Programming assignment 3 udp pinger lab solution

Network Design/ For Martin Writer

James F. Kurose University of Massachusetts, Amherst

Keith W. Ross Polytechnic Institute of NYU

COMPUTER NETWORKING

A Top-Down Approach

SIXTH EDITION

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Library of Congress Cataloging-in-Publication Data Kurose, James F.

Computer networking : a top-down approach / James F. Kurose, Keith W. Ross.—6th ed. p. cm.

Includes bibliographical references and index. ISBN-13: 978-0-13-285620-1 ISBN-10: 0-13-285620-4

1. Internet. 2. Computer networks. I. Ross, Keith W., 1956- II. Title. TK5105.875.I57K88 2012 004.6—dc23

2011048215 10 9 8 7 6 5 4 3 2 1

ISBN-13: 978-0-13-285620-1 ISBN-10: 0-13-285620-4

This book was composed in Quark. Basal font is Times. Display font is Berkeley.

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About the Authors Jim Kurose Jim Kurose is a Distinguished University Professor of Computer Science at the University of Massachusetts, Amherst.

Dr. Kurose has received a number of recognitions for his educational activities including Outstanding Teacher Awards from the National Technological University (eight times), the University of Massachusetts, and the Northeast Association of Graduate Schools. He received the IEEE Taylor Booth Education Medal and was recognized for his leadership of Massachusetts’ Commonwealth Information Technology Initiative. He has been the recipient of a GE Fellowship, an IBM Faculty Development Award, and a Lilly Teaching Fellowship.

Dr. Kurose is a former Editor-in-Chief of IEEE Transactions on Communications and of IEEE/ACM Transactions on Networking. He has been active in the program committees for IEEE Infocom, ACM SIGCOMM, ACM Internet Measurement Conference, and ACM SIGMETRICS for a number of years and has served as Technical Program Co-Chair for those conferences. He is a Fellow of the IEEE and the ACM. His research interests include network protocols and architecture, network measurement, sensor networks, multimedia communication, and modeling and performance evaluation. He holds a PhD in Computer Science from Columbia University.

Keith Ross Keith Ross is the Leonard J. Shustek Chair Professor and Head of the Computer Science Department at Polytechnic Institute of NYU. Before joining NYU-Poly in 2003, he was a professor at the University of Pennsylvania (13 years) and a professor at Eurecom Institute (5 years). He received a B.S.E.E from Tufts University, a M.S.E.E. from Columbia University, and a Ph.D. in Computer and Control Engineering from The University of Michigan. Keith Ross is also the founder and original CEO of Wimba, which develops online multimedia applications for e-learning and was acquired by Blackboard in 2010.

Professor Ross’s research interests are in security and privacy, social networks, peer-to-peer networking, Internet measurement, video streaming, content distribution networks, and stochastic modeling. He is an IEEE Fellow, recipient of the Infocom 2009 Best Paper Award, and recipient of 2011 and 2008 Best Paper Awards for Multimedia Communications (awarded by IEEE Communications Society). He has served on numerous journal editorial boards and conference program commit- tees, including IEEE/ACM Transactions on Networking, ACM SIGCOMM, ACM CoNext, and ACM Internet Measurement Conference. He also has served as an advisor to the Federal Trade Commission on P2P file sharing.

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To Julie and our three precious ones—Chris, Charlie, and Nina

JFK

A big THANKS to my professors, colleagues, and students all over the world.

KWR

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Preface Welcome to the sixth edition of Computer Networking: A Top-Down Approach. Since the publication of the first edition 12 years ago, our book has been adopted for use at many hundreds of colleges and universities, translated into 14 languages, and used by over one hundred thousand students and practitioners worldwide. We’ve heard from many of these readers and have been overwhelmed by the positive response.

What’s New in the Sixth Edition?

We think one important reason for this success has been that our book continues to offer a fresh and timely approach to computer networking instruction. We’ve made changes in this sixth edition, but we’ve also kept unchanged what we believe (and the instruc- tors and students who have used our book have confirmed) to be the most important aspects of this book: its top-down approach, its focus on the Internet and a modern treatment of computer networking, its attention to both principles and practice, and its accessible style and approach toward learning about computer networking. Neverthe- less, the sixth edition has been revised and updated substantially:

• The Companion Web site has been significantly expanded and enriched to include VideoNotes and interactive exercises, as discussed later in this Preface.

• In Chapter 1, the treatment of access networks has been modernized, and the description of the Internet ISP ecosystem has been substantially revised, account- ing for the recent emergence of content provider networks, such as Google’s. The presentation of packet switching and circuit switching has also been reorganized, providing a more topical rather than historical orientation.

• In Chapter 2, Python has replaced Java for the presentation of socket program- ming. While still explicitly exposing the key ideas behind the socket API, Python code is easier to understand for the novice programmer. Moreover, unlike Java, Python provides access to raw sockets, enabling students to build a larger variety of network applications. Java-based socket programming labs have been replaced with corresponding Python labs, and a new Python-based ICMP Ping lab has been added. As always, when material is retired from the book, such as Java-based socket programming material, it remains available on the book’s Companion Web site (see following text).

• In Chapter 3, the presentation of one of the reliable data transfer protocols has been simplified and a new sidebar on TCP splitting, commonly used to optimize the performance of cloud services, has been added.

