  • Utilizing extensions for MPLS-based traffic engineering, IPv6, and multi-topology routing
  • Troubleshooting OSPF and IS-IS log entries, debug output, and LS databases

Doyle's thorough explanations, end-of-chapter review questions, and many wide-ranging examples for both Cisco's IOS and Juniper's JUNOS also make this book an exceptional resource for anyone pursuing a CCIE or JNCIE certification.

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Product Details

  • ISBN-13: 9780321168795
  • Publisher: Addison-Wesley
  • Publication date: 11/1/2005
  • Edition description: New Edition
  • Edition number: 1
  • Pages: 455
  • Product dimensions: 6.98 (w) x 9.17 (h) x 0.91 (d)

Meet the Author

Specializing in IP routing protocols, MPLS, and IPv6, Jeff Doyle has designed or assisted in the design of large-scale IP service provider networks throughout North America, Europe, Japan, Korea, and the People s Republic of China. Jeff is the author of CCIE Professional Development: Routing TCP/IP, Volumes I and II, is an editor and contributing author of Juniper Networks Routers: The Complete Reference. Jeff has presented numerous corporate seminars for Juniper Networks, and has also spoken at NANOG, JANOG, APRICOT, and at IPv6 Forum conferences.

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Read an Excerpt

OSPF and IS-ISOSPF and IS-IS: Choosing an IGP for Large-Scale NetworksPreface

This book is an expansion of a PowerPoint presentation I have used for years in design meetings for IP carrier and service provider networks, comparing and contrasting OSPF and IS-IS. The PowerPoint presentation in turn grew from numerous informal chalk talks I had done for years before that, to address the concerns of engineers who had worked extensively with OSPF but had little or no experience with IS-IS and therefore wondered whether that protocol might be a better fit for their network.

With the growth of large-scale networks, the demand for information on OSPF and IS-IS—and how they compare—continues to increase. To enhance the ability of engineers to access, retain, and reference this information, creating a book was a logical next step in the evolution of my live presentations and seminars. The information in this book certainly reflects the questions, concerns, and challenges that audiences have consistently articulated. I trust readers will find their own business and technical interests addressed herein.

Audience

I have written this book primarily for those engineers and network architects who know OSPF but would like to learn more about IS-IS. For this reason, you will find that for each chapter or section topic I almost always discuss OSPF first, and then IS-IS: The idea is to start you off on familiar ground, discussing how the topic at hand is implemented in OSPF, and then showing how it is implemented in IS-IS.

The book will also be helpful to those who do not already have a rich understanding of OSPF. Perhaps you know OSPF or IS-IS or both at an intermediate level, and you want to deepen your understanding of both protocols. Or perhaps you are one of those rare engineers who knows IS-IS but not OSPF. I have detailed both protocols equally to meet any of these needs.

You will also find the book educational if you are a beginning-level networker who wants to expand your knowledge of link state protocols. Chapter 2 in particular is written for you, to get you through the essential concepts of routing protocols in general and link state protocols in particular, so that you are prepared to tackle the remainder of the book. The chapters are arranged so that they take you from foundation concepts to increasingly more complex ones.

Finally, if you are preparing for a network certification such as Cisco Systems' CCIE or Juniper Networks' JNCIE, you will gain in these pages the essential conceptual understanding of OSPF and IS-IS needed to do well on the test. Configuration and troubleshooting exercises are beyond the scope of this book, however, and you will need to practice solving them via another resource to prepare fully for a certification exam. Review questions are included at the end of each chapter of this book to help you test your comprehension and retention of concepts before moving on to the next chapter. Because the answers are well documented in the body of the chapter, a separate answer sheet is not included.

What Is a Large-Scale Data Communications Network?

OSPF and IS-IS are uniquely appropriate routing protocols for large-scale data communication networks. But how exactly do we characterize such a network? It might be useful to approach a definition by remembering that data communication networks and ideas about them have been around since long before the advent of computers.

