This page intentionally left blank COMPUTER NETWORKS FIFTH EDITION This page intentionally left blank COMPUTER NETWORKS FIFTH EDITION ANDREW S. TANENBAUM Vrije Universiteit Amsterdam, The Netherlands DAVID J. WETHERALL University of Washington Seattle, WA PRENTICE HALL Boston Columbus Indianapolis New York San Francisco Upper Saddle River Amsterdam Cape Town Dubai London Madrid Milan Paris Montreal Toronto Delhi Mexico City Sao Paulo Sydney Hong Kong Seoul Singapore Tapei Tokyo Editorial Director: Marcia Horton Editor-in-Chief: Michael Hirsch Executive Editor: Tracy Dunkelberger Assistant Editor: Melinda Haggerty Editorial Assistant: Allison Michael Vice President, Marketing: Patrice Jones Marketing Manager: Yezan Alayan Marketing Coordinator: Kathryn Ferranti Vice President, Production: Vince O’Brien Managing Editor: Jeff Holcomb Senior Operations Supervisor: Alan Fischer Manufacturing Buyer: Lisa McDowell Cover Direction: Andrew S. Tanenbaum, David J. Wetherall, Tracy Dunkelberger Art Director: Linda Knowles Cover Designer: Susan Paradise Cover Illustration: Jason Consalvo Interior Design: Andrew S. Tanenbaum AV Production Project Manager: Gregory L. Dulles Interior Illustrations: Laserwords, Inc. Media Editor: Daniel Sandin Composition: Andrew S. Tanenbaum Copyeditor: Rachel Head Proofreader: Joe Ruddick Printer/Binder: Courier/Westford Cover Printer: Lehigh-Phoenix Color/ Hagerstown Credits and acknowledgments borrowed from other sources and reproduced, with permission, in this textbook appear on appropriate page within text. Many of the designations by manufacturers and sellers to distinguish their products are claimed as trademarks. Where those designations appear in this book, and the publisher was aware of a trademark claim, the designations have been printed in initial caps or all caps. Copyright © 2011, 2003, 1996, 1989, 1981 Pearson Education, Inc., publishing as Prentice Hall. All rights reserved. Manufactured in the United States of America. This publication is protected by Copyright, and permission should be obtained from the publisher prior to any prohibited reproduction, storage in a retrieval system, or transmission in any form or by any means, electronic, mechanical, photocopying, recording, or likewise. To obtain permission(s) to use material from this work, please submit a written request to Pearson Education, Inc., Permissions Department, 501 Boylston Street, Suite 900, Boston, Massachusetts 02116. Library of Congress Cataloging-in-Publication Data Tanenbaum, Andrew S., 1944Computer networks / Andrew S. Tanenbaum, David J. Wetherall. -- 5th ed. p. cm. Includes bibliographical references and index. ISBN-13: 978-0-13-212695-3 (alk. paper) ISBN-10: 0-13-212695-8 (alk. paper) 1. Computer networks. I. Wetherall, D. (David) II. Title. TK5105.5.T36 2011 004.6--dc22 2010034366 10 9 8 7 6 5 4 3 2 1—CRW—14 13 12 11 10 To Suzanne, Barbara, Daniel, Aron, Marvin, Matilde, and the memory of Bram, and Sweetie π (AST) To Katrin, Lucy, and Pepper (DJW) This page intentionally left blank CONTENTS xix PREFACE 1 1 INTRODUCTION 1.1 USES OF COMPUTER NETWORKS, 3 1.1.1 Business Applications, 3 1.1.2 Home Applications, 6 1.1.3 Mobile Users, 10 1.1.4 Social Issues, 14 1.2 NETWORK HARDWARE, 17 1.2.1 Personal Area Networks, 18 1.2.2 Local Area Networks, 19 1.2.3 Metropolitan Area Networks, 23 1.2.4 Wide Area Networks, 23 1.2.5 Internetworks, 28 1.3 NETWORK SOFTWARE, 29 1.3.1 Protocol Hierarchies, 29 1.3.2 Design Issues for the Layers, 33 1.3.3 Connection-Oriented Versus Connectionless Service, 35 1.3.4 Service Primitives, 38 1.3.5 The Relationship of Services to Protocols, 40 1.4 REFERENCE MODELS, 41 1.4.1 The OSI Reference Model, 41 1.4.2 The TCP/IP Reference Model, 45 1.4.3 The Model Used in This Book, 48 vii viii CONTENTS 1.4.4 A Comparison of the OSI and TCP/IP Reference Models*, 49 1.4.5 A Critique of the OSI Model and Protocols*, 51 1.4.6 A Critique of the TCP/IP Reference Model*, 53 1.5 EXAMPLE NETWORKS, 54 1.5.1 The Internet, 54 1.5.2 Third-Generation Mobile Phone Networks*, 65 1.5.3 Wireless LANs: 802.11*, 70 1.5.4 RFID and Sensor Networks*, 73 1.6 NETWORK STANDARDIZATION*, 75 1.6.1 Who’s Who in the Telecommunications World, 77 1.6.2 Who’s Who in the International Standards World, 78 1.6.3 Who’s Who in the Internet Standards World, 80 1.7 METRIC UNITS, 82 1.8 OUTLINE OF THE REST OF THE BOOK, 83 1.9 SUMMARY, 84 2 THE PHYSICAL LAYER 89 2.1 THE THEORETICAL BASIS FOR DATA COMMUNICATION, 90 2.1.1 Fourier Analysis, 90 2.1.2 Bandwidth-Limited Signals, 90 2.1.3 The Maximum Data Rate of a Channel, 94 2.2 GUIDED TRANSMISSION MEDIA, 95 2.2.1 Magnetic Media, 95 2.2.2 Twisted Pairs, 96 2.2.3 Coaxial Cable, 97 2.2.4 Power Lines, 98 2.2.5 Fiber Optics, 99 2.3 WIRELESS TRANSMISSION, 105 2.3.1 The Electromagnetic Spectrum, 105 2.3.2 Radio Transmission, 109 2.3.3 Microwave Transmission, 110 2.3.4 Infrared Transmission, 114 2.3.5 Light Transmission, 114 CONTENTS ix 2.4 COMMUNICATION SATELLITES*, 116 2.4.1 Geostationary Satellites, 117 2.4.2 Medium-Earth Orbit Satellites, 121 2.4.3 Low-Earth Orbit Satellites, 121 2.4.4 Satellites Versus Fiber, 123 2.5 DIGITAL MODULATION AND MULTIPLEXING, 125 2.5.1 Baseband Transmission, 125 2.5.2 Passband Transmission, 130 2.5.3 Frequency Division Multiplexing, 132 2.5.4 Time Division Multiplexing, 135 2.5.5 Code Division Multiplexing, 135 2.6 THE PUBLIC SWITCHED TELEPHONE NETWORK, 138 2.6.1 Structure of the Telephone System, 139 2.6.2 The Politics of Telephones, 142 2.6.3 The Local Loop: Modems, ADSL, and Fiber, 144 2.6.4 Trunks and Multiplexing, 152 2.6.5 Switching, 161 2.7 THE MOBILE TELEPHONE SYSTEM*, 164 2.7.1 First-Generation (coco1G) Mobile Phones: Analog Voice, 166 2.7.2 Second-Generation (2G) Mobile Phones: Digital Voice, 170 2.7.3 Third-Generation (3G) Mobile Phones: Digital Voice and Data, 174 2.8 CABLE TELEVISION*, 179 2.8.1 Community Antenna Television, 179 2.8.2 Internet over Cable, 180 2.8.3 Spectrum Allocation, 182 2.8.4 Cable Modems, 183 2.8.5 ADSL Versus Cable, 185 2.9 SUMMARY, 186 3 THE DATA LINK LAYER 3.1 DATA LINK LAYER DESIGN ISSUES, 194 3.1.1 Services Provided to the Network Layer, 194 3.1.2 Framing, 197 3.1.3 Error Control, 200 3.1.4 Flow Control, 201 193 x CONTENTS 3.2 ERROR DETECTION AND CORRECTION, 202 3.2.1 Error-Correcting Codes, 204 3.2.2 Error-Detecting Codes, 209 3.3 ELEMENTARY DATA LINK PROTOCOLS, 215 3.3.1 A Utopian Simplex Protocol, 220 3.3.2 A Simplex Stop-and-Wait Protocol for an Error-Free Channel, 221 3.3.3 A Simplex Stop-and-Wait Protocol for a Noisy Channel, 222 3.4 SLIDING WINDOW PROTOCOLS, 226 3.4.1 A One-Bit Sliding Window Protocol, 229 3.4.2 A Protocol Using Go-Back-N, 232 3.4.3 A Protocol Using Selective Repeat, 239 3.5 EXAMPLE DATA LINK PROTOCOLS, 244 3.5.1 Packet over SONET, 245 3.5.2 ADSL (Asymmetric Digital Subscriber Loop), 248 3.6 SUMMARY, 251 4 THE MEDIUM ACCESS CONTROL SUBLAYER 257 4.1 THE CHANNEL ALLOCATION PROBLEM, 258 4.1.1 Static Channel Allocation, 258 4.1.2 Assumptions for Dynamic Channel Allocation, 260 4.2 MULTIPLE ACCESS PROTOCOLS, 261 4.2.1 ALOHA, 262 4.2.2 Carrier Sense Multiple Access Protocols, 266 4.2.3 Collision-Free Protocols, 269 4.2.4 Limited-Contention Protocols, 274 4.2.5 Wireless LAN Protocols, 277 4.3 ETHERNET, 280 4.3.1 Classic Ethernet Physical Layer, 281 4.3.2 Classic Ethernet MAC Sublayer Protocol, 282 4.3.3 Ethernet Performance, 286 4.3.4 Switched Ethernet, 288 CONTENTS xi 4.3.5 Fast Ethernet, 290 4.3.6 Gigabit Ethernet, 293 4.3.7 10-Gigabit Ethernet, 296 4.3.8 Retrospective on Ethernet, 298 4.4 WIRELESS LANS, 299 4.4.1 The 802.11 Architecture and Protocol Stack, 299 4.4.2 The 802.11 Physical Layer, 301 4.4.3 The 802.11 MAC Sublayer Protocol, 303 4.4.4 The 802.11 Frame Structure, 309 4.4.5 Services, 311 4.5 BROADBAND WIRELESS*, 312 4.5.1 Comparison of 802.16 with 802.11 and 3G, 313 4.5.2 The 802.16 Architecture and Protocol Stack, 314 4.5.3 The 802.16 Physical Layer, 316 4.5.4 The 802.16 MAC Sublayer Protocol, 317 4.5.5 The 802.16 Frame Structure, 319 4.6 BLUETOOTH*, 320 4.6.1 Bluetooth Architecture, 320 4.6.2 Bluetooth Applications, 321 4.6.3 The Bluetooth Protocol Stack, 322 4.6.4 The Bluetooth Radio Layer, 324 4.6.5 The Bluetooth Link Layers, 324 4.6.6 The Bluetooth Frame Structure, 325 4.7 RFID*, 327 4.7.1 EPC Gen 2 Architecture, 327 4.7.2 EPC Gen 2 Physical Layer, 328 4.7.3 EPC Gen 2 Tag Identification Layer, 329 4.7.4 Tag Identification Message Formats, 331 4.8 DATA LINK LAYER SWITCHING, 332 4.8.1 Uses of Bridges, 332 4.8.2 Learning Bridges, 334 4.8.3 Spanning Tree Bridges, 337 4.8.4 Repeaters, Hubs, Bridges, Switches, Routers, and Gateways, 340 4.8.5 Virtual LANs, 342 4.9 SUMMARY, 349 xii 5 CONTENTS THE NETWORK LAYER 355 5.1 NETWORK LAYER DESIGN ISSUES, 355 5.1.1 Store-and-Forward Packet Switching, 356 5.1.2 Services Provided to the Transport Layer, 356 5.1.3 Implementation of Connectionless Service, 358 5.1.4 Implementation of Connection-Oriented Service, 359 5.1.5 Comparison of Virtual-Circuit and Datagram Networks, 361 5.2 ROUTING ALGORITHMS, 362 5.2.1 The Optimality Principle, 364 5.2.2 Shortest Path Algorithm, 366 5.2.3 Flooding, 368 5.2.4 Distance Vector Routing, 370 5.2.5 Link State Routing, 373 5.2.6 Hierarchical Routing, 378 5.2.7 Broadcast Routing, 380 5.2.8 Multicast Routing, 382 5.2.9 Anycast Routing, 385 5.2.10 Routing for Mobile Hosts, 386 5.2.11 Routing in Ad Hoc Networks, 389 5.3 CONGESTION CONTROL ALGORITHMS, 392 5.3.1 Approaches to Congestion Control, 394 5.3.2 Traffic-Aware Routing, 395 5.3.3 Admission Control, 397 5.3.4 Traffic Throttling, 398 5.3.5 Load Shedding, 401 5.4 QUALITY OF SERVICE, 404 5.4.1 Application Requirements, 405 5.4.2 Traffic Shaping, 407 5.4.3 Packet Scheduling, 411 5.4.4 Admission Control, 415 5.4.5 Integrated Services, 418 5.4.6 Differentiated Services, 421 5.5 INTERNETWORKING, 424 5.5.1 How Networks Differ, 425 5.5.2 How Networks Can Be Connected, 426 5.5.3 Tunneling, 429 CONTENTS xiii 5.5.4 Internetwork Routing, 431 5.5.5 Packet Fragmentation, 432 5.6 THE NETWORK LAYER IN THE INTERNET, 436 5.6.1 The IP Version 4 Protocol, 439 5.6.2 IP Addresses, 442 5.6.3 IP Version 6, 455 5.6.4 Internet Control Protocols, 465 5.6.5 Label Switching and MPLS, 470 5.6.6 OSPF—An Interior Gateway Routing Protocol, 474 5.6.7 BGP—The Exterior Gateway Routing Protocol, 479 5.6.8 Internet Multicasting, 484 5.6.9 Mobile IP, 485 5.7 SUMMARY, 488 6 THE TRANSPORT LAYER 495 6.1 THE TRANSPORT SERVICE, 495 6.1.1 Services Provided to the Upper Layers, 496 6.1.2 Transport Service Primitives, 498 6.1.3 Berkeley Sockets, 500 6.1.4 An Example of Socket Programming: An Internet File Server, 503 6.2 ELEMENTS OF TRANSPORT PROTOCOLS, 507 6.2.1 Addressing, 509 6.2.2 Connection Establishment, 512 6.2.3 Connection Release, 517 6.2.4 Error Control and Flow Control, 522 6.2.5 Multiplexing, 527 6.2.6 Crash Recovery, 527 6.3 CONGESTION CONTROL, 530 6.3.1 Desirable Bandwidth Allocation, 531 6.3.2 Regulating the Sending Rate, 535 6.3.3 Wireless Issues, 539 6.4 THE INTERNET TRANSPORT PROTOCOLS: UDP, 541 6.4.1 Introduction to UDP, 541 6.4.2 Remote Procedure Call, 543 6.4.3 Real-Time Transport Protocols, 546 xiv CONTENTS 6.5 THE INTERNET TRANSPORT PROTOCOLS: TCP, 552 6.5.1 Introduction to TCP, 552 6.5.2 The TCP Service Model, 553 6.5.3 The TCP Protocol, 556 6.5.4 The TCP Segment Header, 557 6.5.5 TCP Connection Establishment, 560 6.5.6 TCP Connection Release, 562 6.5.7 TCP Connection Management Modeling, 562 6.5.8 TCP Sliding Window, 565 6.5.9 TCP Timer Management, 568 6.5.10 TCP Congestion Control, 571 6.5.11 The Future of TCP, 581 6.6 PERFORMANCE ISSUES*, 582 6.6.1 Performance Problems in Computer Networks, 583 6.6.2 Network Performance Measurement, 584 6.6.3 Host Design for Fast Networks, 586 6.6.4 Fast Segment Processing, 590 6.6.5 Header Compression, 593 6.6.6 Protocols for Long Fat Networks, 595 6.7 DELAY-TOLERANT NETWORKING*, 599 6.7.1 DTN Architecture, 600 6.7.2 The Bundle Protocol, 603 6.8 SUMMARY, 605 7 THE APPLICATION LAYER 7.1 DNS—THE DOMAIN NAME SYSTEM, 611 7.1.1 The DNS Name Space, 612 7.1.2 Domain Resource Records, 616 7.1.3 Name Servers, 619 7.2 ELECTRONIC MAIL*, 623 7.2.1 Architecture and Services, 624 7.2.2 The User Agent, 626 7.2.3 Message Formats, 630 7.2.4 Message Transfer, 637 7.2.5 Final Delivery, 643 611 CONTENTS xv 7.3 THE WORLD WIDE WEB, 646 7.3.1 Architectural Overview, 647 7.3.2 Static Web Pages, 662 7.3.3 Dynamic Web Pages and Web Applications, 672 7.3.4 HTTP—The