• In Chapter 4, the section on router architectures has been significantly updated, reflecting recent developments and practices in the field. Several new integrative sidebars involving DNS, BGP, and OSPF are included.

• Chapter 5 has been reorganized and streamlined, accounting for the ubiquity of switched Ethernet in local area networks and the consequent increased use of Ethernet in point-to-point scenarios. Also, a new section on data center network- ing has been added.

• Chapter 6 has been updated to reflect recent advances in wireless networks, par- ticularly cellular data networks and 4G services and architecture.

• Chapter 7, which focuses on multimedia networking, has gone through a major revision. The chapter now includes an in-depth discussion of streaming video, including adaptive streaming, and an entirely new and modernized discussion of CDNs. A newly added section describes the Netflix, YouTube, and Kankan video streaming systems. The material that has been removed to make way for these new topics is still available on the Companion Web site.

• Chapter 8 now contains an expanded discussion on endpoint authentication.

• Significant new material involving end-of-chapter problems has been added. As with all previous editions, homework problems have been revised, added, and removed.

Audience

This textbook is for a first course on computer networking. It can be used in both computer science and electrical engineering departments. In terms of programming languages, the book assumes only that the student has experience with C, C++, Java, or Python (and even then only in a few places). Although this book is more precise and analytical than many other introductory computer networking texts, it rarely uses any mathematical concepts that are not taught in high school. We have made a deliberate effort to avoid using any advanced calculus, probability, or stochastic process concepts (although we’ve included some homework problems for students with this advanced background). The book is therefore appropriate for undergradu- ate courses and for first-year graduate courses. It should also be useful to practition- ers in the telecommunications industry.

What Is Unique about This Textbook?

The subject of computer networking is enormously complex, involving many concepts, protocols, and technologies that are woven together in an intricate manner. To cope with this scope and complexity, many computer networking texts are often organized around the “layers” of a network architecture. With a layered organization, students can see through the complexity of computer networking— they learn about the distinct concepts and protocols in one part of the architecture while seeing the big picture of how all parts fit together. From a pedagogical perspective, our personal experience has been that such a layered approach

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indeed works well. Nevertheless, we have found that the traditional approach of teaching—bottom up; that is, from the physical layer towards the application layer—is not the best approach for a modern course on computer networking.

A Top-Down Approach

Our book broke new ground 12 years ago by treating networking in a top-down manner—that is, by beginning at the application layer and working its way down toward the physical layer. The feedback we received from teachers and students alike have confirmed that this top-down approach has many advantages and does indeed work well pedagogically. First, it places emphasis on the application layer (a “high growth area” in networking). Indeed, many of the recent revolutions in computer networking—including the Web, peer-to-peer file sharing, and media streaming—have taken place at the application layer. An early emphasis on application- layer issues differs from the approaches taken in most other texts, which have only a small amount of material on network applications, their requirements, application-layer paradigms (e.g., client-server and peer-to-peer), and application programming inter- faces. Second, our experience as instructors (and that of many instructors who have used this text) has been that teaching networking applications near the beginning of the course is a powerful motivational tool. Students are thrilled to learn about how networking applications work—applications such as e-mail and the Web, which most students use on a daily basis. Once a student understands the applications, the student can then understand the network services needed to support these applications. The student can then, in turn, examine the various ways in which such services might be provided and implemented in the lower layers. Covering applications early thus pro- vides motivation for the remainder of the text.

Third, a top-down approach enables instructors to introduce network appli- cation development at an early stage. Students not only see how popular applica- tions and protocols work, but also learn how easy it is to create their own network applications and application-level protocols. With the top-down approach, students get early exposure to the notions of socket programming, serv- ice models, and protocols—important concepts that resurface in all subsequent layers. By providing socket programming examples in Python, we highlight the central ideas without confusing students with complex code. Undergraduates in electrical engineering and computer science should not have difficulty following the Python code.

An Internet Focus

Although we dropped the phrase “Featuring the Internet” from the title of this book with the fourth edition, this doesn’t mean that we dropped our focus on the Internet! Indeed, nothing could be further from the case! Instead, since the Internet has become so pervasive, we felt that any networking textbook must have a significant

focus on the Internet, and thus this phrase was somewhat unnecessary. We continue to use the Internet’s architecture and protocols as primary vehicles for studying fun- damental computer networking concepts. Of course, we also include concepts and protocols from other network architectures. But the spotlight is clearly on the Inter- net, a fact reflected in our organizing the book around the Internet’s five-layer archi- tecture: the application, transport, network, link, and physical layers.

Another benefit of spotlighting the Internet is that most computer science and electrical engineering students are eager to learn about the Internet and its protocols. They know that the Internet has been a revolutionary and disruptive technology and can see that it is profoundly changing our world. Given the enormous relevance of the Internet, students are naturally curious about what is “under the hood.” Thus, it is easy for an instructor to get students excited about basic principles when using the Internet as the guiding focus.

Teaching Networking Principles

Two of the unique features of the book—its top-down approach and its focus on the Internet—have appeared in the titles of our book. If we could have squeezed a third phrase into the subtitle, it would have contained the word principles. The field of networking is now mature enough that a number of fundamentally important issues can be identified. For example, in the transport layer, the fundamental issues include reliable communication over an unreliable network layer, connection establishment/ teardown and handshaking, congestion and flow control, and multiplexing. Two fun- damentally important network-layer issues are determining “good” paths between two routers and interconnecting a large number of heterogeneous networks. In the link layer, a fundamental problem is sharing a multiple access channel. In network security, techniques for providing confidentiality, authentication, and message integrity are all based on cryptographic fundamentals. This text identifies fundamen- tal networking issues and studies approaches towards addressing these issues. The student learning these principles will gain knowledge with a long “shelf life”—long after today’s network standards and protocols have become obsolete, the principles they embody will remain important and relevant. We believe that the combination of using the Internet to get the student’s foot in the door and then emphasizing funda- mental issues and solution approaches will allow the student to quickly understand just about any networking technology.

