OSPF: Anatomy of an Internet Routing Protocol / Edition 1

by John Moy

Overview

Written for TCP/IP network administrators, protocol designers, and network application developers, this book gives the most complete and practical view ever into the inner workings of Internet routing. The book focuses on OSPF (Open Shortest Path First), a common TCP/IP routing protocol that provides robust and efficient routing support in the most demanding Internet environments. A methodical and detailed description of the protocol is offered and OSPF's role within the wider context of a TCP/IP network is demonstrated.

Practical throughout, the book provides not only a theoretical description of Internet routing, but also a real-world look into how theory translates into practice. It shows how algorithms are implemented, and how the routing protocols function in a working network where transmission lines and routers routinely break down.

You will find clear explanations of such routing fundamentals as how a router forwards packets, IP addressing, CIDR (Classless Inter-Domain Routing), the routing table, Internet routing architecture, and the two main routing technologies: Distance Vector and link-state algorithms. OSPF is discussed in depth, with an examination of the rationale behind OSPF's design decisions and how it has evolved to keep pace with the rapidly changing Internet environment. OSPF topics covered by the book include the following:

• OSPF areas and virtual links
• NBMA (Nonbroadcast multi-access) and Point-to-MultiPoint network segments
• OSPF configuration and management
• Interaction with other routing protocols
• OSPF cryptographic authentication
• OSPF protocol extensions, including the Demand Circuit extensions and the multicast extensions to OSPF (MOSPF)
• An OSPF FAQ

IP multicast and multicast routing are also discussed. Methods for debugging routing problems are explained, including a catalog of available debugging tools. The book also offers side-by-side comparisons of all the unicast and multicast routing protocols currently in use in the Internet.

You will come away from this book with a sophisticated understanding of Internet routing and of the OSPF protocol in particular. Moreover, the book's practical focus will enable you to put this deeper understanding to work in your network environment.

Product Details

ISBN-13:	9780201634723
Publisher:	Pearson Education
Publication date:	02/02/1998
Edition description:	New Edition
Pages:	360
Product dimensions:	7.45(w) x 9.50(h) x 0.85(d)

About the Author

John T. Moy is a Senior Consulting Engineer at Ascend Communications. He is the author of the OSPF and MOSPF protocol specifications and currently chairs the OSPF and MOSPF Working Groups in the Internet Engineering Task Force. Mr. Moy has been involved in the design and development of router software for 15 years, currently at Ascend, and previously at Proteon and at Bolt Beranek and Newman. Mr. Moy holds a Master of Arts in Mathematics from Princeton University and a Bachelor of Engineering in Mathematics from the University of Minnesota.

0201634724AB04062001

Read an Excerpt

Introduction The Internet is a global communications network. With connections in more than 100 countries, tens of millions of people use the Internet for business, education, and recreation. Electronic commerce is beginning on the Internet as businesses connect to sell their products and services. Academics collaborate over the Internet by exchanging electronic mail. People can converse using Internet phones, send faxes, participate in online chats and bulletin boards, play multiuser games, and experiment with virtual environments. Special-purpose computers called routers connect the Internet together. As data is forwarded from one place in the Internet to another, it is the routers that make the decisions as to where and how the data is forwarded. The protocols that dynamically inform the routers of the paths that the data should take are called routing protocols. It is the job of these protocols to react quickly to changes in the Internet's infrastructure, such as transmission lines going in and out of service, routers crashing, changes in network policies, and so on. Routing is what makes the Internet tick. Although many users of the Internet and the World Wide Web are unaware of the machinery underlying the network applications, routing is an interesting but complicated subject. Routing protocols are sophisticated distributed algorithms that must also be extremely robust to keep a large, decentralized network like the Internet running smoothly. Audience This book is for students of data communications, TCP/IP network administrators, protocol designers, developers of routing protocol software, and other professionals involved in the design, development, and managementof TCP/IP networks. This book is a practical, hands-on description of Internet routing rather than a theoretical treatment. Although we describe how the various protocols were intended to work, we also describe how well the design has translated into practice. Internet protocol design is a practical undertaking itself, with efficiency of implementation often dictating design choices. For this reason, this book gives an in-depth treatment of how a router really works. Instead of just describing the algorithms, this book goes beyond to show how the algorithms are implemented. We often present ideas in a historical context, showing how Internet protocols have evolved. This is done for two reasons. First, you can learn a lot from the mistakes (and successes) of the past. Second, in order to participate in Internet discussion groups, many of which are dominated by old-timers, it is good to have some context. This book is not an elementary introduction to TCP/IP and its routing. Instead we assume that you have some familiarity with the TCP/IP protocol suite and some exposure to the basic concepts of routing. These assumptions allow us to explore many of the facets of Internet routing in greater detail than possible in an introductory text. Organization of This Book