HyperText Transfer Protocol, 683 7.3.5 The Mobile Web, 693 7.3.6 Web Search, 695 7.4 STREAMING AUDIO AND VIDEO, 697 7.4.1 Digital Audio, 699 7.4.2 Digital Video, 704 7.4.3 Streaming Stored Media, 713 7.4.4 Streaming Live Media, 721 7.4.5 Real-Time Conferencing, 724 7.5 CONTENT DELIVERY, 734 7.5.1 Content and Internet Traffic, 736 7.5.2 Server Farms and Web Proxies, 738 7.5.3 Content Delivery Networks, 743 7.5.4 Peer-to-Peer Networks, 748 7.6 SUMMARY, 757 8 NETWORK SECURITY 8.1 CRYPTOGRAPHY, 766 8.1.1 Introduction to Cryptography, 767 8.1.2 Substitution Ciphers, 769 8.1.3 Transposition Ciphers, 771 8.1.4 One-Time Pads, 772 8.1.5 Two Fundamental Cryptographic Principles, 776 8.2 SYMMETRIC-KEY ALGORITHMS, 778 8.2.1 DES—The Data Encryption Standard, 780 8.2.2 AES—The Advanced Encryption Standard, 783 8.2.3 Cipher Modes, 787 8.2.4 Other Ciphers, 792 8.2.5 Cryptanalysis, 792 763 xvi CONTENTS 8.3 PUBLIC-KEY ALGORITHMS, 793 8.3.1 RSA, 794 8.3.2 Other Public-Key Algorithms, 796 8.4 DIGITAL SIGNATURES, 797 8.4.1 Symmetric-Key Signatures, 798 8.4.2 Public-Key Signatures, 799 8.4.3 Message Digests, 800 8.4.4 The Birthday Attack, 804 8.5 MANAGEMENT OF PUBLIC KEYS, 806 8.5.1 Certificates, 807 8.5.2 X.509, 809 8.5.3 Public Key Infrastructures, 810 8.6 COMMUNICATION SECURITY, 813 8.6.1 IPsec, 814 8.6.2 Firewalls, 818 8.6.3 Virtual Private Networks, 821 8.6.4 Wireless Security, 822 8.7 AUTHENTICATION PROTOCOLS, 827 8.7.1 Authentication Based on a Shared Secret Key, 828 8.7.2 Establishing a Shared Key: The Diffie-Hellman Key Exchange, 833 8.7.3 Authentication Using a Key Distribution Center, 835 8.7.4 Authentication Using Kerberos, 838 8.7.5 Authentication Using Public-Key Cryptography, 840 8.8 EMAIL SECURITY*, 841 8.8.1 PGP—Pretty Good Privacy, 842 8.8.2 S/MIME, 846 8.9 WEB SECURITY, 846 8.9.1 Threats, 847 8.9.2 Secure Naming, 848 8.9.3 SSL—The Secure Sockets Layer, 853 8.9.4 Mobile Code Security, 857 8.10 SOCIAL ISSUES, 860 8.10.1 Privacy, 860 8.10.2 Freedom of Speech, 863 8.10.3 Copyright, 867 8.11 SUMMARY, 869 CONTENTS 9 READING LIST AND BIBLIOGRAPHY xvii 877 9.1 SUGGESTIONS FOR FURTHER READING*, 877 9.1.1 Introduction and General Works, 878 9.1.2 The Physical Layer, 879 9.1.3 The Data Link Layer, 880 9.1.4 The Medium Access Control Sublayer, 880 9.1.5 The Network Layer, 881 9.1.6 The Transport Layer, 882 9.1.7 The Application Layer, 882 9.1.8 Network Security, 883 9.2 ALPHABETICAL BIBLIOGRAPHY*, 884 INDEX 905 This page intentionally left blank PREFACE This book is now in its fifth edition. Each edition has corresponded to a different phase in the way computer networks were used. When the first edition appeared in 1980, networks were an academic curiosity. When the second edition appeared in 1988, networks were used by universities and large businesses. When the third edition appeared in 1996, computer networks, especially the Internet, had become a daily reality for millions of people. By the fourth edition, in 2003, wireless networks and mobile computers had become commonplace for accessing the Web and the Internet. Now, in the fifth edition, networks are about content distribution (especially videos using CDNs and peer-to-peer networks) and mobile phones are small computers on the Internet. New in the Fifth Edition Among the many changes in this book, the most important one is the addition of Prof. David J. Wetherall as a co-author. David brings a rich background in networking, having cut his teeth designing metropolitan-area networks more than 20 years ago. He has worked with the Internet and wireless networks ever since and is a professor at the University of Washington, where he has been teaching and doing research on computer networks and related topics for the past decade. Of course, the book also has many changes to keep up with the: ever-changing world of computer networks. Among these are revised and new material on Wireless networks (802.12 and 802.16) The 3G networks used by smart phones RFID and sensor networks Content distribution using CDNs Peer-to-peer networks Real-time media (from stored, streaming, and live sources) Internet telephony (voice over IP) Delay-tolerant networks A more detailed chapter-by-chapter list follows. xix xx PREFACE Chapter 1 has the same introductory function as in the fourth edition, but the contents have been revised and brought up to date. The Internet, mobile phone networks, 802.11, and RFID and sensor networks are discussed as examples of computer networks. Material on the original Ethernet—with its vampire taps— has been removed, along with the material on ATM. Chapter 2, which covers the physical layer, has expanded coverage of digital modulation (including OFDM as widely used in wireless networks) and 3G networks (based on CDMA). New technologies are discussed, including Fiber to the Home and power-line networking. Chapter 3, on point-to-point links, has been improved in two ways. The material on codes for error detection and correction has been updated, and also includes a brief description of the modern codes that are important in practice (e.g., convolutional and LDPC codes). The examples of protocols now use Packet over SONET and ADSL. Sadly, the material on protocol verification has been removed as it is little used. In Chapter 4, on the MAC sublayer, the principles are timeless but the technologies have changed. Sections on the example networks have been redone accordingly, including gigabit Ethernet, 802.11, 802.16, Bluetooth, and RFID. Also updated is the coverage of LAN switching, including VLANs. Chapter 5, on the network layer, covers the same ground as in the fourth edition. The revisions have been to update material and add depth, particularly for quality of service (relevant for real-time media) and internetworking. The sections on BGP, OSPF and CIDR have been expanded, as has the treatment of multicast routing. Anycast routing is now included. Chapter 6, on the transport layer, has had material added, revised, and removed. New material describes delay-tolerant networking and congestion control in general. The revised material updates and expands the coverage of TCP congestion control. The material removed described connection-oriented network layers, something rarely seen any more. Chapter 7, on applications, has also been updated and enlarged. While material on DNS and email is similar to that in the fourth edition, in the past few years there have been many developments in the use of the Web, streaming media and content delivery. Accordingly, sections on the Web and streaming media have been brought up to date. A new section covers content distribution, including CDNs and peer-to-peer networks. Chapter 8, on security, still covers both symmetric and public-key cryptography for confidentiality and authenticity. Material on the techniques used in practice, including firewalls and VPNs, has been updated, with new material on 802.11 security and Kerberos V5 added. Chapter 9 contains a renewed list of suggested readings and a comprehensive bibliography of over 300 citations to the current literature. More than half of these are to papers and books written in 2000 or later, and the rest are citations to classic papers. PREFACE xxi List of Acronyms Computer books are full of acronyms. This one is no exception. By the time you are finished reading this one, the following should ring a bell: ADSL, AES, AJAX, AODV, AP, ARP, ARQ, AS, BGP, BOC, CDMA, CDN, CGI, CIDR, CRL, CSMA, CSS, DCT, DES, DHCP, DHT, DIFS, DMCA, DMT, DMZ, DNS, DOCSIS, DOM, DSLAM, DTN, FCFS, FDD, FDDI, FDM, FEC, FIFO, FSK, FTP, GPRS, GSM, HDTV, HFC, HMAC, HTTP, IAB, ICANN, ICMP, IDEA, IETF, IMAP, IMP, IP, IPTV, IRTF, ISO, ISP, ITU, JPEG, JSP, JVM, LAN, LATA, LEC, LEO, LLC, LSR, LTE, MAN, MFJ, MIME, MPEG, MPLS, MSC, MTSO, MTU, NAP, NAT, NRZ, NSAP, OFDM, OSI, OSPF, PAWS, PCM, PGP, PIM, PKI, POP, POTS, PPP, PSTN, QAM, QPSK, RED, RFC, RFID, RPC, RSA, RTSP, SHA, SIP, SMTP, SNR, SOAP, SONET, SPE, SSL, TCP, TDD, TDM, TSAP, UDP, UMTS, URL, VLAN, VSAT, WAN, WDM, and XML. But don’t worry. Each will appear in boldface type and be carefully defined before it is used. As a fun test, see how many you can identify before reading the book, write the number in the margin, then try again after reading the book. How to Use the Book To help instructors use this book as a text for courses ranging in length from quarters to semesters, we have structured the chapters into core and optional material. The sections marked with a ‘‘*’’ in the table of contents are the optional ones. If a major section (e.g., 2.7) is so marked, all of its subsections are optional. They provide material on network technologies that is useful but can be omitted from a short course without loss of continuity. Of course, students should be encouraged to read those sections as well, to the extent they have time, as all the material is up to date and of value. Instructors’ Resource Materials The following protected instructors’ resource materials are available on the publisher’s Web site at www.pearsonhighered.com/tanenbaum. For a username and password, please contact your local Pearson representative. Solutions manual PowerPoint lecture slides Students’ Resource Materials Resources for students are available through the open-access Companion Web site link on www.pearsonhighered.com/tanenbaum, including Web resources, links to tutorials, organizations, FAQs, and more Figures, tables, and programs from the book Steganography demo Protocol simulators xxii PREFACE Acknowledgements Many people helped us during the course of the fifth edition. We would especially like to thank Emmanuel Agu (Worcester Polytechnic Institute), Yoris Au (University of Texas at Antonio), Nikhil Bhargava (Aircom International, Inc.), Michael Buettner (University of Washington), John Day (Boston University), Kevin Fall (Intel Labs), Ronald Fulle (Rochester Institute of Technology), Ben Greenstein (Intel Labs), Daniel Halperin (University of Washington), Bob Kinicki (Worcester Polytechnic Institute), Tadayoshi Kohno (University of Washington), Sarvish Kulkarni (Villanova University), Hank Levy (University of Washington), Ratul Mahajan (Microsoft Research), Craig Partridge (BBN), Michael Piatek (University of Washington), Joshua Smith (Intel Labs), Neil Spring (University of Maryland), David Teneyuca (University of Texas at Antonio), Tammy VanDegrift (University of Portland), and Bo Yuan (Rochester Institute of Technology), for providing ideas and feedback. Melody Kadenko and Julie Svendsen provided administrative support to David. Shivakant Mishra (University of Colorado at Boulder) and Paul Nagin (Chimborazo Publishing, Inc.) thought of many new and challenging end-of-chapter problems. Our editor at Pearson, Tracy Dunkelberger, was her usual helpful self in many ways large and small. Melinda Haggerty and Jeff Holcomb did a good job of keeping things running smoothly. Steve Armstrong (LeTourneau University) prepared the PowerPoint slides. Stephen Turner (University of Michigan at Flint) artfully revised the Web resources and the simulators that accompany the text. Our copyeditor, Rachel Head, is an odd hybrid: she has the eye of an eagle and the memory of an elephant. After reading all her corrections, both of us wondered how we ever made it past third grade. Finally, we come to the most important people. Suzanne has been through this 19 times now and still has endless patience and love. Barbara and Marvin now know the difference between good textbooks and bad ones and are always an inspiration to produce good ones. Daniel and Matilde are welcome additions to our family. Aron is unlikely to read this book soon, but he likes the nice pictures on page 866 (AST). Katrin and Lucy provided endless support and always managed to keep a smile on my face. Thank you (DJW). ANDREW S. TANENBAUM DAVID J. WETHERALL 1 INTRODUCTION Each of the past three centuries was dominated by a single new technology. The 18th century was the era of the great mechanical systems accompanying the Industrial Revolution. The 19th century was the age of the steam engine. During the 20th century, the key technology was information gathering, processing, and distribution. Among other developments, we saw the installation of worldwide telephone networks, the invention of radio and television, the birth and unprecedented growth of the computer industry, the launching of communication satellites, and, of