The Web Site

Each new copy of this textbook includes six months of access to a Companion Web site for all book readers at http://www.pearsonhighered.com/kurose-ross, which includes:

• Interactive learning material. An important new component of the sixth edition is the significantly expanded online and interactive learning material. The book’s Companion Web site now contains VideoNotes—video presentations of

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important topics thoughout the book done by the authors, as well as walk- throughs of solutions to problems similar to those at the end of the chapter. We’ve also added Interactive Exercises that can create (and present solutions for) problems similar to selected end-of-chapter problems. Since students can generate (and view solutions for) an unlimited number of similar problem instances, they can work until the material is truly mastered. We’ve seeded the Web site with VideoNotes and online problems for chapters 1 through 5 and will continue to actively add and update this material over time. As in earlier edi- tions, the Web site contains the interactive Java applets that animate many key networking concepts. The site also has interactive quizzes that permit students to check their basic understanding of the subject matter. Professors can integrate these interactive features into their lectures or use them as mini labs.

• Additional technical material. As we have added new material in each edition of our book, we’ve had to remove coverage of some existing topics to keep the book at manageable length. For example, to make room for the new material in this edition, we’ve removed material on ATM networks and the RTSP protocol for multimedia. Material that appeared in earlier editions of the text is still of interest, and can be found on the book’s Web site.

• Programming assignments. The Web site also provides a number of detailed programming assignments, which include building a multithreaded Web server, building an e-mail client with a GUI interface, programming the sender and receiver sides of a reliable data transport protocol, programming a distrib- uted routing algorithm, and more.

• Wireshark labs. One’s understanding of network protocols can be greatly deep- ened by seeing them in action. The Web site provides numerous Wireshark assignments that enable students to actually observe the sequence of messages exchanged between two protocol entities. The Web site includes separate Wire- shark labs on HTTP, DNS, TCP, UDP, IP, ICMP, Ethernet, ARP, WiFi, SSL, and on tracing all protocols involved in satisfying a request to fetch a web page. We’ll continue to add new labs over time.

Pedagogical Features

We have each been teaching computer networking for more than 20 years. Together, we bring more than 50 years of teaching experience to this text, during which time we have taught many thousands of students. We have also been active researchers in computer networking during this time. (In fact, Jim and Keith first met each other as master’s students in a computer networking course taught by Mischa Schwartz in 1979 at Columbia University.) We think all this gives us a good perspective on where networking has been and where it is likely to go in the future. Nevertheless, we have resisted temptations to bias the material in this book

towards our own pet research projects. We figure you can visit our personal Web sites if you are interested in our research. Thus, this book is about modern com- puter networking—it is about contemporary protocols and technologies as well as the underlying principles behind these protocols and technologies. We also believe that learning (and teaching!) about networking can be fun. A sense of humor, use of analogies, and real-world examples in this book will hopefully make this mate- rial more fun.

Supplements for Instructors

We provide a complete supplements package to aid instructors in teaching this course. This material can be accessed from Pearson’s Instructor Resource Center (http://www.pearsonhighered.com/irc). Visit the Instructor Resource Center or send e-mail to computing@aw.com for information about accessing these instructor’s supplements.

• PowerPoint® slides. We provide PowerPoint slides for all nine chapters. The slides have been completely updated with this sixth edition. The slides cover each chapter in detail. They use graphics and animations (rather than relying only on monotonous text bullets) to make the slides interesting and visually appealing. We provide the original PowerPoint slides so you can customize them to best suit your own teaching needs. Some of these slides have been contributed by other instructors who have taught from our book.

• Homework solutions. We provide a solutions manual for the homework problems in the text, programming assignments, and Wireshark labs. As noted earlier, we’ve introduced many new homework problems in the first five chapters of the book.

Chapter Dependencies

The first chapter of this text presents a self-contained overview of computer net- working. Introducing many key concepts and terminology, this chapter sets the stage for the rest of the book. All of the other chapters directly depend on this first chap- ter. After completing Chapter 1, we recommend instructors cover Chapters 2 through 5 in sequence, following our top-down philosophy. Each of these five chap- ters leverages material from the preceding chapters. After completing the first five chapters, the instructor has quite a bit of flexibility. There are no interdependencies among the last four chapters, so they can be taught in any order. However, each of the last four chapters depends on the material in the first five chapters. Many instructors first teach the first five chapters and then teach one of the last four chap- ters for “dessert.”

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One Final Note: We’d Love to Hear from You

We encourage students and instructors to e-mail us with any comments they might have about our book. It’s been wonderful for us to hear from so many instructors and students from around the world about our first four editions. We’ve incorporated many of these suggestions into later editions of the book. We also encourage instructors to send us new homework problems (and solutions) that would complement the current homework problems. We’ll post these on the instructor-only portion of the Web site. We also encourage instructors and students to create new Java applets that illustrate the concepts and protocols in this book. If you have an applet that you think would be appropriate for this text, please submit it to us. If the applet (including notation and terminology) is appropriate, we’ll be happy to include it on the text’s Web site, with an appropriate reference to the applet’s authors.

