The Network Press - Encyclopedia of Networking - Second Edition

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As in the ﬁrst edition, I’ve tried to make this Encyclopedia a comprehensive source of informa- tion about matters relating to networking. I’ve also tried to present the information in a clear and useful manner. This book contains comprehensive, straightforward summaries of the major concepts, issues, and approaches related to networking. Networking is deﬁned broadly to encompass conﬁgurations ranging from a couple of connected computers just a few feet apart to a network of several thousand machines (of all types and sizes) scattered around the world. You’ll ﬁnd discussions of networking as it’s done by servers and clients, managers and agents, peers,...

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T HE N ETWORK P RESS ENCYCLOPEDIA OF NETWORKING S ECOND E DITION W ERNER F EIBEL N OW I MPROVED — THE M OST C OMPREHENSIVE C OMPENDIUM OF N ETWORKING C ONCEPTS , I SSUES , AND T ERMS C OVERS N ET W ARE 4.1, W INDOWS 95, W INDOWS NT S ERVER 3.51, U NIX W ARE 2, AND OS/2 W ARP C ONNECT F ULL T EXT ON CD-ROM FOR Q UICK E LECTRONIC R EFERENCE
The Encyclopedia of Networking
The Encyclopedia of Networking Second Edition The First Edition of this book was published under the title Novell’s® Complete Encyclopedia of Networking Werner Feibel San Francisco s Paris s Düsseldorf s Soest
Acquisitions Editor: Kristine Plachy Developmental Editor: Guy Hart-Davis Editors: Kristen Vanberg-Wolff and Maureen Adams Technical Editor: Mary Madden Book Designer: Seventeenth Street Studios Technical Illustrators: Cuong Le, Heather Lewis, and Alan Smith Desktop Publisher: London Road Design Production Coordinator: Nathan Johanson Indexer: Matthew Spence Cover Designer: Archer Design Cover Photographer: Dewitt Jones SYBEX is a registered trademark of SYBEX Inc. Network Press and the Network Press logo are trademarks of SYBEX Inc. TRADEMARKS: SYBEX has attempted throughout this book to distinguish proprietary trademarks from descriptive terms by following the capitalization style used by the manufacturer. Every effort has been made to supply complete and accurate information. However, SYBEX assumes no responsibility for its use, nor for any infringement of the intellectual property rights of third parties which would result from such use. The ﬁrst edition of this book was published under the title Novell’s ® Complete Encyclopedia of Networking ©1995 SYBEX Inc. Copyright ©1996 SYBEX Inc., 2021 Challenger Drive, Alameda, CA 94501. World rights reserved. No part of this publication may be stored in a retrieval system, transmitted, or reproduced in any way, including but not limited to photocopy, photograph, magnetic or other record, without the prior agreement and written permission of the publisher. Library of Congress Card Number: 95-72476 ISBN: 0-7821-1829-1 Manufactured in the United States of America 10 9 8 7 6 5 4 3 2 1
w Warranty SYBEX warrants the enclosed CD-ROM to be free of physical defects for a period of ninety (90) days after purchase. If you discover a defect in the CD during this warranty period, you can obtain a replacement CD at no charge by sending the defective CD, postage prepaid, with proof of purchase to: SYBEX Inc. Customer Service Department 2021 Challenger Drive Alameda, CA 94501 (800) 227-2346 Fax: (510) 523-2373 After the 90-day period, you can obtain a replacement CD by sending us the defective CD, proof of purchase, and a check or money order for $10, payable to SYBEX. w Disclaimer SYBEX makes no warranty or representation, either express or implied, with respect to this medium or its contents, quality, performance, merchantability, or ﬁtness for a particular purpose. In no event will SYBEX, its distributors, or dealers be liable for direct, indirect, special, incidental, or consequential damages arising out of the use of or inability to use the software even if advised of the possibility of such damage. The exclusion of implied warranties is not permitted by some states. Therefore, the above exclusion may not apply to you. This warranty provides you with speciﬁc legal rights; there may be other rights that you may have that vary from state to state. w Copy Protection None of the material on the CD is copy-protected. However, in all cases, reselling or making copies of these programs without authorization is expressly forbidden.