course, the Internet. As a result of rapid technological progress, these areas are rapidly converging in the 21st century and the differences between collecting, transporting, storing, and processing information are quickly disappearing. Organizations with hundreds of offices spread over a wide geographical area routinely expect to be able to examine the current status of even their most remote outpost at the push of a button. As our ability to gather, process, and distribute information grows, the demand for ever more sophisticated information processing grows even faster. Although the computer industry is still young compared to other industries (e.g., automobiles and air transportation), computers have made spectacular progress in a short time. During the first two decades of their existence, computer systems were highly centralized, usually within a single large room. Not infrequently, this room had glass walls, through which visitors could gawk at the great electronic wonder inside. A medium-sized company or university might have had 1 2 INTRODUCTION CHAP. 1 one or two computers, while very large institutions had at most a few dozen. The idea that within forty years vastly more powerful computers smaller than postage stamps would be mass produced by the billions was pure science fiction. The merging of computers and communications has had a profound influence on the way computer systems are organized. The once-dominant concept of the ‘‘computer center’’ as a room with a large computer to which users bring their work for processing is now totally obsolete (although data centers holding thousands of Internet servers are becoming common). The old model of a single computer serving all of the organization’s computational needs has been replaced by one in which a large number of separate but interconnected computers do the job. These systems are called computer networks. The design and organization of these networks are the subjects of this book. Throughout the book we will use the term ‘‘computer network’’ to mean a collection of autonomous computers interconnected by a single technology. Two computers are said to be interconnected if they are able to exchange information. The connection need not be via a copper wire; fiber optics, microwaves, infrared, and communication satellites can also be used. Networks come in many sizes, shapes and forms, as we will see later. They are usually connected together to make larger networks, with the Internet being the most well-known example of a network of networks. There is considerable confusion in the literature between a computer network and a distributed system. The key distinction is that in a distributed system, a collection of independent computers appears to its users as a single coherent system. Usually, it has a single model or paradigm that it presents to the users. Often a layer of software on top of the operating system, called middleware, is responsible for implementing this model. A well-known example of a distributed system is the World Wide Web. It runs on top of the Internet and presents a model in which everything looks like a document (Web page). In a computer network, this coherence, model, and software are absent. Users are exposed to the actual machines, without any attempt by the system to make the machines look and act in a coherent way. If the machines have different hardware and different operating systems, that is fully visible to the users. If a user † wants to run a program on a remote machine, he has to log onto that machine and run it there. In effect, a distributed system is a software system built on top of a network. The software gives it a high degree of cohesiveness and transparency. Thus, the distinction between a network and a distributed system lies with the software (especially the operating system), rather than with the hardware. Nevertheless, there is considerable overlap between the two subjects. For example, both distributed systems and computer networks need to move files around. The difference lies in who invokes the movement, the system or the user. † ‘‘He’’ should be read as ‘‘he or she’’ throughout this book. SEC. 1.1 USES OF COMPUTER NETWORKS 3 Although this book primarily focuses on networks, many of the topics are also important in distributed systems. For more information about distributed systems, see Tanenbaum and Van Steen (2007). 1.1 USES OF COMPUTER NETWORKS Before we start to examine the technical issues in detail, it is worth devoting some time to pointing out why people are interested in computer networks and what they can be used for. After all, if nobody were interested in computer networks, few of them would be built. We will start with traditional uses at companies, then move on to home networking and recent developments regarding mobile users, and finish with social issues. 1.1.1 Business Applications Most companies have a substantial number of computers. For example, a company may have a computer for each worker and use them to design products, write brochures, and do the payroll. Initially, some of these computers may have worked in isolation from the others, but at some point, management may have decided to connect them to be able to distribute information throughout the company. Put in slightly more general form, the issue here is resource sharing. The goal is to make all programs, equipment, and especially data available to anyone on the network without regard to the physical location of the resource or the user. An obvious and widespread example is having a group of office workers share a common printer. None of the individuals really needs a private printer, and a high-volume networked printer is often cheaper, faster, and easier to maintain than a large collection of individual printers. However, probably even more important than sharing physical resources such as printers, and tape backup systems, is sharing information. Companies small and large are vitally dependent on computerized information. Most companies have customer records, product information, inventories, financial statements, tax information, and much more online. If all of its computers suddenly went down, a bank could not last more than five minutes. A modern manufacturing plant, with a computer-controlled assembly line, would not last even 5 seconds. Even a small travel agency or three-person law firm is now highly dependent on computer networks for allowing employees to access relevant information and documents instantly. For smaller companies, all the computers are likely to be in a single office or perhaps a single building, but for larger ones, the computers and employees may be scattered over dozens of offices and plants in many countries. Nevertheless, a sales person in New York might sometimes need access to a product inventory 4 INTRODUCTION CHAP. 1 database in Singapore. Networks called VPNs (Virtual Private Networks) may be used to join the individual networks at different sites into one extended network. In other words, the mere fact that a user happens to be 15,000 km away from his data should not prevent him from using the data as though they were local. This goal may be summarized by saying that it is an attempt to end the ‘‘tyranny of geography.’’ In the simplest of terms, one can imagine a company’s information system as consisting of one or more databases with company information and some number of employees who need to access them remotely. In this model, the data are stored on powerful computers called servers. Often these are centrally housed and maintained by a system administrator. In contrast, the employees have simpler machines, called clients, on their desks, with which they access remote data, for example, to include in spreadsheets they are constructing. (Sometimes we will refer to the human user of the client machine as the ‘‘client,’’ but it should be clear from the context whether we mean the computer or its user.) The client and server machines are connected by a network, as illustrated in Fig. 1-1. Note that we have shown the network as a simple oval, without any detail. We will use this form when we mean a network in the most abstract sense. When more detail is required, it will be provided. Client Server Network Figure 1-1. A network with two clients and one server. This whole arrangement is called the client-server model. It is widely used and forms the basis of much network usage. The most popular realization is that of a Web application, in which the server generates Web pages based on its database in response to client requests that may update the database. The client-server model is applicable when the client and server are both in the same building (and belong to the same company), but also when they are far apart. For example, when a person at home accesses a page on the World Wide Web, the same model is employed, with the remote Web server being the server and the user’s personal SEC. 1.1 5 USES OF COMPUTER NETWORKS computer being the client. Under most conditions, one server can handle a large number (hundreds or thousands) of clients simultaneously. If we look at the client-server model in detail, we see that two processes (i.e., running programs) are involved, one on the client machine and one on the server machine. Communication takes the form of the client process sending a message over the network to the server process. The client process then waits for a reply message. When the server process gets the request, it performs the requested work or looks up the requested data and sends back a reply. These messages are shown in Fig. 1-2. Client machine Server machine Request Network Reply Client process Server process Figure 1-2. The client-server model involves requests and replies. A second goal of setting up a computer network has to do with people rather than information or even computers. A computer network can provide a powerful communication medium among employees. Virtually every company that has two or more computers now has email (electronic mail), which employees generally use for a great deal of daily communication. In fact, a common gripe around the water cooler is how much email everyone has to deal with, much of it quite meaningless because bosses have discovered that they can send the same (often content-free) message to all their subordinates at the push of a button. Telephone calls between employees may be carried by the computer network instead of by the phone company. This technology is called IP telephony or Voice over IP (VoIP) when Internet technology is used. The microphone and speaker at each end may belong to a VoIP-enabled phone or the employee’s computer. Companies find this a wonderful way to save on their telephone bills. Other, richer forms of communication are made possible by computer networks. Video can be added to audio so that employees at distant locations can see and hear each other as they hold a meeting. This technique is a powerful tool for eliminating the cost and time previously devoted to travel. Desktop sharing lets remote workers see and interact with a graphical computer screen. This makes it easy for two or more people who work far apart to read and write a shared blackboard or write a report together. When one worker makes a change to an online document, the others can see the change immediately, instead of waiting several days for a letter. Such a speedup makes cooperation among far-flung groups of people easy where it previously had been impossible. More ambitious forms of remote coordination such as telemedicine are only now starting to be used (e.g., 6 INTRODUCTION CHAP. 1 remote patient monitoring) but may become much more important. It is sometimes said that communication and transportation are having a race, and whichever wins will make the other obsolete. A third goal for many companies is doing business electronically, especially with customers and suppliers. This new model is called e-commerce (electronic commerce) and it has grown rapidly in recent years. Airlines, bookstores, and other retailers have discovered that many customers like the convenience of shopping from home. Consequently, many companies provide catalogs of their goods and services online and take orders online. Manufacturers of automobiles, aircraft, and computers, among others, buy subsystems from a variety of suppliers and then assemble the parts. Using computer networks, manufacturers can place orders electronically as needed. This reduces the need for large inventories and enhances efficiency. 