So, as the saying goes, “Keep those cards and letters coming!” Seriously, please do continue to send us interesting URLs, point out typos, disagree with any of our claims, and tell us what works and what doesn’t work. Tell us what you think should or shouldn’t be included in the next edition. Send your e-mail to kurose@cs.umass.edu and ross@poly.edu.

Acknowledgments

Since we began writing this book in 1996, many people have given us invaluable help and have been influential in shaping our thoughts on how to best organize and teach a networking course. We want to say A BIG THANKS to everyone who has helped us from the earliest first drafts of this book, up to this fifth edition. We are also very thankful to the many hundreds of readers from around the world—students, fac- ulty, practitioners—who have sent us thoughts and comments on earlier editions of the book and suggestions for future editions of the book. Special thanks go out to:

Al Aho (Columbia University) Hisham Al-Mubaid (University of Houston-Clear Lake) Pratima Akkunoor (Arizona State University) Paul Amer (University of Delaware) Shamiul Azom (Arizona State University) Lichun Bao (University of California at Irvine) Paul Barford (University of Wisconsin) Bobby Bhattacharjee (University of Maryland) Steven Bellovin (Columbia University) Pravin Bhagwat (Wibhu) Supratik Bhattacharyya (previously at Sprint) Ernst Biersack (Eurécom Institute)

Shahid Bokhari (University of Engineering & Technology, Lahore) Jean Bolot (Technicolor Research) Daniel Brushteyn (former University of Pennsylvania student) Ken Calvert (University of Kentucky) Evandro Cantu (Federal University of Santa Catarina) Jeff Case (SNMP Research International) Jeff Chaltas (Sprint) Vinton Cerf (Google) Byung Kyu Choi (Michigan Technological University) Bram Cohen (BitTorrent, Inc.) Constantine Coutras (Pace University) John Daigle (University of Mississippi) Edmundo A. de Souza e Silva (Federal University of Rio de Janeiro) Philippe Decuetos (Eurécom Institute) Christophe Diot (Technicolor Research) Prithula Dhunghel (Akamai) Deborah Estrin (University of California, Los Angeles) Michalis Faloutsos (University of California at Riverside) Wu-chi Feng (Oregon Graduate Institute) Sally Floyd (ICIR, University of California at Berkeley) Paul Francis (Max Planck Institute) Lixin Gao (University of Massachusetts) JJ Garcia-Luna-Aceves (University of California at Santa Cruz) Mario Gerla (University of California at Los Angeles) David Goodman (NYU-Poly) Yang Guo (Alcatel/Lucent Bell Labs) Tim Griffin (Cambridge University) Max Hailperin (Gustavus Adolphus College) Bruce Harvey (Florida A&M University, Florida State University) Carl Hauser (Washington State University) Rachelle Heller (George Washington University) Phillipp Hoschka (INRIA/W3C) Wen Hsin (Park University) Albert Huang (former University of Pennsylvania student) Cheng Huang (Microsoft Research) Esther A. Hughes (Virginia Commonwealth University) Van Jacobson (Xerox PARC) Pinak Jain (former NYU-Poly student) Jobin James (University of California at Riverside) Sugih Jamin (University of Michigan) Shivkumar Kalyanaraman (IBM Research, India) Jussi Kangasharju (University of Helsinki) Sneha Kasera (University of Utah) Parviz Kermani (formerly of IBM Research)

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Hyojin Kim (former University of Pennsylvania student) Leonard Kleinrock (University of California at Los Angeles) David Kotz (Dartmouth College) Beshan Kulapala (Arizona State University) Rakesh Kumar (Bloomberg) Miguel A. Labrador (University of South Florida) Simon Lam (University of Texas) Steve Lai (Ohio State University) Tom LaPorta (Penn State University) Tim-Berners Lee (World Wide Web Consortium) Arnaud Legout (INRIA) Lee Leitner (Drexel University) Brian Levine (University of Massachusetts) Chunchun Li (former NYU-Poly student) Yong Liu (NYU-Poly) William Liang (former University of Pennsylvania student) Willis Marti (Texas A&M University) Nick McKeown (Stanford University) Josh McKinzie (Park University) Deep Medhi (University of Missouri, Kansas City) Bob Metcalfe (International Data Group) Sue Moon (KAIST) Jenni Moyer (Comcast) Erich Nahum (IBM Research) Christos Papadopoulos (Colorado Sate University) Craig Partridge (BBN Technologies) Radia Perlman (Intel) Jitendra Padhye (Microsoft Research) Vern Paxson (University of California at Berkeley) Kevin Phillips (Sprint) George Polyzos (Athens University of Economics and Business) Sriram Rajagopalan (Arizona State University) Ramachandran Ramjee (Microsoft Research) Ken Reek (Rochester Institute of Technology) Martin Reisslein (Arizona State University) Jennifer Rexford (Princeton University) Leon Reznik (Rochester Institute of Technology) Pablo Rodrigez (Telefonica) Sumit Roy (University of Washington) Avi Rubin (Johns Hopkins University) Dan Rubenstein (Columbia University) Douglas Salane (John Jay College) Despina Saparilla (Cisco Systems) John Schanz (Comcast)