For example, when Alexander Graham Bell and Theodore Vail founded American Telephone and Telegraph Company (AT&T) in 1885, they had no intention of providing telegraph services. But they understood that the network they were building could be used for transmitting more than just telephony signals. They had no clear vision of what those other signals might be, other than knowing that they would be, like telephony signals, representations of information. "Telegraph" reflected Bell's and Vail's anticipation of data communication networks within the understanding of their time.

Wide-area data communication has been around for as long as man has had a need to share information over a range greater than voice can cover. Many ancient civilizations used signal fires to communicate quickly over a long distance. In feudal Japan, villages sent paper lanterns aloft in the evenings, rising on hot air created by the same fire that illuminated them, to notify nearby villages of their safety. Throughout the southwestern United States you can find petroglyphs—carvings of figures and symbols on the sides of rocks—created by hunting, warring, or traveling parties of Native Americans over the centuries. Although some might have been intended as merely decorative, many petroglyphs are thought to be signals and messages left by one party for other parties expected to pass that way. These carvings put an interesting twist on data communications: The signal remains stationary while the transmitting and receiving nodes move around.

Telegraph networks were the first data communications networks using electrical, digital signals. And although they were certainly wide-area—connecting countries and even spanning continents via transoceanic cables—they did not entail the complexity that we assume today when we talk about large-scale networks. Signals were easily originated, routed, and received by human operators, and the network was maintained through human monitoring and intervention.

How, then, do we define a large-scale network? Although the number of nodes and links certainly influences the definition, a definition based solely on numbers is too narrow. Instead, the scale of a network is defined by its complexity. A small-scale network is one that can be easily managed by direct human intervention; hence early telegraph networks could be considered small-scale even though they covered a very large geographic area. In terms of IP networks, a small-scale network is one that can be routed statically and requires no automated management systems. Notice I say can be: A given small-scale network might in fact run a routing protocol or automated management software; just because it can be routed statically does not mean it must be.

As a network grows in complexity, automation becomes more necessary. A mid-sized network is one in which static routing and management by direct human monitoring and intervention are no longer practical. In IP networks, a routing protocol such as RIP is required to maintain forwarding over many paths.

As the network continues to grow, however, the capabilities of automated systems themselves become an issue. As Chapter 2 explains, simple routing protocols such as RIP present problems in complex networks with many routing variables. A large-scale network, then, is one in which the automated network systems must be able to manage the network as a single entity rather than managing individual connections between nodes. Factors to consider in a large-scale network include the following:

  • Complex interactions between individual nodes
  • Complex path diversity requiring load balancing, traffic monitoring and distribution, and strong loop avoidance
  • Complex link metrics
  • Diverse data transport requirements
  • Stringent requirements for security and reliability

OSPF and IS-IS can easily be run in a network of any size. But their true value is in their capability to perform consistently as their network domain grows large. No other IGP for IP networks can reliably route the world's largest networks.

A Word on IOS and JUNOS

The examples used throughout this book are provided in either Cisco Systems IOS or Juniper Networks JUNOS. In the early chapters, I occasionally provide examples from both operating systems. However, my intention is to provide you with an understanding of the protocols themselves, not to attempt to teach you a specific operating system. I have used IOS and JUNOS simply because they are the router operating systems I know and have access to. Armed with the information in this book you should be able to pick up a manual from any router vendor and easily configure and troubleshoot OSPF and IS-IS on that vendor's system.

I first and foremost want to thank Catherine Nolan and all of the editorial, production, and marketing staff at Addison-Wesley for the extraordinary patience they had with me and with my long string of missed deadlines. I also want to thank my development editor, Laurie McGuire. This is not the first book Laurie and I have worked on together, and as usual she has made me appear to be a much better writer than I am.

Thanks to my technical reviewers: Eural Authement, Hannes Gredler, Dave Humphrey, Pete Moyer, Mike Shand, and Rena Yang. The experience and depth of knowledge this group represents are incomparable, and no author could ask for a better review team. I would also like to thank Ross Callon, Vint Cerf, Steve Crocker, Paul Goyette, Matt Kolon, Chelian Pandian, and Russ White, and all of my colleagues in the Professional Services group of Juniper Networks, for comments and advice on specific sections of the book.