1.1.2 Home Applications In 1977, Ken Olsen was president of the Digital Equipment Corporation, then the number two computer vendor in the world (after IBM). When asked why Digital was not going after the personal computer market in a big way, he said: ‘‘There is no reason for any individual to have a computer in his home.’’ History showed otherwise and Digital no longer exists. People initially bought computers for word processing and games. Recently, the biggest reason to buy a home computer was probably for Internet access. Now, many consumer electronic devices, such as set-top boxes, game consoles, and clock radios, come with embedded computers and computer networks, especially wireless networks, and home networks are broadly used for entertainment, including listening to, looking at, and creating music, photos, and videos. Internet access provides home users with connectivity to remote computers. As with companies, home users can access information, communicate with other people, and buy products and services with e-commerce. The main benefit now comes from connecting outside of the home. Bob Metcalfe, the inventor of Ethernet, hypothesized that the value of a network is proportional to the square of the number of users because this is roughly the number of different connections that may be made (Gilder, 1993). This hypothesis is known as ‘‘Metcalfe’s law.’’ It helps to explain how the tremendous popularity of the Internet comes from its size. Access to remote information comes in many forms. It can be surfing the World Wide Web for information or just for fun. Information available includes the arts, business, cooking, government, health, history, hobbies, recreation, science, sports, travel, and many others. Fun comes in too many ways to mention, plus some ways that are better left unmentioned. Many newspapers have gone online and can be personalized. For example, it is sometimes possible to tell a newspaper that you want everything about corrupt SEC. 1.1 USES OF COMPUTER NETWORKS 7 politicians, big fires, scandals involving celebrities, and epidemics, but no football, thank you. Sometimes it is possible to have the selected articles downloaded to your computer while you sleep. As this trend continues, it will cause massive unemployment among 12-year-old paperboys, but newspapers like it because distribution has always been the weakest link in the whole production chain. Of course, to make this model work, they will first have to figure out how to make money in this new world, something not entirely obvious since Internet users expect everything to be free. The next step beyond newspapers (plus magazines and scientific journals) is the online digital library. Many professional organizations, such as the ACM (www.acm.org) and the IEEE Computer Society (www.computer.org), already have all their journals and conference proceedings online. Electronic book readers and online libraries may make printed books obsolete. Skeptics should take note of the effect the printing press had on the medieval illuminated manuscript. Much of this information is accessed using the client-server model, but there is different, popular model for accessing information that goes by the name of peer-to-peer communication (Parameswaran et al., 2001). In this form, individuals who form a loose group can communicate with others in the group, as shown in Fig. 1-3. Every person can, in principle, communicate with one or more other people; there is no fixed division into clients and servers. Figure 1-3. In a peer-to-peer system there are no fixed clients and servers. Many peer-to-peer systems, such BitTorrent (Cohen, 2003), do not have any central database of content. Instead, each user maintains his own database locally and provides a list of other nearby people who are members of the system. A new user can then go to any existing member to see what he has and get the names of other members to inspect for more content and more names. This lookup process can be repeated indefinitely to build up a large local database of what is out there. It is an activity that would get tedious for people but computers excel at it. 8 INTRODUCTION CHAP. 1 Peer-to-peer communication is often used to share music and videos. It really hit the big time around 2000 with a music sharing service called Napster that was shut down after what was probably the biggest copyright infringement case in all of recorded history (Lam and Tan, 2001; and Macedonia, 2000). Legal applications for peer-to-peer communication also exist. These include fans sharing public domain music, families sharing photos and movies, and users downloading public software packages. In fact, one of the most popular Internet applications of all, email, is inherently peer-to-peer. This form of communication is likely to grow considerably in the future. All of the above applications involve interactions between a person and a remote database full of information. The second broad category of network use is person-to-person communication, basically the 21st century’s answer to the 19th century’s telephone. E-mail is already used on a daily basis by millions of people all over the world and its use is growing rapidly. It already routinely contains audio and video as well as text and pictures. Smell may take a while. Any teenager worth his or her salt is addicted to instant messaging. This facility, derived from the UNIX talk program in use since around 1970, allows two people to type messages at each other in real time. There are multi-person messaging services too, such as the Twitter service that lets people send short text messages called ‘‘tweets’’ to their circle of friends or other willing audiences. The Internet can be used by applications to carry audio (e.g., Internet radio stations) and video (e.g., YouTube). Besides being a cheap way to call to distant friends, these applications can provide rich experiences such as telelearning, meaning attending 8 A.M. classes without the inconvenience of having to get out of bed first. In the long run, the use of networks to enhance human-to-human communication may prove more important than any of the others. It may become hugely important to people who are geographically challenged, giving them the same access to services as people living in the middle of a big city. Between person-to-person communications and accessing information are social network applications. Here, the flow of information is driven by the relationships that people declare between each other. One of the most popular social networking sites is Facebook. It lets people update their personal profiles and shares the updates with other people who they have declared to be their friends. Other social networking applications can make introductions via friends of friends, send news messages to friends such as Twitter above, and much more. Even more loosely, groups of people can work together to create content. A wiki, for example, is a collaborative Web site that the members of a community edit. The most famous wiki is the Wikipedia, an encyclopedia anyone can edit, but there are thousands of other wikis. Our third category is electronic commerce in the broadest sense of the term. Home shopping is already popular and enables users to inspect the online catalogs of thousands of companies. Some of these catalogs are interactive, showing products from different viewpoints and in configurations that can be personalized. SEC. 1.1 USES OF COMPUTER NETWORKS 9 After the customer buys a product electronically but cannot figure out how to use it, online technical support may be consulted. Another area in which e-commerce is widely used is access to financial institutions. Many people already pay their bills, manage their bank accounts, and handle their investments electronically. This trend will surely continue as networks become more secure. One area that virtually nobody foresaw is electronic flea markets (e-flea?). Online auctions of second-hand goods have become a massive industry. Unlike traditional e-commerce, which follows the client-server model, online auctions are peer-to-peer in the sense that consumers can act as both buyers and sellers. Some of these forms of e-commerce have acquired cute little tags based on the fact that ‘‘to’’ and ‘‘2’’ are pronounced the same. The most popular ones are listed in Fig. 1-4. Tag Full name Example B2C Business-to-consumer Ordering books online B2B Business-to-business Car manufacturer ordering tires from supplier G2C Government-to-consumer Government distributing tax forms electronically C2C Consumer-to-consumer Auctioning second-hand products online P2P Peer-to-peer Music sharing Figure 1-4. Some forms of e-commerce. Our fourth category is entertainment. This has made huge strides in the home in recent years, with the distribution of music, radio and television programs, and movies over the Internet beginning to rival that of traditional mechanisms. Users can find, buy, and download MP3 songs and DVD-quality movies and add them to their personal collection. TV shows now reach many homes via IPTV (IP TeleVision) systems that are based on IP technology instead of cable TV or radio transmissions. Media streaming applications let users tune into Internet radio stations or watch recent episodes of their favorite TV shows. Naturally, all of this content can be moved around your house between different devices, displays and speakers, usually with a wireless network. Soon, it may be possible to search for any movie or television program ever made, in any country, and have it displayed on your screen instantly. New films may become interactive, where the user is occasionally prompted for the story direction (should Macbeth murder Duncan or just bide his time?) with alternative scenarios provided for all cases. Live television may also become interactive, with the audience participating in quiz shows, choosing among contestants, and so on. Another form of entertainment is game playing. Already we have multiperson real-time simulation games, like hide-and-seek in a virtual dungeon, and flight 10 INTRODUCTION CHAP. 1 simulators with the players on one team trying to shoot down the players on the opposing team. Virtual worlds provide a persistent setting in which thousands of users can experience a shared reality with three-dimensional graphics. Our last category is ubiquitous computing, in which computing is embedded into everyday life, as in the vision of Mark Weiser (1991). Many homes are already wired with security systems that include door and window sensors, and there are many more sensors that can be folded in to a smart home monitor, such as energy consumption. Your electricity, gas and water meters could also report usage over the network. This would save money as there would be no need to send out meter readers. And your smoke detectors could call the fire department instead of making a big noise (which has little value if no one is home). As the cost of sensing and communication drops, more and more measurement and reporting will be done with networks. Increasingly, consumer electronic devices are networked. For example, some high-end cameras already have a wireless network capability and use it to send photos to a nearby display for viewing. Professional sports photographers can also send their photos to their editors in real-time, first wirelessly to an access point then over the Internet. Devices such as televisions that plug into the wall can use power-line networks to send information throughout the house over the wires that carry electricity. It may not be very surprising to have these objects on the network, but objects that we do not think of as computers may sense and communicate information too. For example, your shower may record water usage, give you visual feedback while you lather up, and report to a home environmental monitoring application when you are done to help save on your water bill. A technology called RFID (Radio Frequency IDentification) will push this idea even further in the future. RFID tags are passive (i.e., have no battery) chips the size of stamps and they can already be affixed to books, passports, pets, credit cards, and other items in the home and out. This lets RFID readers locate and communicate with the items over a distance of up to several meters, depending on the kind of RFID. Originally, RFID was commercialized to replace barcodes. It has not succeeded yet because barcodes are free and RFID tags cost a few cents. Of course, RFID tags offer much more and their price is rapidly declining. They may turn the real world into the Internet of things (ITU, 2005). 