Henning Schulzrinne (Columbia University) Mischa Schwartz (Columbia University) Ardash Sethi (University of Delaware) Harish Sethu (Drexel University) K. Sam Shanmugan (University of Kansas) Prashant Shenoy (University of Massachusetts) Clay Shields (Georgetown University) Subin Shrestra (University of Pennsylvania) Bojie Shu (former NYU-Poly student) Mihail L. Sichitiu (NC State University) Peter Steenkiste (Carnegie Mellon University) Tatsuya Suda (University of California at Irvine) Kin Sun Tam (State University of New York at Albany) Don Towsley (University of Massachusetts) David Turner (California State University, San Bernardino) Nitin Vaidya (University of Illinois) Michele Weigle (Clemson University) David Wetherall (University of Washington) Ira Winston (University of Pennsylvania) Di Wu (Sun Yat-sen University) Shirley Wynn (NYU-Poly) Raj Yavatkar (Intel) Yechiam Yemini (Columbia University) Ming Yu (State University of New York at Binghamton) Ellen Zegura (Georgia Institute of Technology) Honggang Zhang (Suffolk University) Hui Zhang (Carnegie Mellon University) Lixia Zhang (University of California at Los Angeles) Meng Zhang (former NYU-Poly student) Shuchun Zhang (former University of Pennsylvania student) Xiaodong Zhang (Ohio State University) ZhiLi Zhang (University of Minnesota) Phil Zimmermann (independent consultant) Cliff C. Zou (University of Central Florida)

We also want to thank the entire Addison-Wesley team—in particular, Michael Hirsch, Marilyn Lloyd, and Emma Snider—who have done an absolutely outstanding job on this sixth edition (and who have put up with two very finicky authors who seem con- genitally unable to meet deadlines!). Thanks also to our artists, Janet Theurer and Patrice Rossi Calkin, for their work on the beautiful figures in this book, and to Andrea Stefanowicz and her team at PreMediaGlobal for their wonderful production work on this edition. Finally, a most special thanks go to Michael Hirsch, our editor at Addison- Wesley, and Susan Hartman, our former editor at Addison-Wesley. This book would not be what it is (and may well not have been at all) without their graceful manage- ment, constant encouragement, nearly infinite patience, good humor, and perseverance.

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Table of Contents

Chapter 1 Computer Networks and the Internet 1 1.1 What Is the Internet? 2

1.1.1 A Nuts-and-Bolts Description 2 1.1.2 A Services Description 5 1.1.3 What Is a Protocol? 7

1.2 The Network Edge 9 1.2.1 Access Networks 12 1.2.2 Physical Media 18

1.3 The Network Core 22 1.3.1 Packet Switching 22 1.3.2 Circuit Switching 27 1.3.3 A Network of Networks 32

1.4 Delay, Loss, and Throughput in Packet-Switched Networks 35 1.4.1 Overview of Delay in Packet-Switched Networks 35 1.4.2 Queuing Delay and Packet Loss 39 1.4.3 End-to-End Delay 42 1.4.4 Throughput in Computer Networks 44

1.5 Protocol Layers and Their Service Models 47 1.5.1 Layered Architecture 47 1.5.2 Encapsulation 53

1.6 Networks Under Attack 55 1.7 History of Computer Networking and the Internet 60

1.7.1 The Development of Packet Switching: 1961–1972 60 1.7.2 Proprietary Networks and Internetworking: 1972–1980 62 1.7.3 A Proliferation of Networks: 1980–1990 63 1.7.4 The Internet Explosion: The 1990s 64 1.7.5 The New Millennium 65

1.8 Summary 66 Homework Problems and Questions 68 Wireshark Lab 78 Interview: Leonard Kleinrock 80

Chapter 2 Application Layer 83 2.1 Principles of Network Applications 84

2.1.1 Network Application Architectures 86 2.1.2 Processes Communicating 88 2.1.3 Transport Services Available to Applications 91 2.1.4 Transport Services Provided by the Internet 93 2.1.5 Application-Layer Protocols 96 2.1.6 Network Applications Covered in This Book 97

2.2 The Web and HTTP 98 2.2.1 Overview of HTTP 98 2.2.2 Non-Persistent and Persistent Connections 100 2.2.3 HTTP Message Format 103 2.2.4 User-Server Interaction: Cookies 108 2.2.5 Web Caching 110 2.2.6 The Conditional GET 114

2.3 File Transfer: FTP 116 2.3.1 FTP Commands and Replies 118

2.4 Electronic Mail in the Internet 118 2.4.1 SMTP 121 2.4.2 Comparison with HTTP 124 2.4.3 Mail Message Format 125 2.4.4 Mail Access Protocols 125

2.5 DNS—The Internet’s Directory Service 130 2.5.1 Services Provided by DNS 131 2.5.2 Overview of How DNS Works 133 2.5.3 DNS Records and Messages 139

2.6 Peer-to-Peer Applications 144 2.6.1 P2P File Distribution 145 2.6.2 Distributed Hash Tables (DHTs) 151

2.7 Socket Programming: Creating Network Applications 156 2.7.1 Socket Programming with UDP 157 2.7.2 Socket Programming with TCP 163

2.8 Summary 168 Homework Problems and Questions 169 Socket Programming Assignments 179 Wireshark Labs: HTTP, DNS 181 Interview: Marc Andreessen 182

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Chapter 3 Transport Layer 185 3.1 Introduction and Transport-Layer Services 186