My wife, Sara, and my children, Anna, Carol, James, and Katherine, are my bedrock. Their support and encouragement have been essential to the success of this book project; and the love, teasing, and laughter they bring into my life is priceless. Without them, I would be adrift.

Finally, I want to thank all of the readers of my previous books. I am honored by the kind comments and generous compliments I have received from around the world, and I hope you find this book to be equally useful.

© Copyright Pearson Education. All rights reserved.

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Table of Contents

About the Author.

Preface.

Audience.

What Is a Large-Scale Data Communications Network?

A Word on IOS and JUNOS.

Acknowledgments.

1. The Roots of Link State Protocols.

An Intergalactic Network.

ARPANET.

The Network Working Group.

The Birth of the Internet.

Routing in the ARPANET.

The European Invasion.

Separate But Equal.

Conclusion.

2. Link State Basics.

Vector Protocol Basics.

Vector Protocol Convergence.

Common Characteristics of Vector Protocols .

Routing Loops.

Fundamental Link State Concepts.

Adjacencies.

Flooding.

Announcement Headers.

Database Synchronization.

SPF Calculations.

Areas.

Review Questions.

3. Message Types.

Comparative Terminology.

Message Encapsulation.

Message Architecture.

Message Types.

LSAs and LSPs.

Subnetwork Dependent and Independent Functions.

Subnetwork Dependent Functions.

Subnetwork Independent Functions.

Review Questions.

4. Addressing, Neighbor Discovery, and Adjacencies.

Router and Area IDs.

OSPF Router IDs.

Troubleshooting: Duplicate Router IDs.

OSPF Area IDs.

IS-IS System and Area IDs.

The Hello Protocol.

OSPF Hello Protocol Basics.

IS-IS Hello Protocol Basics.

IS-IS Dynamic Hostname Exchange.

OSPF Domain Name Lookup.

Adjacencies.

OSPF Adjacencies.

IS-IS Adjacencies.

Designated Routers.

OSPF Designated Routers.

IS-IS Designated Intermediate Systems.

Media Types.

OSPF Network Types.

IS-IS Network Types.

Interface Databases.

The OSPF Interface Data Structure.

OSPF Interface States.

The IS-IS Interface Data Structure.

Review Questions.

5. Flooding.

Flooding Components.

OSPF Flooding.

IS-IS Flooding.

Areas and Router Types.

OSPF Areas and Router Types.

IS-IS Areas and Router Types.

Metric Types.

OSPF Metrics.

IS-IS Metrics.

Essential LSAs.

Router LSAs.

Network LSAs.

Network Summary LSAs.

ASBR Summary LSAs.

AS-External LSAs.

Essential TLVs.

Area Addresses TLV.

IS Neighbors TLV.

Protocols Supported TLV.

IP Interface Addresses TLV.

IP Internal Reachability Information TLV.

IP External Reachability Information TLV.

Extended IS Reachability TLV.

Extended IP Reachability TLV.

Review Questions.

6. Link State Database Synchronization.

OSPF Database Synchronization.

OSPF Packets Used in Database Synchronization.

The Options Field.

The OSPF Neighbor Data Structure.

LSA Lists for Database Exchange and Flooding.

Database Exchange Management: Masters and Slaves.

The OSPF Neighbor State Machine.

Troubleshooting: Reading OSPF Log Entries and Debug Output.

Troubleshooting: Comparing OSPF LS Databases.

IS-IS Database Synchronization.

IS-IS PDUs Used in Synchronization.

Send Routing Message and Send Sequence Number Flags.

Synchronization on Point-to-Point Networks.

Synchronization on Broadcast Networks.

Troubleshooting: Reading IS-IS Log Entries and Debug Output.

Troubleshooting: Comparing IS-IS LS Databases.

Review Questions.

7. Area Design.

Area Scalability.

Area Reliability.

OSPF Areas.

Backbone and Non-Backbone Areas.

Factors for Scaling OSPF Areas.