[In] a certain Chinese encyclopedia…it is written that animals are divided into: (a) those belonging to the Emperor (b) those that are embalmed (c) tame ones (d) suckling pigs (e) sirens (f) fabulous ones (g) stray dogs (h) those included in the present classiﬁcation (i) those that tremble as if mad (j) innumerable ones (k) those drawn with a very ﬁne camelhair brush (l) others (m) those that have just broken the water pitcher (n) those that look like ﬂies from a long way off Jorge Luis Borges
Acknowledgments As with the ﬁrst edition, this book would never have been completed without the help of many people. These people deserve thanks for all their efforts and energy. Guy Hart-Davis convinced me that it was time for a revision and set me to work. Several people did splen- did work during the production process: Kris Vanberg-Wolff, a veteran of the ﬁrst edition, worked on the revision until her planned departure for the calmer (and tastier) world of cooking school. Maureen Adams, Laura Arendal, and Nathan Johanson took over the pro- duction chores after Kris left. They did an excellent job, especially considering the short notice and even shorter revision schedule. My heartfelt thanks to all these folks. Mary Madden’s technical reviews were always full of gentle, constructive corrections and useful suggestions for improvements. Although I may not have been smart enough to act on all of them, the suggestions have improved the book immensely—for which I’m very grateful. Kris Vanberg-Wolff’s eagle eyes and infallible grammatical sense found and ﬁxed my awkward phrasings, stylistic inconsistencies, and grammatical aberrations. I shudder to think what the book would have looked like without the beneﬁt of these efforts. As always, I’m very grateful to all the people who worked between and behind the scenes to make this book, and also to those who created the compact disc. Thanks also to the many people who sent me information about their products and who took the time to answer my questions. Finally, I dedicate this book to my wife Luanne and my daughter Molly—for all the joy and fun they provide, during both work and play hours.
Table of Contents Introduction ix Entries (Listed Alphabetically) 1 Appendix A: Acronyms and Abbreviations 1113 Appendix B: Bibliography and Other Resources 1235 Index 1251
Introduction Introduction Introduction w What You’ll Find in This Book As in the ﬁrst edition, I’ve tried to make this Encyclopedia a comprehensive source of informa- tion about matters relating to networking. I’ve also tried to present the information in a clear and useful manner. This book contains comprehensive, straightforward summaries of the major concepts, issues, and approaches related to networking. Networking is deﬁned broadly to encompass conﬁgurations ranging from a couple of connected computers just a few feet apart to a network of several thousand machines (of all types and sizes) scattered around the world. You’ll ﬁnd discussions of networking as it’s done by servers and clients, managers and agents, peers, and even over the telephone. You probably won’t ﬁnd anything here that you can’t ﬁnd in other places. However, I don’t know of any other book or source that collects so much network-related information in one place. To ﬁnd all the information summarized here, you would need to check hundreds of books, disks, articles, Web pages, or other documents. Despite its hefty size, this encyclopedia just scratches the surface of what there is to know about networking. After all, how complete can any book be if just the World Wide Web on the Internet has over 10 million hypertext documents. I do think, however, that this book scratches deeper than most other references you’ll ﬁnd. This revised edition updates entries for concepts and technologies that change rapidly or where there have been major developments. I’ve also added considerable material about the Internet (and especially about the World Wide Web), since interest in this networking phenom- enon is growing at an astounding pace. Concepts, Not Instructions As in the ﬁrst edition, I’ve tried to cover concepts rather than making this a how-to book. Thus, you won’t learn how to install networks or run speciﬁc programs. However, you will learn about different types of programs and what they do. For example, you can read about browsers and how they make exploring the World Wide Web possible; you’ll also learn about programs such as network operating systems and how they differ from ordinary operating systems.
x Introduction An Anchor in an Ocean of Words This book was obsolete from the moment it was written. That’s because nothing changes faster than vocabulary in a ﬁeld where there is money to be made. Since major breakthroughs and advances are still happening in the area of networking, there are new network-related words and concepts to be found in almost every issue of every computer magazine. If you include acronyms and abbreviations, the speed with which the vernacular expands is even faster. For example, the ﬁrst edition of this book was published under a year ago and it was no trouble ﬁnding almost 2,000 new entries for Appendix A. Given the futility of even trying to stay completely up-to-date, I’ve chosen to focus on the more enduring concepts and facts—those that provide the foundations and background that underlie the constantly changing terminology. This makes the Encyclopedia more generally useful and enduring. Helping the Book Grow While core networking concepts change very little, the core does grow. For example, ten years ago there was much less need to know about wireless communications because there were fewer wireless products, as well as less public interest in the technology. Because of such progress, the body of essential fundamentals grows with each year. I expect to update and add to the material in the book, and hope to make the Encyclopedia always effective, comprehensive, and useful. Fortunately, an electronic medium makes it easier to grow in this way. If you need to ﬁnd out something about networking, look for it in this book. If you ﬁnd an entry for the topic, we hope you’ll be more informed after you’ve read it. On the other hand, if you can’t ﬁnd the information you need, didn’t understand it, or don’t think you learned what you should have, please drop us a line and tell us. Also, if there are concepts or terms you would like to see included, please let us know. If you can provide references, that would be helpful. Even under the best of circumstances, there’s lit- tle chance that you’ll get a reply to individual queries. However, we will read your comments and suggestions and will try to use them to improve future versions of the book.