1.1.3 Mobile Users Mobile computers, such as laptop and handheld computers, are one of the fastest-growing segments of the computer industry. Their sales have already overtaken those of desktop computers. Why would anyone want one? People on the go often want to use their mobile devices to read and send email, tweet, watch movies, download music, play games, or simply to surf the Web for information. They want to do all of the things they do at home and in the office. Naturally, they want to do them from anywhere on land, sea or in the air. SEC. 1.1 USES OF COMPUTER NETWORKS 11 Connectivity to the Internet enables many of these mobile uses. Since having a wired connection is impossible in cars, boats, and airplanes, there is a lot of interest in wireless networks. Cellular networks operated by the telephone companies are one familiar kind of wireless network that blankets us with coverage for mobile phones. Wireless hotspots based on the 802.11 standard are another kind of wireless network for mobile computers. They have sprung up everywhere that people go, resulting in a patchwork of coverage at cafes, hotels, airports, schools, trains and planes. Anyone with a laptop computer and a wireless modem can just turn on their computer on and be connected to the Internet through the hotspot, as though the computer were plugged into a wired network. Wireless networks are of great value to fleets of trucks, taxis, delivery vehicles, and repairpersons for keeping in contact with their home base. For example, in many cities, taxi drivers are independent businessmen, rather than being employees of a taxi company. In some of these cities, the taxis have a display the driver can see. When a customer calls up, a central dispatcher types in the pickup and destination points. This information is displayed on the drivers’ displays and a beep sounds. The first driver to hit a button on the display gets the call. Wireless networks are also important to the military. If you have to be able to fight a war anywhere on Earth at short notice, counting on using the local networking infrastructure is probably not a good idea. It is better to bring your own. Although wireless networking and mobile computing are often related, they are not identical, as Fig. 1-5 shows. Here we see a distinction between fixed wireless and mobile wireless networks. Even notebook computers are sometimes wired. For example, if a traveler plugs a notebook computer into the wired network jack in a hotel room, he has mobility without a wireless network. Wireless Mobile No No Desktop computers in offices Typical applications No Yes A notebook computer used in a hotel room Yes No Networks in unwired buildings Yes Yes Store inventory with a handheld computer Figure 1-5. Combinations of wireless networks and mobile computing. Conversely, some wireless computers are not mobile. In the home, and in offices or hotels that lack suitable cabling, it can be more convenient to connect desktop computers or media players wirelessly than to install wires. Installing a wireless network may require little more than buying a small box with some electronics in it, unpacking it, and plugging it in. This solution may be far cheaper than having workmen put in cable ducts to wire the building. Finally, there are also true mobile, wireless applications, such as people walking around stores with a handheld computers recording inventory. At many busy 12 INTRODUCTION CHAP. 1 airports, car rental return clerks work in the parking lot with wireless mobile computers. They scan the barcodes or RFID chips of returning cars, and their mobile device, which has a built-in printer, calls the main computer, gets the rental information, and prints out the bill on the spot. Perhaps the key driver of mobile, wireless applications is the mobile phone. Text messaging or texting is tremendously popular. It lets a mobile phone user type a short message that is then delivered by the cellular network to another mobile subscriber. Few people would have predicted ten years ago that having teenagers tediously typing short text messages on mobile phones would be an immense money maker for telephone companies. But texting (or Short Message Service as it is known outside the U.S.) is very profitable since it costs the carrier but a tiny fraction of one cent to relay a text message, a service for which they charge far more. The long-awaited convergence of telephones and the Internet has finally arrived, and it will accelerate the growth of mobile applications. Smart phones, such as the popular iPhone, combine aspects of mobile phones and mobile computers. The (3G and 4G) cellular networks to which they connect can provide fast data services for using the Internet as well as handling phone calls. Many advanced phones connect to wireless hotspots too, and automatically switch between networks to choose the best option for the user. Other consumer electronics devices can also use cellular and hotspot networks to stay connected to remote computers. Electronic book readers can download a newly purchased book or the next edition of a magazine or today’s newspaper wherever they roam. Electronic picture frames can update their displays on cue with fresh images. Since mobile phones know their locations, often because they are equipped with GPS (Global Positioning System) receivers, some services are intentionally location dependent. Mobile maps and directions are an obvious candidate as your GPS-enabled phone and car probably have a better idea of where you are than you do. So, too, are searches for a nearby bookstore or Chinese restaurant, or a local weather forecast. Other services may record location, such as annotating photos and videos with the place at which they were made. This annotation is known as ‘‘geo-tagging.’’ An area in which mobile phones are now starting to be used is m-commerce (mobile-commerce) (Senn, 2000). Short text messages from the mobile are used to authorize payments for food in vending machines, movie tickets, and other small items instead of cash and credit cards. The charge then appears on the mobile phone bill. When equipped with NFC (Near Field Communication) technology the mobile can act as an RFID smartcard and interact with a nearby reader for payment. The driving forces behind this phenomenon are the mobile device makers and network operators, who are trying hard to figure out how to get a piece of the e-commerce pie. From the store’s point of view, this scheme may save them most of the credit card company’s fee, which can be several percent. SEC. 1.1 USES OF COMPUTER NETWORKS 13 Of course, this plan may backfire, since customers in a store might use the RFID or barcode readers on their mobile devices to check out competitors’ prices before buying and use them to get a detailed report on where else an item can be purchased nearby and at what price. One huge thing that m-commerce has going for it is that mobile phone users are accustomed to paying for everything (in contrast to Internet users, who expect everything to be free). If an Internet Web site charged a fee to allow its customers to pay by credit card, there would be an immense howling noise from the users. If, however, a mobile phone operator its customers to pay for items in a store by waving the phone at the cash register and then tacked on a fee for this convenience, it would probably be accepted as normal. Time will tell. No doubt the uses of mobile and wireless computers will grow rapidly in the future as the size of computers shrinks, probably in ways no one can now foresee. Let us take a quick look at some possibilities. Sensor networks are made up of nodes that gather and wirelessly relay information they sense about the state of the physical world. The nodes may be part of familiar items such as cars or phones, or they may be small separate devices. For example, your car might gather data on its location, speed, vibration, and fuel efficiency from its on-board diagnostic system and upload this information to a database (Hull et al., 2006). Those data can help find potholes, plan trips around congested roads, and tell you if you are a ‘‘gas guzzler’’ compared to other drivers on the same stretch of road. Sensor networks are revolutionizing science by providing a wealth of data on behavior that could not previously be observed. One example is tracking the migration of individual zebras by placing a small sensor on each animal (Juang et al., 2002). Researchers have packed a wireless computer into a cube 1 mm on edge (Warneke et al., 2001). With mobile computers this small, even small birds, rodents, and insects can be tracked. Even mundane uses, such as in parking meters, can be significant because they make use of data that were not previously available. Wireless parking meters can accept credit or debit card payments with instant verification over the wireless link. They can also report when they are in use over the wireless network. This would let drivers download a recent parking map to their car so they can find an available spot more easily. Of course, when a meter expires, it might also check for the presence of a car (by bouncing a signal off it) and report the expiration to parking enforcement. It has been estimated that city governments in the U.S. alone could collect an additional $10 billion this way (Harte et al., 2000). Wearable computers are another promising application. Smart watches with radios have been part of our mental space since their appearance in the Dick Tracy comic strip in 1946; now you can buy them. Other such devices may be implanted, such as pacemakers and insulin pumps. Some of these can be controlled over a wireless network. This lets doctors test and reconfigure them more easily. It could also lead to some nasty problems if the devices are as insecure as the average PC and can be hacked easily (Halperin et al., 2008). 14 INTRODUCTION CHAP. 1 1.1.4 Social Issues Computer networks, like the printing press 500 years ago, allow ordinary citizens to distribute and view content in ways that were not previously possible. But along with the good comes the bad, as this new-found freedom brings with it many unsolved social, political, and ethical issues. Let us just briefly mention a few of them; a thorough study would require a full book, at least. Social networks, message boards, content sharing sites, and a host of other applications allow people to share their views with like-minded individuals. As long as the subjects are restricted to technical topics or hobbies like gardening, not too many problems will arise. The trouble comes with topics that people actually care about, like politics, religion, or sex. Views that are publicly posted may be deeply offensive to some people. Worse yet, they may not be politically correct. Furthermore, opinions need not be limited to text; high-resolution color photographs and video clips are easily shared over computer networks. Some people take a live-and-let-live view, but others feel that posting certain material (e.g., verbal attacks on particular countries or religions, pornography, etc.) is simply unacceptable and that such content must be censored. Different countries have different and conflicting laws in this area. Thus, the debate rages. In the past, people have sued network operators, claiming that they are responsible for the contents of what they carry, just as newspapers and magazines are. The inevitable response is that a network is like a telephone company or the post office and cannot be expected to police what its users say. It should now come only as a slight surprise to learn that some network operators block content for their own reasons. Some users of peer-to-peer applications had their network service cut off because the network operators did not find it profitable to carry the large amounts of traffic sent by those applications. Those same operators would probably like to treat