External Prefixes and OSPF Scaling.

Stub Areas.

Totally Stubby Areas.

Not-So-Stubby Areas.

Address Summarization.

Virtual Links.

IS-IS Areas.

Backbone and Non-Backbone Areas.

Factors for Scaling IS-IS Areas.

Default IS-IS L1 Area Behavior.

Redundant L1/L2 Routers.

Address Summarization, Again.

L2 to L1 Route Leaking.

Redistributing External Prefixes into IS-IS.

Multiple Area IDs.

IS-IS Virtual Links.

BGP and Area Design.

Review Questions.

8. Scaling.

SPF Enhancements.

Equal-Cost Multipath.

Pseudonodes and ECMP.

Incremental SPF Calculations.

Partial Route Calculations.

SPF Delay.

Flooding Enhancements.

Transmit Pacing.

Retransmit Pacing.

Mesh Groups.

Demand Circuits and Flood Reduction.

Fragmentation.

Overloading.

Review Questions.

9. Security and Reliability.

Routing Protocol Vulnerabilities.

Malicious Threats.

Non-Malicious Threats.

Security and Reliability Features.

Inherent Security.

Authentication.

Checksums.

Graceful Restart.

Bidirectional Forwarding Detection.

Designing for Security and Reliability.

Redundancy.

Protecting the Domain Edge.

Protecting the Router.

Operating for Security and Reliability.

Configuration Management.

Change Management.

The Network Lab.

Review Questions.

10. Extensibility.

Extending OSPF.

The OSPF Extensibility Problem.

Opaque LSAs.

The Router Information Opaque LSA.

Extending IS-IS.

The IS-IS Extensibility Advantage.

The Protocols Supported TLV.

Route Tagging.

Review Questions.

11. Extensions for MPLS Traffic Engineering.

MPLS: An Overview.

Labels and Label Switching.

Forwarding Equivalence Classes and Label Binding.

Label Distribution.

The MPLS Header.

Traffic Engineering: An Overview.

TE Link Parameters.

Constrained Shortest Path First.

OSPF Extensions for Traffic Engineering.

IS-IS Extensions for Traffic Engineering.

Review Questions.

12. Extensions for IPv6.

IPv6: An Overview.

IPv6 Features and Functions.

IPv6 Address Format.

IPv6 Address Representation.

The Neighbor Discovery Protocol.

Stateless Address Autoconfiguration.

IPv6 Header Format.

Extension Headers.

OSPFv3.

IPv4 and IPv6 Compatibility in OSPF.

Differences from OSPFv2.

OSPFv3 LSAs.

The Options Field.

OSPFv3 Packets.

Future Extensions to OSPFv3.

IS-IS Extensions for IPv6.

Review Questions.

13. Extensions for Multi-Topology Routing.

OSPF Extensions for Multi-Topology Routing.

MT-OSPF Procedures.

MT-OSPF LSAs.

Link Exclusion.

IS-IS Extensions for Multi-Topology Routing.

MT-ISIS Procedures.

MT-ISIS TLVs.

Review Questions.

AFTERWORD: The Future of Link State Protocols.

Index.

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Preface

OSPF and IS-IS: Choosing an IGP for Large-Scale Networks

Preface

This book is an expansion of a PowerPoint presentation I have used for years in design meetings for IP carrier and service provider networks, comparing and contrasting OSPF and IS-IS. The PowerPoint presentation in turn grew from numerous informal chalk talks I had done for years before that, to address the concerns of engineers who had worked extensively with OSPF but had little or no experience with IS-IS and therefore wondered whether that protocol might be a better fit for their network.

With the growth of large-scale networks, the demand for information on OSPF and IS-IS—and how they compare—continues to increase. To enhance the ability of engineers to access, retain, and reference this information, creating a book was a logical next step in the evolution of my live presentations and seminars. The information in this book certainly reflects the questions, concerns, and challenges that audiences have consistently articulated. I trust readers will find their own business and technical interests addressed herein.