This book is organized into five parts. Part I sets the groundwork for a discussion of Internet routing. After a brief description of how routing fits together with the rest of the Internet's protocols, Chapter 1 describes in depth how a router forwards packets. This discussion naturally leads to an explanation of IP addressing and CIDR, as well as of the interaction of hosts and routers. Internet routing protocols are introduced in Chapter 2, beginning with a treatment of the end product of all routing protocols: the router's routing table. Chapter 2 ends with an overview of the Internet's routing architecture and the two main routing technologies in use in today's Internet: Distance Vector and link-state algorithms.

Part II describes the Internet's OSPF routing protocol. We start in Chapter 3 with an explanation of why the OSPF protocol was developed in the first place. Chapter 4 discusses the basics of link-state routing; Chapter 5, how OSPF behaves over various subnet technologies; Chapter 6, its use of hierarchical routing; and Chapter 7, extensions to OSPF. Each chapter not only describes how OSPF works but also explains why it works that way. We explore the reasons behind OSPF's design decisions and how the OSPF protocol has evolved to keep pace with the rapidly changing Internet environment. Part II concludes with an OSPF FAQ (Chapter 8).

Part III (Chapters 9 and 10) describes TCP/IP multicast routing, including broadcast and multicast forwarding, the MBONE, and the two distinct types of multicast routing protocols: source-based trees and shared-tree algorithms. As we did with unicast routing, we go further into the subject of multicast routing through the examination of a particular multicast routing protocol: the Multicast Extensions to OSPF (MOSPF).

Part IV covers the configuration and management of Internet routing. The configuration and management of OSPF is explained in detail in Chapter 11. Chapter 12 describes the tools used to monitor and debug routing in a TCP/IP network. For each tool, we describe its use, how it works, and its advantages and drawbacks.

Part V is a comparison of Internet routing protocols. Chapter 13 compares and contrasts the routing protocols in use in the Internet: RIP, OSPF, BGP, IGRP, and IS-IS. In Chapter 14, we examine the available multicast protocols: DVMRP, MOSPF, PIM Dense and Sparse, and CBT.

Following Chapter 14 is an extensive bibliography arranged and numbered in alphabetical order. Within the text, the citation 85, for example, refers to item 85 in the bibliography. Companion Book: OSPF Complete Implementation

The companion book OSPF Complete Implementation, in keeping with the Internet tradition that reveres "working code" over all else, explores even further the mechanics of Internet routing through examination of a real, working OSPF implementation. The book contains a complete implementation of OSPF on CD. Written in C++, the OSPF implementation is intended to be portable to a wide range of environments. Two sample ports are included: an OSPF routing daemon (called ospfd) for FreeBSD 2.1 and an OSPF routing simulator that can be run on Windows 95. The OSPF implementation has been developed using publicly available tools. Acknowledgments

I would like to thank the technical reviewers who improved this book through their thoughtful and timely reviews: Ran Atkinson, Eural Authement, Fred Baker, Howard Berkowitz, Jeffrey Burgan, Joel Halpern, Mukesh Kacker, Robert Minnear, Jim Reid, and W. Richard Stevens. Thanks also to Tim Stoddard and the Arkansas Public School Computer Network (APSCN) for letting me collect OSPF statistics on the APSCN network and use that network as an example of OSPF configuration in Chapter 11, Configuration and Management. Thanks to S. Randall McLamb for drawing the figures.