3.1.1 Relationship Between Transport and Network Layers 186 3.1.2 Overview of the Transport Layer in the Internet 189

3.2 Multiplexing and Demultiplexing 191 3.3 Connectionless Transport: UDP 198

3.3.1 UDP Segment Structure 202 3.3.2 UDP Checksum 202

3.4 Principles of Reliable Data Transfer 204 3.4.1 Building a Reliable Data Transfer Protocol 206 3.4.2 Pipelined Reliable Data Transfer Protocols 215 3.4.3 Go-Back-N (GBN) 218 3.4.4 Selective Repeat (SR) 223

3.5 Connection-Oriented Transport: TCP 230 3.5.1 The TCP Connection 231 3.5.2 TCP Segment Structure 233 3.5.3 Round-Trip Time Estimation and Timeout 238 3.5.4 Reliable Data Transfer 242 3.5.5 Flow Control 250 3.5.6 TCP Connection Management 252

3.6 Principles of Congestion Control 259 3.6.1 The Causes and the Costs of Congestion 259 3.6.2 Approaches to Congestion Control 265 3.6.3 Network-Assisted Congestion-Control Example:

ATM ABR Congestion Control 266 3.7 TCP Congestion Control 269

3.7.1 Fairness 279 3.8 Summary 283 Homework Problems and Questions 285 Programming Assignments 300 Wireshark Labs: TCP, UDP 301 Interview: Van Jacobson 302

Chapter 4 The Network Layer 305 4.1 Introduction 306

4.1.1 Forwarding and Routing 308 4.1.2 Network Service Models 310

4.2 Virtual Circuit and Datagram Networks 313 4.2.1 Virtual-Circuit Networks 314 4.2.2 Datagram Networks 317 4.2.3 Origins of VC and Datagram Networks 319

4.3 What’s Inside a Router? 320 4.3.1 Input Processing 322 4.3.2 Switching 324 4.3.3 Output Processing 326 4.3.4 Where Does Queuing Occur? 327 4.3.5 The Routing Control Plane 331

4.4 The Internet Protocol (IP): Forwarding and Addressing in the Internet 331 4.4.1 Datagram Format 332 4.4.2 IPv4 Addressing 338 4.4.3 Internet Control Message Protocol (ICMP) 353 4.4.4 IPv6 356 4.4.5 A Brief Foray into IP Security 362

4.5 Routing Algorithms 363 4.5.1 The Link-State (LS) Routing Algorithm 366 4.5.2 The Distance-Vector (DV) Routing Algorithm 371 4.5.3 Hierarchical Routing 379

4.6 Routing in the Internet 383 4.6.1 Intra-AS Routing in the Internet: RIP 384 4.6.2 Intra-AS Routing in the Internet: OSPF 388 4.6.3 Inter-AS Routing: BGP 390

4.7 Broadcast and Multicast Routing 399 4.7.1 Broadcast Routing Algorithms 400 4.7.2 Multicast 405

4.8 Summary 412 Homework Problems and Questions 413 Programming Assignments 429 Wireshark Labs: IP, ICMP 430 Interview: Vinton G. Cerf 431

Chapter 5 The Link Layer: Links, Access Networks, and LANs 433 5.1 Introduction to the Link Layer 434

5.1.1 The Services Provided by the Link Layer 436 5.1.2 Where Is the Link Layer Implemented? 437

5.2 Error-Detection and -Correction Techniques 438 5.2.1 Parity Checks 440 5.2.2 Checksumming Methods 442 5.2.3 Cyclic Redundancy Check (CRC) 443

5.3 Multiple Access Links and Protocols 445 5.3.1 Channel Partitioning Protocols 448 5.3.2 Random Access Protocols 449 5.3.3 Taking-Turns Protocols 459 5.3.4 DOCSIS: The Link-Layer Protocol for Cable Internet Access 460

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5.4 Switched Local Area Networks 461 5.4.1 Link-Layer Addressing and ARP 462 5.4.2 Ethernet 469 5.4.3 Link-Layer Switches 476 5.4.4 Virtual Local Area Networks (VLANs) 482

5.5 Link Virtualization: A Network as a Link Layer 486 5.5.1 Multiprotocol Label Switching (MPLS) 487

5.6 Data Center Networking 490 5.7 Retrospective: A Day in the Life of a Web Page Request 495

5.7.1 Getting Started: DHCP, UDP, IP, and Ethernet 495 5.7.2 Still Getting Started: DNS and ARP 497 5.7.3 Still Getting Started: Intra-Domain Routing to the DNS Server 498 5.7.4 Web Client-Server Interaction: TCP and HTTP 499

5.8 Summary 500 Homework Problems and Questions 502 Wireshark Labs: Ethernet and ARP, DHCP 510 Interview: Simon S. Lam 511

Chapter 6 Wireless and Mobile Networks 513 6.1 Introduction 514 6.2 Wireless Links and Network Characteristics 519

6.2.1 CDMA 522 6.3 WiFi: 802.11 Wireless LANs 526

6.3.1 The 802.11 Architecture 527 6.3.2 The 802.11 MAC Protocol 531 6.3.3 The IEEE 802.11 Frame 537 6.3.4 Mobility in the Same IP Subnet 541 6.3.5 Advanced Features in 802.11 542 6.3.6 Personal Area Networks: Bluetooth and Zigbee 544