different companies differently. If you are a big company and pay well then you get good service, but if you are a small-time player, you get poor service. Opponents of this practice argue that peer-to-peer and other content should be treated in the same way because they are all just bits to the network. This argument for communications that are not differentiated by their content or source or who is providing the content is known as network neutrality (Wu, 2003). It is probably safe to say that this debate will go on for a while. Many other parties are involved in the tussle over content. For instance, pirated music and movies fueled the massive growth of peer-to-peer networks, which did not please the copyright holders, who have threatened (and sometimes taken) legal action. There are now automated systems that search peer-to-peer networks and fire off warnings to network operators and users who are suspected of infringing copyright. In the United States, these warnings are known as DMCA takedown notices after the Digital Millennium Copyright Act. This SEC. 1.1 USES OF COMPUTER NETWORKS 15 search is an arms’ race because it is hard to reliably catch copyright infringement. Even your printer might be mistaken for a culprit (Piatek et al., 2008). Computer networks make it very easy to communicate. They also make it easy for the people who run the network to snoop on the traffic. This sets up conflicts over issues such as employee rights versus employer rights. Many people read and write email at work. Many employers have claimed the right to read and possibly censor employee messages, including messages sent from a home computer outside working hours. Not all employees agree with this, especially the latter part. Another conflict is centered around government versus citizen’s rights. The FBI has installed systems at many Internet service providers to snoop on all incoming and outgoing email for nuggets of interest. One early system was originally called Carnivore, but bad publicity caused it to be renamed to the more innocent-sounding DCS1000 (Blaze and Bellovin, 2000; Sobel, 2001; and Zacks, 2001). The goal of such systems is to spy on millions of people in the hope of perhaps finding information about illegal activities. Unfortunately for the spies, the Fourth Amendment to the U.S. Constitution prohibits government searches without a search warrant, but the government often ignores it. Of course, the government does not have a monopoly on threatening people’s privacy. The private sector does its bit too by profiling users. For example, small files called cookies that Web browsers store on users’ computers allow companies to track users’ activities in cyberspace and may also allow credit card numbers, social security numbers, and other confidential information to leak all over the Internet (Berghel, 2001). Companies that provide Web-based services may maintain large amounts of personal information about their users that allows them to study user activities directly. For example, Google can read your email and show you advertisements based on your interests if you use its email service, Gmail. A new twist with mobile devices is location privacy (Beresford and Stajano, 2003). As part of the process of providing service to your mobile device the network operators learn where you are at different times of day. This allows them to track your movements. They may know which nightclub you frequent and which medical center you visit. Computer networks also offer the potential to increase privacy by sending anonymous messages. In some situations, this capability may be desirable. Beyond preventing companies from learning your habits, it provides, for example, a way for students, soldiers, employees, and citizens to blow the whistle on illegal behavior on the part of professors, officers, superiors, and politicians without fear of reprisals. On the other hand, in the United States and most other democracies, the law specifically permits an accused person the right to confront and challenge his accuser in court so anonymous accusations cannot be used as evidence. The Internet makes it possible to find information quickly, but a great deal of it is ill considered, misleading, or downright wrong. That medical advice you 16 INTRODUCTION CHAP. 1 plucked from the Internet about the pain in your chest may have come from a Nobel Prize winner or from a high-school dropout. Other information is frequently unwanted. Electronic junk mail (spam) has become a part of life because spammers have collected millions of email addresses and would-be marketers can cheaply send computer-generated messages to them. The resulting flood of spam rivals the flow messages from real people. Fortunately, filtering software is able to read and discard the spam generated by other computers, with lesser or greater degrees of success. Still other content is intended for criminal behavior. Web pages and email messages containing active content (basically, programs or macros that execute on the receiver’s machine) can contain viruses that take over your computer. They might be used to steal your bank account passwords, or to have your computer send spam as part of a botnet or pool of compromised machines. Phishing messages masquerade as originating from a trustworthy party, for example, your bank, to try to trick you into revealing sensitive information, for example, credit card numbers. Identity theft is becoming a serious problem as thieves collect enough information about a victim to obtain credit cards and other documents in the victim’s name. It can be difficult to prevent computers from impersonating people on the Internet. This problem has led to the development of CAPTCHAs, in which a computer asks a person to solve a short recognition task, for example, typing in the letters shown in a distorted image, to show that they are human (von Ahn, 2001). This process is a variation on the famous Turing test in which a person asks questions over a network to judge whether the entity responding is human. A lot of these problems could be solved if the computer industry took computer security seriously. If all messages were encrypted and authenticated, it would be harder to commit mischief. Such technology is well established and we will study it in detail in Chap. 8. The problem is that hardware and software vendors know that putting in security features costs money and their customers are not demanding such features. In addition, a substantial number of the problems are caused by buggy software, which occurs because vendors keep adding more and more features to their programs, which inevitably means more code and thus more bugs. A tax on new features might help, but that might be a tough sell in some quarters. A refund for defective software might be nice, except it would bankrupt the entire software industry in the first year. Computer networks raise new legal problems when they interact with old laws. Electronic gambling provides an example. Computers have been simulating things for decades, so why not simulate slot machines, roulette wheels, blackjack dealers, and more gambling equipment? Well, because it is illegal in a lot of places. The trouble is, gambling is legal in a lot of other places (England, for example) and casino owners there have grasped the potential for Internet gambling. What happens if the gambler, the casino, and the server are all in different countries, with conflicting laws? Good question. SEC. 1.2 NETWORK HARDWARE 17 1.2 NETWORK HARDWARE It is now time to turn our attention from the applications and social aspects of networking (the dessert) to the technical issues involved in network design (the spinach). There is no generally accepted taxonomy into which all computer networks fit, but two dimensions stand out as important: transmission technology and scale. We will now examine each of these in turn. Broadly speaking, there are two types of transmission technology that are in widespread use: broadcast links and point-to-point links. Point-to-point links connect individual pairs of machines. To go from the source to the destination on a network made up of point-to-point links, short messages, called packets in certain contexts, may have to first visit one or more intermediate machines. Often multiple routes, of different lengths, are possible, so finding good ones is important in point-to-point networks. Point-to-point transmission with exactly one sender and exactly one receiver is sometimes called unicasting. In contrast, on a broadcast network, the communication channel is shared by all the machines on the network; packets sent by any machine are received by all the others. An address field within each packet specifies the intended recipient. Upon receiving a packet, a machine checks the address field. If the packet is intended for the receiving machine, that machine processes the packet; if the packet is intended for some other machine, it is just ignored. A wireless network is a common example of a broadcast link, with communication shared over a coverage region that depends on the wireless channel and the transmitting machine. As an analogy, consider someone standing in a meeting room and shouting ‘‘Watson, come here. I want you.’’ Although the packet may actually be received (heard) by many people, only Watson will respond; the others just ignore it. Broadcast systems usually also allow the possibility of addressing a packet to all destinations by using a special code in the address field. When a packet with this code is transmitted, it is received and processed by every machine on the network. This mode of operation is called broadcasting. Some broadcast systems also support transmission to a subset of the machines, which known as multicasting. An alternative criterion for classifying networks is by scale. Distance is important as a classification metric because different technologies are used at different scales. In Fig. 1-6 we classify multiple processor systems by their rough physical size. At the top are the personal area networks, networks that are meant for one person. Beyond these come longer-range networks. These can be divided into local, metropolitan, and wide area networks, each with increasing scale. Finally, the connection of two or more networks is called an internetwork. The worldwide Internet is certainly the best-known (but not the only) example of an internetwork. 18 INTRODUCTION CHAP. 1 Soon we will have even larger internetworks with the Interplanetary Internet that connects networks across space (Burleigh et al., 2003). Interprocessor distance 1m 10 m 100 m 1 km 10 km 100 km 1000 km 10,000 km Processors located in same Square meter Example Personal area network Room Building Local area network Campus City Metropolitan area network Country Wide area network Continent Planet The Internet Figure 1-6. Classification of interconnected processors by scale. In this book we will be concerned with networks at all these scales. In the following sections, we give a brief introduction to network hardware by scale. 1.2.1 Personal Area Networks PANs (Personal Area Networks) let devices communicate over the range of a person. A common example is a wireless network that connects a computer with its peripherals. Almost every computer has an attached monitor, keyboard, mouse, and printer. Without using wireless, this connection must be done with cables. So many new users have a hard time finding the right cables and plugging them into the right little holes (even though they are usually color coded) that most computer vendors offer the option of sending a technician to the user’s home to do it. To help these users, some companies got together to design a short-range wireless network called Bluetooth to connect these components without wires. The idea is that if your devices have Bluetooth, then you need no cables. You just put them down, turn them on, and they work together. For many people, this ease of operation is a big plus. In the simplest form, Bluetooth networks use the master-slave paradigm of Fig. 1-7. The system unit (the PC) is normally the master, talking to the mouse, keyboard, etc., as slaves. The master tells the slaves what addresses to use, when they can broadcast, how long they can transmit, what frequencies they can use, and so on. Bluetooth can be used in other settings, too. It is often used to connect a headset to a mobile phone without cords and it can allow your digital music player SEC. 1.2 NETWORK HARDWARE 19 Figure 1-7. Bluetooth PAN configuration. to connect to your car merely being brought within range. A completely different kind of PAN is formed when an embedded medical device such as a pacemaker, insulin pump, or hearing aid talks to a user-operated remote control. We will discuss Bluetooth in more detail in Chap. 4. PANs can also be built with other technologies that communicate over short ranges, such as RFID on smartcards and library books. We will study RFID in Chap. 4. 