I would also like to acknowledge the help of my editors at Addison Wesley Longman over the long life of this project: Carol Long, Karen Gettman, and Mary Harrington.

And special thanks to my wife, Sonya Keene, who designed the book, edited rough drafts, created the index, and gave encouragement while this book was being written. J.M.
October, 1997

Table of Contents

List of Tables ..... ix

List of Figures ..... xi

Preface ..... xv

Part I. Internet Routing Overview ..... 1

Chapter 1. Role of Routers in the Internet ..... 3

1.1 The Internet Protocol Suite ..... 4
1.2 Forwarding IP Datagrams ..... 9
1.3 IPv6 ..... 22

Chapter 2. Internet Routing Protocols ..... 27

2.1 Routing Tables ..... 27
2.2 Internet Routing Architecture ..... 32
2.3 Distance Vector Algorithms ..... 35
2.4 Link-State Algorithms ..... 39

Part II. The OSPF Protocol ..... 41

Chapter 3. Developing the OSPF Protocol ..... 43

3.1 Functional Requirements ..... 43
3.2 Design Decisions ..... 47
3.3 OSPFv1: A False Start ..... 56
3.4 Interoperability Testing ..... 57
3.5 Field Trials ..... 61
3.6 On Becoming a Standard ..... 66
3.7 The Internet Evolves ..... 67

Chapter 4. OSPF Basics ..... 71

4.1 An OSPF Example ..... 72
4.2 Link State Advertisements (LSAs) ..... 74
4.3 A Sample LSA: The Router-LSA ..... 81
4.4 The Link-State Database ..... 83
4.5 Communicating between OSPF Routers: OSPF Packets ..... 85
4.6 Neighbor Discovery and Maintenance ..... 86
4.7 Database Synchronization ..... 87
4.8 Routing Calculations ..... 95

Chapter 5. OSPF Network Types ..... 101

5.1 The IP Subnet Model ..... 102
5.2 Broadcast Subnets ..... 104
5.3 NBMA Subnets ..... 111
5.4 Point-to-MultiPoint Subnets ..... 114

Chapter 6. Hierarchical Routing in OSPF ..... 119

6.1 OSPF Areas ..... 122
6.2 Incorporating External Routing Information ..... 127
6.3OSPF Area Types ..... 132

Chapter 7. OSPF Extensions ..... 135

7.1 TOS-Based Routing ..... 137
7.2 Stub Areas ..... 139
7.3 Demand Circuit Extensions ..... 140
7.4 NSSA Areas ..... 143
7.5 Database Overflow Support ..... 145
7.6 The External-Attributes-LSA ..... 146

Chapter 8. An OSPF FAQ ..... 151

Part III. Internet Multicast Routing ..... 169

Chapter 9. Internet Multicast Routing ..... 171

9.1 Internet Multicast Model ..... 171
9.2 The Multicast Protocol Stack ..... 173
9.3 Broadcast Forwarding ..... 178
9.4 MBONE ..... 184

Chapter 10. MOSPF ..... 187

10.1 An Extended Example ..... 188
10.2 Group-Membership-LSAs ..... 192
10.3 MOSPF Routing Calculations ..... 194
10.4 Hierarchical Multicast in MOSPF ..... 199
10.5 Backward Compatibility: Mixing with Nonmulticast Routers ..... 203
10.6 MOSPF in the MBONE ..... 206

Part IV. Configuration and Management ..... 211

Chapter 11. OSPF Management ..... 213

11.1 SNMP ..... 214
11.2 OSPF MIB ..... 218
11.3 Configuring OSPF ..... 222
11.4 An Example: The Arkansas Public School Computer Network ..... 228
11.5 Monitoring the OSPF Protocol ..... 230
11.6 Interactions with Other Routing Protocols ..... 233
11.7 OSPF Security ..... 236

Chapter 12. Debugging Routing Problems ..... 243

12.1 War Stories ..... 244
12.2 Finding Tools for Debugging Routing Problems ..... 245
12.3 Tool Interpretation ..... 245
12.4 The ping Tool ..... 247
12.5 The traceroute Tool ..... 250
12.6 SNMP MIBs ..... 254
12.7 MIB-Based Tools ..... 257
12.8 Network Analyzers ..... 259
12.9 Protocol-Specific Tools ..... 263
12.10 Product-Specific Monitoring and Tracing ..... 265
12.11 Multicast Traceroute ..... 266