6.4 Cellular Internet Access 546 6.4.1 An Overview of Cellular Network Architecture 547 6.4.2 3G Cellular Data Networks: Extending the Internet to Cellular

Subscribers 550 6.4.3 On to 4G: LTE 553

6.5 Mobility Management: Principles 555 6.5.1 Addressing 557 6.5.2 Routing to a Mobile Node 559

6.6 Mobile IP 564 6.7 Managing Mobility in Cellular Networks 570

6.7.1 Routing Calls to a Mobile User 571 6.7.2 Handoffs in GSM 572

6.8 Wireless and Mobility: Impact on Higher-Layer Protocols 575 6.9 Summary 578 Homework Problems and Questions 578 Wireshark Lab: IEEE 802.11 (WiFi) 583 Interview: Deborah Estrin 584

Chapter 7 Multimedia Networking 587 7.1 Multimedia Networking Applications 588

7.1.1 Properties of Video 588 7.1.2 Properties of Audio 590 7.1.3 Types of Multimedia Network Applications 591

7.2 Streaming Stored Video 593 7.2.1 UDP Streaming 595 7.2.2 HTTP Streaming 596 7.2.3 Adaptive Streaming and DASH 600 7.2.4 Content Distribution Networks 602 7.2.5 Case Studies: Netflix, YouTube, and Kankan 608

7.3 Voice-over-IP 612 7.3.1 Limitations of the Best-Effort IP Service 612 7.3.2 Removing Jitter at the Receiver for Audio 614 7.3.3 Recovering from Packet Loss 617 7.3.4 Case Study: VoIP with Skype 620

7.4 Protocols for Real-Time Conversational Applications 623 7.4.1 RTP 624 7.4.2 SIP 627

7.5 Network Support for Multimedia 632 7.5.1 Dimensioning Best-Effort Networks 634 7.5.2 Providing Multiple Classes of Service 636 7.5.3 Diffserv 648 7.5.4 Per-Connection Quality-of-Service (QoS) Guarantees:

Resource Reservation and Call Admission 652 7.6 Summary 655 Homework Problems and Questions 656 Programming Assignment 666 Interview: Henning Schulzrinne 668

Chapter 8 Security in Computer Networks 671 8.1 What Is Network Security? 672 8.2 Principles of Cryptography 675

8.2.1 Symmetric Key Cryptography 676 8.2.2 Public Key Encryption 683

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8.3 Message Integrity and Digital Signatures 688 8.3.1 Cryptographic Hash Functions 689 8.3.2 Message Authentication Code 691 8.3.3 Digital Signatures 693

8.4 End-Point Authentication 700 8.4.1 Authentication Protocol ap1.0 700 8.4.2 Authentication Protocol ap2.0 701 8.4.3 Authentication Protocol ap3.0 702 8.4.4 Authentication Protocol ap3.1 703 8.4.5 Authentication Protocol ap4.0 703

8.5 Securing E-Mail 705 8.5.1 Secure E-Mail 706 8.5.2 PGP 710

8.6 Securing TCP Connections: SSL 711 8.6.1 The Big Picture 713 8.6.2 A More Complete Picture 716

8.7 Network-Layer Security: IPsec and Virtual Private Networks 718 8.7.1 IPsec and Virtual Private Networks (VPNs) 718 8.7.2 The AH and ESP Protocols 720 8.7.3 Security Associations 720 8.7.4 The IPsec Datagram 721 8.7.5 IKE: Key Management in IPsec 725

8.8 Securing Wireless LANs 726 8.8.1 Wired Equivalent Privacy (WEP) 726 8.8.2 IEEE 802.11i 728

8.9 Operational Security: Firewalls and Intrusion Detection Systems 731 8.9.1 Firewalls 731 8.9.2 Intrusion Detection Systems 739

8.10 Summary 742 Homework Problems and Questions 744 Wireshark Lab: SSL 752 IPsec Lab 752 Interview: Steven M. Bellovin 753

Chapter 9 Network Management 755 9.1 What Is Network Management? 756 9.2 The Infrastructure for Network Management 760 9.3 The Internet-Standard Management Framework 764

9.3.1 Structure of Management Information: SMI 766 9.3.2 Management Information Base: MIB 770

9.3.3 SNMP Protocol Operations and Transport Mappings 772 9.3.4 Security and Administration 775

9.4 ASN.1 778 9.5 Conclusion 783 Homework Problems and Questions 783 Interview: Jennifer Rexford 786

References 789 Index 823

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COMPUTER NETWORKING

A Top-Down Approach

SIXTH EDITION

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CHAPTER 1 Computer Networks and the Internet

1

Today’s Internet is arguably the largest engineered system ever created by mankind, with hundreds of millions of connected computers, communication links, and switches; with billions of users who connect via laptops, tablets, and smartphones; and with an array of new Internet-connected devices such as sensors, Web cams, game consoles, picture frames, and even washing machines. Given that the Internet is so large and has so many diverse components and uses, is there any hope of understanding how it works? Are there guiding principles and structure that can pro- vide a foundation for understanding such an amazingly large and complex system? And if so, is it possible that it actually could be both interesting and fun to learn about computer networks? Fortunately, the answers to all of these questions is a resounding YES! Indeed, it’s our aim in this book to provide you with a modern introduction to the dynamic field of computer networking, giving you the principles and practical insights you’ll need to understand not only today’s networks, but tomorrow’s as well.