1.2.2 Local Area Networks The next step up is the LAN (Local Area Network). A LAN is a privately owned network that operates within and nearby a single building like a home, office or factory. LANs are widely used to connect personal computers and consumer electronics to let them share resources (e.g., printers) and exchange information. When LANs are used by companies, they are called enterprise networks. Wireless LANs are very popular these days, especially in homes, older office buildings, cafeterias, and other places where it is too much trouble to install cables. In these systems, every computer has a radio modem and an antenna that it uses to communicate with other computers. In most cases, each computer talks to a device in the ceiling as shown in Fig. 1-8(a). This device, called an AP (Access Point), wireless router, or base station, relays packets between the wireless computers and also between them and the Internet. Being the AP is like being the popular kid as school because everyone wants to talk to you. However, if other computers are close enough, they can communicate directly with one another in a peer-to-peer configuration. There is a standard for wireless LANs called IEEE 802.11, popularly known as WiFi, which has become very widespread. It runs at speeds anywhere from 11 20 INTRODUCTION Access point To wired network Ports CHAP. 1 Ethernet switch To rest of network Figure 1-8. Wireless and wired LANs. (a) 802.11. (b) Switched Ethernet. to hundreds of Mbps. (In this book we will adhere to tradition and measure line speeds in megabits/sec, where 1 Mbps is 1,000,000 bits/sec, and gigabits/sec, where 1 Gbps is 1,000,000,000 bits/sec.) We will discuss 802.11 in Chap. 4. Wired LANs use a range of different transmission technologies. Most of them use copper wires, but some use optical fiber. LANs are restricted in size, which means that the worst-case transmission time is bounded and known in advance. Knowing these bounds helps with the task of designing network protocols. Typically, wired LANs run at speeds of 100 Mbps to 1 Gbps, have low delay (microseconds or nanoseconds), and make very few errors. Newer LANs can operate at up to 10 Gbps. Compared to wireless networks, wired LANs exceed them in all dimensions of performance. It is just easier to send signals over a wire or through a fiber than through the air. The topology of many wired LANs is built from point-to-point links. IEEE 802.3, popularly called Ethernet, is, by far, the most common type of wired LAN. Fig. 1-8(b) shows a sample topology of switched Ethernet. Each computer speaks the Ethernet protocol and connects to a box called a switch with a point-to-point link. Hence the name. A switch has multiple ports, each of which can connect to one computer. The job of the switch is to relay packets between computers that are attached to it, using the address in each packet to determine which computer to send it to. To build larger LANs, switches can be plugged into each other using their ports. What happens if you plug them together in a loop? Will the network still work? Luckily, the designers thought of this case. It is the job of the protocol to sort out what paths packets should travel to safely reach the intended computer. We will see how this works in Chap. 4. It is also possible to divide one large physical LAN into two smaller logical LANs. You might wonder why this would be useful. Sometimes, the layout of the network equipment does not match the organization’s structure. For example, the SEC. 1.2 NETWORK HARDWARE 21 engineering and finance departments of a company might have computers on the same physical LAN because they are in the same wing of the building but it might be easier to manage the system if engineering and finance logically each had its own network Virtual LAN or VLAN. In this design each port is tagged with a ‘‘color,’’ say green for engineering and red for finance. The switch then forwards packets so that computers attached to the green ports are separated from the computers attached to the red ports. Broadcast packets sent on a red port, for example, will not be received on a green port, just as though there were two different LANs. We will cover VLANs at the end of Chap. 4. There are other wired LAN topologies too. In fact, switched Ethernet is a modern version of the original Ethernet design that broadcast all the packets over a single linear cable. At most one machine could successfully transmit at a time, and a distributed arbitration mechanism was used to resolve conflicts. It used a simple algorithm: computers could transmit whenever the cable was idle. If two or more packets collided, each computer just waited a random time and tried later. We will call that version classic Ethernet for clarity, and as you suspected, you will learn about it in Chap. 4. Both wireless and wired broadcast networks can be divided into static and dynamic designs, depending on how the channel is allocated. A typical static allocation would be to divide time into discrete intervals and use a round-robin algorithm, allowing each machine to broadcast only when its time slot comes up. Static allocation wastes channel capacity when a machine has nothing to say during its allocated slot, so most systems attempt to allocate the channel dynamically (i.e., on demand). Dynamic allocation methods for a common channel are either centralized or decentralized. In the centralized channel allocation method, there is a single entity, for example, the base station in cellular networks, which determines who goes next. It might do this by accepting multiple packets and prioritizing them according to some internal algorithm. In the decentralized channel allocation method, there is no central entity; each machine must decide for itself whether to transmit. You might think that this approach would lead to chaos, but it does not. Later we will study many algorithms designed to bring order out of the potential chaos. It is worth spending a little more time discussing LANs in the home. In the future, it is likely that every appliance in the home will be capable of communicating with every other appliance, and all of them will be accessible over the Internet. This development is likely to be one of those visionary concepts that nobody asked for (like TV remote controls or mobile phones), but once they arrived nobody can imagine how they lived without them. Many devices are already capable of being networked. These include computers, entertainment devices such as TVs and DVDs, phones and other consumer electronics such as cameras, appliances like clock radios, and infrastructure like utility meters and thermostats. This trend will only continue. For instance, the average home probably has a dozen clocks (e.g., in appliances), all of which could 22 INTRODUCTION CHAP. 1 adjust to daylight savings time automatically if the clocks were on the Internet. Remote monitoring of the home is a likely winner, as many grown children would be willing to spend some money to help their aging parents live safely in their own homes. While we could think of the home network as just another LAN, it is more likely to have different properties than other networks. First, the networked devices have to be very easy to install. Wireless routers are the most returned consumer electronic item. People buy one because they want a wireless network at home, find that it does not work ‘‘out of the box,’’ and then return it rather than listen to elevator music while on hold on the technical helpline. Second, the network and devices have to be foolproof in operation. Air conditioners used to have one knob with four settings: OFF, LOW, MEDIUM, and HIGH. Now they have 30-page manuals. Once they are networked, expect the chapter on security alone to be 30 pages. This is a problem because only computer users are accustomed to putting up with products that do not work; the car-, television-, and refrigerator-buying public is far less tolerant. They expect products to work 100% without the need to hire a geek. Third, low price is essential for success. People will not pay a $50 premium for an Internet thermostat because few people regard monitoring their home temperature from work that important. For $5 extra, though, it might sell. Fourth, it must be possible to start out with one or two devices and expand the reach of the network gradually. This means no format wars. Telling consumers to buy peripherals with IEEE 1394 (FireWire) interfaces and a few years later retracting that and saying USB 2.0 is the interface-of-the-month and then switching that to 802.11g—oops, no, make that 802.11n—I mean 802.16 (different wireless networks)—is going to make consumers very skittish. The network interface will have to remain stable for decades, like the television broadcasting standards. Fifth, security and reliability will be very important. Losing a few files to an email virus is one thing; having a burglar disarm your security system from his mobile computer and then plunder your house is something quite different. An interesting question is whether home networks will be wired or wireless. Convenience and cost favors wireless networking because there are no wires to fit, or worse, retrofit. Security favors wired networking because the radio waves that wireless networks use are quite good at going through walls. Not everyone is overjoyed at the thought of having the neighbors piggybacking on their Internet connection and reading their email. In Chap. 8 we will study how encryption can be used to provide security, but it is easier said than done with inexperienced users. A third option that may be appealing is to reuse the networks that are already in the home. The obvious candidate is the electric wires that are installed throughout the house. Power-line networks let devices that plug into outlets broadcast information throughout the house. You have to plug in the TV anyway, and this way it can get Internet connectivity at the same time. The difficulty is SEC. 1.2 NETWORK HARDWARE 23 how to carry both power and data signals at the same time. Part of the answer is that they use different frequency bands. In short, home LANs offer many opportunities and challenges. Most of the latter relate to the need for the networks to be easy to manage, dependable, and secure, especially in the hands of nontechnical users, as well as low cost. 1.2.3 Metropolitan Area Networks A MAN (Metropolitan Area Network) covers a city. The best-known examples of MANs are the cable television networks available in many cities. These systems grew from earlier community antenna systems used in areas with poor over-the-air television reception. In those early systems, a large antenna was placed on top of a nearby hill and a signal was then piped to the subscribers’ houses. At first, these were locally designed, ad hoc systems. Then companies began jumping into the business, getting contracts from local governments to wire up entire cities. The next step was television programming and even entire channels designed for cable only. Often these channels were highly specialized, such as all news, all sports, all cooking, all gardening, and so on. But from their inception until the late 1990s, they were intended for television reception only. When the Internet began attracting a mass audience, the cable TV network operators began to realize that with some changes to the system, they could provide two-way Internet service in unused parts of the spectrum. At that point, the cable TV system began to morph from simply a way to distribute television to a metropolitan area network. To a first approximation, a MAN might look something like the system shown in Fig. 1-9. In this figure we see both television signals and Internet being fed into the centralized cable headend for subsequent distribution to people’s homes. We will come back to this subject in detail in Chap. 2. Cable television is not the only MAN, though. Recent developments in highspeed wireless Internet access have resulted in another MAN, which has been standardized as IEEE 802.16 and is popularly known as WiMAX. We will look at it in Chap. 4. 