Part V. Routing Protocol Comparisons ..... 273

Chapter 13. Unicast Routing Protocols ..... 275

13.1 RIP ..... 277
13.2 OSPF ..... 281
13.3 BGP ..... 284
13.4 IGBP ..... 297
13.5 Integrated IS-IS ..... 300
13.6 Historical Protocols ..... 303
13.7 Interaction among Routing Protocols ..... 305

Chapter 14. Multicast Routing Protocols ..... 309

14.1 Broadcast-and-Prune Protocols ..... 310
14

Preface

The Internet is a global communications network. With connections in more than 100 countries, tens of millions of people use the Internet for business, education, and recreation. Electronic commerce is beginning on the Internet as businesses connect to sell their products and services. Academics collaborate over the Internet by exchanging electronic mail. People can converse using Internet phones, send faxes, participate in online chats and bulletin boards, play multiuser games, and experiment with virtual environments. Special-purpose computers called routers connect the Internet together. As data is forwarded from one place in the Internet to another, it is the routers that make the decisions as to where and how the data is forwarded. The protocols that dynamically inform the routers of the paths that the data should take are called routing protocols. It is the job of these protocols to react quickly to changes in the InternetÕs infrastructure, such as transmission lines going in and out of service, routers crashing, changes in network policies, and so on. Routing is what makes the Internet tick. Although many users of the Internet and the World Wide Web are unaware of the machinery underlying the network applications, routing is an interesting but complicated subject. Routing protocols are sophisticated distributed algorithms that must also be extremely robust to keep a large, decentralized network like the Internet running smoothly.

Audience

This book is for students of data communications, TCP/IP network administrators, protocol designers, developers of routing protocol software, and other professionals involved in the design, development, and management of TCP/IP networks. This book is a practical, hands-on description of Internet routing rather than a theoretical treatment. Although we describe how the various protocols were intended to work, we also describe how well the design has translated into practice. Internet protocol design is a practical undertaking itself, with efficiency of implementation often dictating design choices. For this reason, this book gives an in-depth treatment of how a router really works. Instead of just describing the algorithms, this book goes beyond to show how the algorithms are implemented. We often present ideas in a historical context, showing how Internet protocols have evolved. This is done for two reasons. First, you can learn a lot from the mistakes (and successes) of the past. Second, in order to participate in Internet discussion groups, many of which are dominated by old-timers, it is good to have some context. This book is not an elementary introduction to TCP/IP and its routing. Instead we assume that you have some familiarity with the TCP/IP protocol suite and some exposure to the basic concepts of routing. These assumptions allow us to explore many of the facets of Internet routing in greater detail than possible in an introductory text.

Organization of This Book

This book is organized into five parts. Part I sets the groundwork for a discussion of Internet routing. After a brief description of how routing fits together with the rest of the InternetÕs protocols, Chapter 1 describes in depth how a router forwards packets. This discussion naturally leads to an explanation of IP addressing and CIDR, as well as of the interaction of hosts and routers. Internet routing protocols are introduced in Chapter 2, beginning with a treatment of the end product of all routing protocols: the routerÕs routing table. Chapter 2 ends with an overview of the InternetÕs routing architecture and the two main routing technologies in use in todayÕs Internet: Distance Vector and link-state algorithms.

Part II describes the InternetÕs OSPF routing protocol. We start in Chapter 3 with an explanation of why the OSPF protocol was developed in the first place. Chapter 4 discusses the basics of link-state routing; Chapter 5, how OSPF behaves over various subnet technologies; Chapter 6, its use of hierarchical routing; and Chapter 7, extensions to OSPF. Each chapter not only describes how OSPF works but also explains why it works that way. We explore the reasons behind OSPFÕs design decisions and how the OSPF protocol has evolved to keep pace with the rapidly changing Internet environment. Part II concludes with an OSPF FAQ (Chapter 8).