Audience

I have written this book primarily for those engineers and network architects who know OSPF but would like to learn more about IS-IS. For this reason, you will find that for each chapter or section topic I almost always discuss OSPF first, and then IS-IS: The idea is to start you off on familiar ground, discussing how the topic at hand is implemented in OSPF, and then showing how it is implemented in IS-IS.

The book will also be helpful to those who do not already have a rich understanding of OSPF. Perhaps you know OSPF or IS-IS or both at an intermediate level, and you want to deepen your understanding of both protocols. Or perhaps you are one of those rare engineers who knows IS-IS but not OSPF. I have detailed both protocols equally to meet any of these needs.

You will also find the book educational if you are a beginning-level networker who wants to expand your knowledge of link state protocols. Chapter 2 in particular is written for you, to get you through the essential concepts of routing protocols in general and link state protocols in particular, so that you are prepared to tackle the remainder of the book. The chapters are arranged so that they take you from foundation concepts to increasingly more complex ones.

Finally, if you are preparing for a network certification such as Cisco Systems' CCIE or Juniper Networks' JNCIE, you will gain in these pages the essential conceptual understanding of OSPF and IS-IS needed to do well on the test. Configuration and troubleshooting exercises are beyond the scope of this book, however, and you will need to practice solving them via another resource to prepare fully for a certification exam. Review questions are included at the end of each chapter of this book to help you test your comprehension and retention of concepts before moving on to the next chapter. Because the answers are well documented in the body of the chapter, a separate answer sheet is not included.

What Is a Large-Scale Data Communications Network?

OSPF and IS-IS are uniquely appropriate routing protocols for large-scale data communication networks. But how exactly do we characterize such a network? It might be useful to approach a definition by remembering that data communication networks and ideas about them have been around since long before the advent of computers.

For example, when Alexander Graham Bell and Theodore Vail founded American Telephone and Telegraph Company (AT&T) in 1885, they had no intention of providing telegraph services. But they understood that the network they were building could be used for transmitting more than just telephony signals. They had no clear vision of what those other signals might be, other than knowing that they would be, like telephony signals, representations of information. "Telegraph" reflected Bell's and Vail's anticipation of data communication networks within the understanding of their time.

Wide-area data communication has been around for as long as man has had a need to share information over a range greater than voice can cover. Many ancient civilizations used signal fires to communicate quickly over a long distance. In feudal Japan, villages sent paper lanterns aloft in the evenings, rising on hot air created by the same fire that illuminated them, to notify nearby villages of their safety. Throughout the southwestern United States you can find petroglyphs—carvings of figures and symbols on the sides of rocks—created by hunting, warring, or traveling parties of Native Americans over the centuries. Although some might have been intended as merely decorative, many petroglyphs are thought to be signals and messages left by one party for other parties expected to pass that way. These carvings put an interesting twist on data communications: The signal remains stationary while the transmitting and receiving nodes move around.

Telegraph networks were the first data communications networks using electrical, digital signals. And although they were certainly wide-area—connecting countries and even spanning continents via transoceanic cables—they did not entail the complexity that we assume today when we talk about large-scale networks. Signals were easily originated, routed, and received by human operators, and the network was maintained through human monitoring and intervention.

How, then, do we define a large-scale network? Although the number of nodes and links certainly influences the definition, a definition based solely on numbers is too narrow. Instead, the scale of a network is defined by its complexity. A small-scale network is one that can be easily managed by direct human intervention; hence early telegraph networks could be considered small-scale even though they covered a very large geographic area. In terms of IP networks, a small-scale network is one that can be routed statically and requires no automated management systems. Notice I say can be: A given small-scale network might in fact run a routing protocol or automated management software; just because it can be routed statically does not mean it must be.

As a network grows in complexity, automation becomes more necessary. A mid-sized network is one in which static routing and management by direct human monitoring and intervention are no longer practical. In IP networks, a routing protocol such as RIP is required to maintain forwarding over many paths.

