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`COMPUTER NETWORKS
`
`FIFTH EDITION
`
`ANDREWS. TANENBAUM
`Vrije Universiteit
`Amsterdam, The Netherlands
`
`DAVID J. WETHERALL
`University of Washington
`Seattle, WA
`
`PRENTICE HALL
`
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`Ta$~
`and th
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`ToKai
`
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`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.
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`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
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`Library of Congress Cataloging-in-Pnblication Data
`
`Tanenbaum, Andrew S., 1944-
`Computer networks I 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
`
`7 8 9 10 V092 16 15 14
`
`PEARSON
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`-------
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`tj>, I
`
`The
`:Stage
`
`1ence
`ff the
`their
`thou(cid:173)
`com(cid:173)
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`= job.
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`a.tion.
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`t user
`f and
`
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`' : files
`-user.
`
`SEC. 1.1
`
`USES OF COMPUTER NETWORKS
`
`3
`
`Although this book primarily focuses on networks, many of the topics are also im(cid:173)
`p01tant 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 net(cid:173)
`works, few of them would be built. We will start with traditional uses at com(cid:173)
`panies, 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 com(cid:173)
`pany.
`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 net(cid:173)
`works 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
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`4
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`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 net(cid:173)
`work. 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 stor(cid:173)
`ed on powerlul 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.
`
`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 data(cid:173)
`base 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
`
`EC
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`relIIOie coc
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`CHAP. 1
`
`;ks) may
`ded net(cid:173)
`;m away
`iey were
`r.end the
`
`ystem as
`,number
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`!¢rsona1
`
`SEC. 1.1
`
`USES OF COMPUTER NETWORKS
`
`5
`
`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
`
`Request ~
`
`Server machine
`
`a--1-1 __..!.-..-;cr=---==:~ork )::----i-.1 D I
`
`-;-
`
`Client process
`
`Reply ~
`Server process
`
`Figure 1-2. The client-server model involves requests and rei;>lies.
`
`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 gener(cid:173)
`ally 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 com(cid:173)
`puter. Companies find this a wonderful way to save on their telephone bills.
`Other, richer forms of communication are made possible by computer net(cid:173)
`works. 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 black(cid:173)
`board 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.,
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