1.2.4 Wide Area Networks A WAN (Wide Area Network) spans a large geographical area, often a country or continent. We will begin our discussion with wired WANs, using the example of a company with branch offices in different cities. The WAN in Fig. 1-10 is a network that connects offices in Perth, Melbourne, and Brisbane. Each of these offices contains computers intended for running user (i.e., application) programs. We will follow traditional usage and call these machines hosts. The rest of the network that connects these hosts is then called the 24 INTRODUCTION CHAP. 1 Junction box Antenna Head end Internet Figure 1-9. A metropolitan area network based on cable TV. communication subnet, or just subnet for short. The job of the subnet is to carry messages from host to host, just as the telephone system carries words (really just sounds) from speaker to listener. In most WANs, the subnet consists of two distinct components: transmission lines and switching elements. Transmission lines move bits between machines. They can be made of copper wire, optical fiber, or even radio links. Most companies do not have transmission lines lying about, so instead they lease the lines from a telecommunications company. Switching elements, or just switches, are specialized computers that connect two or more transmission lines. When data arrive on an incoming line, the switching element must choose an outgoing line on which to forward them. These switching computers have been called by various names in the past; the name router is now most commonly used. Unfortunately, some people pronounce it ‘‘rooter’’ while others have it rhyme with ‘‘doubter.’’ Determining the correct pronunciation will be left as an exercise for the reader. (Note: the perceived correct answer may depend on where you live.) A short comment about the term ‘‘subnet’’ is in order here. Originally, its only meaning was the collection of routers and communication lines that moved packets from the source host to the destination host. Readers should be aware that it has acquired a second, more recent meaning in conjunction with network addressing. We will discuss that meaning in Chap. 5 and stick with the original meaning (a collection of lines and routers) until then. The WAN as we have described it looks similar to a large wired LAN, but there are some important differences that go beyond long wires. Usually in a WAN, the hosts and subnet are owned and operated by different people. In our SEC. 1.2 25 NETWORK HARDWARE Subnet Transmission line Brisbane Router Perth Melbourne Figure 1-10. WAN that connects three branch offices in Australia. example, the employees might be responsible for their own computers, while the company’s IT department is in charge of the rest of the network. We will see clearer boundaries in the coming examples, in which the network provider or telephone company operates the subnet. Separation of the pure communication aspects of the network (the subnet) from the application aspects (the hosts) greatly simplifies the overall network design. A second difference is that the routers will usually connect different kinds of networking technology. The networks inside the offices may be switched Ethernet, for example, while the long-distance transmission lines may be SONET links (which we will cover in Chap. 2). Some device needs to join them. The astute reader will notice that this goes beyond our definition of a network. This means that many WANs will in fact be internetworks, or composite networks that are made up of more than one network. We will have more to say about internetworks in the next section. A final difference is in what is connected to the subnet. This could be individual computers, as was the case for connecting to LANs, or it could be entire LANs. This is how larger networks are built from smaller ones. As far as the subnet is concerned, it does the same job. We are now in a position to look at two other varieties of WANs. First, rather than lease dedicated transmission lines, a company might connect its offices to the Internet This allows connections to be made between the offices as virtual links 26 INTRODUCTION CHAP. 1 that use the underlying capacity of the Internet. This arrangement, shown in Fig. 1-11, is called a VPN (Virtual Private Network). Compared to the dedicated arrangement, a VPN has the usual advantage of virtualization, which is that it provides flexible reuse of a resource (Internet connectivity). Consider how easy it is to add a fourth office to see this. A VPN also has the usual disadvantage of virtualization, which is a lack of control over the underlying resources. With a dedicated line, the capacity is clear. With a VPN your mileage may vary with your Internet service. Internet Link via the internet Brisbane Perth Melbourne Figure 1-11. WAN using a virtual private network. The second variation is that the subnet may be run by a different company. The subnet operator is known as a network service provider and the offices are its customers. This structure is shown in Fig. 1-12. The subnet operator will connect to other customers too, as long as they can pay and it can provide service. Since it would be a disappointing network service if the customers could only send packets to each other, the subnet operator will also connect to other networks that are part of the Internet. Such a subnet operator is called an ISP (Internet Service Provider) and the subnet is an ISP network. Its customers who connect to the ISP receive Internet service. We can use the ISP network to preview some key issues that we will study in later chapters. In most WANs, the network contains many transmission lines, each connecting a pair of routers. If two routers that do not share a transmission line wish to communicate, they must do this indirectly, via other routers. There SEC. 1.2 27 NETWORK HARDWARE ISP network Transmission line Brisbane Customer network Perth Melbourne Figure 1-12. WAN using an ISP network. may be many paths in the network that connect these two routers. How the network makes the decision as to which path to use is called the routing algorithm. Many such algorithms exist. How each router makes the decision as to where to send a packet next is called the forwarding algorithm. Many of them exist too. We will study some of both types in detail in Chap. 5. Other kinds of WANs make heavy use of wireless technologies. In satellite systems, each computer on the ground has an antenna through which it can send data to and receive data from to a satellite in orbit. All computers can hear the output from the satellite, and in some cases they can also hear the upward transmissions of their fellow computers to the satellite as well. Satellite networks are inherently broadcast and are most useful when the broadcast property is important. The cellular telephone network is another example of a WAN that uses wireless technology. This system has already gone through three generations and a fourth one is on the horizon. The first generation was analog and for voice only. The second generation was digital and for voice only. The third generation is digital and is for both voice and data. Each cellular base station covers a distance much larger than a wireless LAN, with a range measured in kilometers rather than tens of meters. The base stations are connected to each other by a backbone network that is usually wired. The data rates of cellular networks are often on the order of 1 Mbps, much smaller than a wireless LAN that can range up to on the order of 100 Mbps. We will have a lot to say about these networks in Chap. 2. 28 INTRODUCTION CHAP. 1 1.2.5 Internetworks Many networks exist in the world, often with different hardware and software. People connected to one network often want to communicate with people attached to a different one. The fulfillment of this desire requires that different, and frequently incompatible, networks be connected. A collection of interconnected networks is called an internetwork or internet. These terms will be used in a generic sense, in contrast to the worldwide Internet (which is one specific internet), which we will always capitalize. The Internet uses ISP networks to connect enterprise networks, home networks, and many other networks. We will look at the Internet in great detail later in this book. Subnets, networks, and internetworks are often confused. The term ‘‘subnet’’ makes the most sense in the context of a wide area network, where it refers to the collection of routers and communication lines owned by the network operator. As an analogy, the telephone system consists of telephone switching offices connected to one another by high-speed lines, and to houses and businesses by low-speed lines. These lines and equipment, owned and managed by the telephone company, form the subnet of the telephone system. The telephones themselves (the hosts in this analogy) are not part of the subnet. A network is formed by the combination of a subnet and its hosts. However, the word ‘‘network’’ is often used in a loose sense as well. A subnet might be described as a network, as in the case of the ‘‘ISP network’’ of Fig. 1-12. An internetwork might also be described as a network, as in the case of the WAN in Fig. 1-10. We will follow similar practice, and if we are distinguishing a network from other arrangements, we will stick with our original definition of a collection of computers interconnected by a single technology. Let us say more about what constitutes an internetwork. We know that an internet is formed when distinct networks are interconnected. In our view, connecting a LAN and a WAN or connecting two LANs is the usual way to form an internetwork, but there is little agreement in the industry over terminology in this area. There are two rules of thumb that are useful. First, if different organizations have paid to construct different parts of the network and each maintains its part, we have an internetwork rather than a single network. Second, if the underlying technology is different in different parts (e.g., broadcast versus point-to-point and wired versus wireless), we probably have an internetwork. To go deeper, we need to talk about how two different networks can be connected. The general name for a machine that makes a connection between two or more networks and provides the necessary translation, both in terms of hardware and software, is a gateway. Gateways are distinguished by the layer at which they operate in the protocol hierarchy. We will have much more to say about layers and protocol hierarchies starting in the next section, but for now imagine that higher layers are more tied to applications, such as the Web, and lower layers are more tied to transmission links, such as Ethernet. SEC. 1.2 NETWORK HARDWARE 29 Since the benefit of forming an internet is to connect computers across networks, we do not want to use too low-level a gateway or we will be unable to make connections between different kinds of networks. We do not want to use too high-level a gateway either, or the connection will only work for particular applications. The level in the middle that is ‘‘just right’’ is often called the network layer, and a router is a gateway that switches packets at the network layer. We can now spot an internet by finding a network that has routers. 1.3 NETWORK SOFTWARE The first computer networks were designed with the hardware as the main concern and the software as an afterthought. This strategy no longer works. Network software is now highly structured. In the following sections we examine the software structuring technique in some detail. The approach described here forms the keystone of the entire book and will occur repeatedly later on. 1.3.1 Protocol Hierarchies To reduce their design complexity, most networks are organized as a stack of layers or levels, each one built upon the one below it. The number of layers, the name o...
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