Part III (Chapters 9 and 10) describes TCP/IP multicast routing, including broadcast and multicast forwarding, the MBONE, and the two distinct types of multicast routing protocols: source-based trees and shared-tree algorithms. As we did with unicast routing, we go further into the subject of multicast routing through the examination of a particular multicast routing protocol: the Multicast Extensions to OSPF (MOSPF).

Part IV covers the configuration and management of Internet routing. The configuration and management of OSPF is explained in detail in Chapter 11. Chapter 12 describes the tools used to monitor and debug routing in a TCP/IP network. For each tool, we describe its use, how it works, and its advantages and drawbacks.

Part V is a comparison of Internet routing protocols. Chapter 13 compares and contrasts the routing protocols in use in the Internet: RIP, OSPF, BGP, IGRP, and IS-IS. In Chapter 14, we examine the available multicast protocols: DVMRP, MOSPF, PIM Dense and Sparse, and CBT.

Following Chapter 14 is an extensive bibliography arranged and numbered in alphabetical order. Within the text, the citation [85], for example, refers to item 85 in the bibliography.

Companion Book: OSPF Complete Implementation

The companion book OSPF Complete Implementation, in keeping with the Internet tradition that reveres Òworking codeÓ over all else, explores even further the mechanics of Internet routing through examination of a real, working OSPF implementation. The book contains a complete implementation of OSPF on CD. Written in C++, the OSPF implementation is intended to be portable to a wide range of environments. Two sample ports are included: an OSPF routing daemon (called ospfd) for FreeBSD 2.1 and an OSPF routing simulator that can be run on Windows 95. The OSPF implementation has been developed using publicly available tools.

Acknowledgments

I would like to thank the technical reviewers who improved this book through their thoughtful and timely reviews: Ran Atkinson, Eural Authement, Fred Baker, Howard Berkowitz, Jeffrey Burgan, Joel Halpern, Mukesh Kacker, Robert Minnear, Jim Reid, and W. Richard Stevens. Thanks also to Tim Stoddard and the Arkansas Public School Computer Network (APSCN) for letting me collect OSPF statistics on the APSCN network and use that network as an example of OSPF configuration in Chapter 11, Configuration and Management. Thanks to S. Randall McLamb for drawing the figures.

I would also like to acknowledge the help of my editors at Addison Wesley Longman over the long life of this project: Carol Long, Karen Gettman, and Mary Harrington.

And special thanks to my wife, Sonya Keene, who designed the book, edited rough drafts, created the index, and gave encouragement while this book was being written.

Editorial Reviews

Fatbrain Review

This excellent publication is one of the few that actually details the Open Shortest Path First (OSPF) routing protocol. It explains the inner workings of routers and provides descriptions of OSPF implementation within the context of TCP/IP protocols. To get the most out of the book, you should be familiar with the TCP/IP suite of protocols.

The author outlines packet routing and forwarding, discusses the interaction of hosts and routers and dissects the OSPF protocol itself. After this, the author guides you through the development of OSPF, link-state routing basics, hierarchical routing and OSPF extensions. An interesting feature of this section is the analysis and explanation of algorithms. The book examines TCP/IP multicast routing and forwarding, source-based trees, shared-tree algorithms and the multicast extensions to OSPF. It details OSPF configuration and management, then discusses OSPF troubleshooting tools and techniques. Also included is a comparison of existing routing protocols such as RIP, BGP, IGRP and IS-IS; and a comparison of multicast protocols such as DVMPR, MOSPF, CBT and PIM.

A good unpublished companion for this volume is OSPF: Complete Implementation. More detailed information on routing algorithms and their implementation is contained in TCP/IP Illustrated, Volume 2, The Implementation and for interdomain routing and network design try Internet Routing Architectures.

Written for TCP/IP network administrators, protocol designers, and network application developers, this introductory text explains the inner workings of the OSPF (Open Shortest Path First) TCP/IP routing protocol for the Internet. Topics covered include: OSBF virtual links, NBMA (nonbroadcast multi-access) network segments, interactions with other routing protocols, and protocol extensions. Annotation c. by Book News, Inc., Portland, Or.

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