As the network continues to grow, however, the capabilities of automated systems themselves become an issue. As Chapter 2 explains, simple routing protocols such as RIP present problems in complex networks with many routing variables. A large-scale network, then, is one in which the automated network systems must be able to manage the network as a single entity rather than managing individual connections between nodes. Factors to consider in a large-scale network include the following:

  • Complex interactions between individual nodes
  • Complex path diversity requiring load balancing, traffic monitoring and distribution, and strong loop avoidance
  • Complex link metrics
  • Diverse data transport requirements
  • Stringent requirements for security and reliability

OSPF and IS-IS can easily be run in a network of any size. But their true value is in their capability to perform consistently as their network domain grows large. No other IGP for IP networks can reliably route the world's largest networks.

A Word on IOS and JUNOS

The examples used throughout this book are provided in either Cisco Systems IOS or Juniper Networks JUNOS. In the early chapters, I occasionally provide examples from both operating systems. However, my intention is to provide you with an understanding of the protocols themselves, not to attempt to teach you a specific operating system. I have used IOS and JUNOS simply because they are the router operating systems I know and have access to. Armed with the information in this book you should be able to pick up a manual from any router vendor and easily configure and troubleshoot OSPF and IS-IS on that vendor's system.

I first and foremost want to thank Catherine Nolan and all of the editorial, production, and marketing staff at Addison-Wesley for the extraordinary patience they had with me and with my long string of missed deadlines. I also want to thank my development editor, Laurie McGuire. This is not the first book Laurie and I have worked on together, and as usual she has made me appear to be a much better writer than I am.

Thanks to my technical reviewers: Eural Authement, Hannes Gredler, Dave Humphrey, Pete Moyer, Mike Shand, and Rena Yang. The experience and depth of knowledge this group represents are incomparable, and no author could ask for a better review team. I would also like to thank Ross Callon, Vint Cerf, Steve Crocker, Paul Goyette, Matt Kolon, Chelian Pandian, and Russ White, and all of my colleagues in the Professional Services group of Juniper Networks, for comments and advice on specific sections of the book.

My wife, Sara, and my children, Anna, Carol, James, and Katherine, are my bedrock. Their support and encouragement have been essential to the success of this book project; and the love, teasing, and laughter they bring into my life is priceless. Without them, I would be adrift.

Finally, I want to thank all of the readers of my previous books. I am honored by the kind comments and generous compliments I have received from around the world, and I hope you find this book to be equally useful.

© Copyright Pearson Education. All rights reserved.

Read More Show Less

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  • Anonymous

    Posted January 25, 2006

    nice chapter on the development of the Internet

    Doyle gives us a very understandable discourse on the OSPF and IS-IS routing methods. These are implemented by Cisco and Juniper routers. Cisco dominates the networking arena, while Juniper is one of the larger secondary players. So understanding OSPF and IS-IS is vital if your duties involve administering networks using those companies' devices. The link state nature of the OSPF and IS-IS protocols is shown to scale much more easily to large networks, as compared to vector protocols. The latter are slower to converge and are susceptible to looping. Interestingly, the book starts off with a detailed chapter on the rise of the Internet. It mentions luminaries like Vinton Cerf, Licklider, Kleinrock, Postel and others. And how the ARPANET was the predecessor of the Internet. However, I do take issue with the claim that the Internet began in 1983, when the ARPANET transitioned to TCP/IP. The chapter itself says that 'almost all the internetworking technologies we use to this day had their start with the ARPANET.' Thus, others who were involved in establishing the ARPANET take the Internet's true beginning to be that of the ARPANET. For example, Kleinrock considers the birth date to be in October 1969, when his group made the first connection between two nodes on the ARPANET, at UCLA and Stanford Research Institute. He and UCLA considers this date to be definitive. Granted, there is hometown boosterism here, but I heard him give a seminar with a strong technical description of the 1969 event, and it seemed very plausible. But I should add that even if you consider Doyle's assertion about the Internet's start to be wrong, it does not detract from his first chapter or the rest of the book. The objective events in that chapter are correctly recounted, and the chapter is useful in showing how all this Internet 'thing' came about. Read it as good cultural background.

    Was this review helpful? Yes  No   Report this review
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