This first chapter presents a broad overview of computer networking and the Internet. Our goal here is to paint a broad picture and set the context for the rest of this book, to see the forest through the trees. We’ll cover a lot of ground in this intro- ductory chapter and discuss a lot of the pieces of a computer network, without los- ing sight of the big picture.

We’ll structure our overview of computer networks in this chapter as follows. After introducing some basic terminology and concepts, we’ll first examine the basic hardware and software components that make up a network. We’ll begin at the network’s edge and look at the end systems and network applications running in the network. We’ll then explore the core of a computer network, examining the links and the switches that transport data, as well as the access networks and phys- ical media that connect end systems to the network core. We’ll learn that the Inter- net is a network of networks, and we’ll learn how these networks connect with each other.

After having completed this overview of the edge and core of a computer net- work, we’ll take the broader and more abstract view in the second half of this chap- ter. We’ll examine delay, loss, and throughput of data in a computer network and provide simple quantitative models for end-to-end throughput and delay: models that take into account transmission, propagation, and queuing delays. We’ll then introduce some of the key architectural principles in computer networking, namely, protocol layering and service models. We’ll also learn that computer networks are vulnerable to many different types of attacks; we’ll survey some of these attacks and consider how computer networks can be made more secure. Finally, we’ll close this chapter with a brief history of computer networking.

1.1 What Is the Internet? In this book, we’ll use the public Internet, a specific computer network, as our prin- cipal vehicle for discussing computer networks and their protocols. But what is the Internet? There are a couple of ways to answer this question. First, we can describe the nuts and bolts of the Internet, that is, the basic hardware and software components that make up the Internet. Second, we can describe the Internet in terms of a net- working infrastructure that provides services to distributed applications. Let’s begin with the nuts-and-bolts description, using Figure 1.1 to illustrate our discussion.

1.1.1 A Nuts-and-Bolts Description

The Internet is a computer network that interconnects hundreds of millions of com- puting devices throughout the world. Not too long ago, these computing devices were primarily traditional desktop PCs, Linux workstations, and so-called servers that store and transmit information such as Web pages and e-mail messages. Increasingly, however, nontraditional Internet end systems such as laptops, smartphones, tablets, TVs, gaming consoles, Web cams, automobiles, environmental sensing devices, picture frames, and home electrical and security systems are being connected to the Internet. Indeed, the term computer network is beginning to sound a bit dated, given the many nontraditional devices that are being hooked up to the Internet. In Internet jar- gon, all of these devices are called hosts or end systems. As of July 2011, there were

2 CHAPTER 1 • COMPUTER NETWORKS AND THE INTERNET

1.1 • WHAT IS THE INTERNET? 3

Figure 1.1 � Some pieces of the Internet

Key:

Host (= end system)

Server Mobile Router Link-Layer switch

Modem Base station

Smartphone Cell phone tower

National or Global ISP

Mobile Network

Local or Regional ISP

Enterprise Network

Home Network

nearly 850 million end systems attached to the Internet [ISC 2012], not counting smartphones, laptops, and other devices that are only intermittently connected to the Internet. Overall, more there are an estimated 2 billion Internet users [ITU 2011].

End systems are connected together by a network of communication links and packet switches. We’ll see in Section 1.2 that there are many types of communica- tion links, which are made up of different types of physical media, including coaxial cable, copper wire, optical fiber, and radio spectrum. Different links can transmit data at different rates, with the transmission rate of a link measured in bits/second. When one end system has data to send to another end system, the sending end sys- tem segments the data and adds header bytes to each segment. The resulting pack- ages of information, known as packets in the jargon of computer networks, are then sent through the network to the destination end system, where they are reassembled into the original data.

A packet switch takes a packet arriving on one of its incoming communication links and forwards that packet on one of its outgoing communication links. Packet switches come in many shapes and flavors, but the two most prominent types in today’s Internet are routers and link-layer switches. Both types of switches for- ward packets toward their ultimate destinations. Link-layer switches are typically used in access networks, while routers are typically used in the network core. The sequence of communication links and packet switches traversed by a packet from the sending end system to the receiving end system is known as a route or path through the network. The exact amount of traffic being carried in the Internet is difficult to estimate but Cisco [Cisco VNI 2011] estimates global Internet traffic will be nearly 40 exabytes per month in 2012.

Packet-switched networks (which transport packets) are in many ways simi- lar to transportation networks of highways, roads, and intersections (which trans- port vehicles). Consider, for example, a factory that needs to move a large amount of cargo to some destination warehouse located thousands of kilometers away. At the factory, the cargo is segmented and loaded into a fleet of trucks. Each of the trucks then independently travels through the network of highways, roads, and intersections to the destination warehouse. At the destination ware- house, the cargo is unloaded and grouped with the rest of the cargo arriving from the same shipment. Thus, in many ways, packets are analogous to trucks, com- munication links are analogous to highways and roads, packet switches are anal- ogous to intersections, and end systems are analogous to buildings. Just as a truck takes a path through the transportation network, a packet takes a path through a computer network.

End systems access the Internet through Internet Service Providers (ISPs), including residential ISPs such as local cable or telephone companies; corporate ISPs; university ISPs; and ISPs that provide WiFi access in airports, hotels, coffee shops, and other public places. Each ISP is in itself a network of packet switches and communication links. ISPs provide a variety of types of network access to the end systems, including residential broadband access such as cable modem or DSL,