Symbols & Numbers &
2 & (Ampersand) Symbols & Numbers w w & (Ampersand) @ (At sign) The ampersand is used to indicate special The at sign is used to separate the username characters in HTML (Hypertext Markup from domain speciﬁers in e-mail addresses. Language) documents—that is, documents For example, mels@golemxiv.mit.edu for the World Wide Web. For example, would indicate someone with username mels & speciﬁes the ampersand character on a computer named golemxiv at MIT. (&); ö speciﬁes a lowercase o with w an umlaut, or dieresis, mark (ö). \ (Backslash) w In some operating systems, such as DOS, < > (Angle Brackets) OS/2, and NetWare, the backslash character Angle brackets are used in pairs to surround separates directory names or directory and markup tags in HTML (Hypertext Markup ﬁle names in a path statement. By itself, the Language) documents for the World Wide backslash represents the root directory in Web. For example, indicates a para- these operating systems. graph break; and indicate the start In various programming and editing con- and end of a section that is to be displayed texts, the backslash is used to escape the in boldface. character that follows. For example, \n is an escape code to indicate a newline character w in many operating environments. * (Asterisk) w In several operating systems, the asterisk // (Double Slash) serves as a wildcard character: to represent one or more characters, such as in a ﬁle In URLs (Uniform Resource Locators), dou- name or extension. For example, a* matches ble slash characters separate the protocol act, actor, and and, but not band. from the site and document names. For In pattern matching involving regular example, if it existed, expressions, the asterisk matches the occur- http://examplehost.ucsc.edu/ rences of the single character immediately ﬁlename.html preceding it. For example, ba*th matches bth, bath, and baaaaath, but not bbath. would refer to a ﬁle named ﬁlename.html In e-mail and in other contexts that use residing on the examplehost machine at the plain text, asterisks are sometimes used University of California at Santa Cruz. To around words or phrases to indicate em- get to this ﬁle, you would use a server that phasis. For example, “I *really* want supports the HTTP (Hypertext Transport to emphasize the second word in this Protocol). sentence.”
4B/5B Encoding 3 w events that have helped deﬁne the computer µ(Mu) culture: Used as an abbreviation for the preﬁx micro, http://www.phil.uni-sb.de/fun/jargon/ as in µsec for microsecond and µm for index.html micrometer. This order of magnitude corre- In this URL, the ﬁle is named index.html, sponds to 2−20, which is roughly 10−6, or and it is located in the /fun/jargon directory one-millionth. on a machine in Germany (de). SEE ALSO In other operating systems, such as DOS, Order of Magnitude OS/2, and NetWare, a slash is sometimes used to indicate or separate command line w switches or options for a command. . and .. (Period and Double Period) w In hierarchically organized directory sys- 1Base5 tems, such as those used by UNIX, DOS, and OS/2, . and .. refer to the current and The IEEE 802.3 committee’s designation the parent directories, respectively. In pat- for an Ethernet network that operates at tern matching involving regular expressions, 1 megabit per second (Mbps) and that the . matches any single character, except a uses unshielded twisted-pair (UTP) cable. newline character. This conﬁguration uses a physical bus, with nodes attached to a common cable. w AT&T’s StarLAN is an example of a 1Base5 ? (Question Mark) network. In many operating systems, a question mark SEE ALSO serves as a wildcard character that repre- 10BaseX; 10Broad36 sents a single character, such as in a ﬁle or directory name. w 4B/5B Encoding w / (Slash) 4B/5B encoding is a data-translation scheme that serves as a preliminary to signal encod- The slash (also known as a forward slash or ing in FDDI (Fiber Distributed Data Inter- a virgule) separates directory levels in some face) networks. In 4B/5B, every group of operating systems (most notably UNIX), in four bits is represented as a ﬁve-bit symbol. addresses for gopher, and in URLs (Uniform This symbol is associated with a bit pattern Resource Locators). For example, the fol- that is then encoded using a standard signal- lowing URL speciﬁes the name and location encoding method, usually NRZI (non-return of a hypertext version of the jargon ﬁle, to zero inverted). which contains deﬁnitions for terms and This preprocessing makes the subsequent electrical encoding 80 percent efﬁcient. For
4 5B/6B Encoding example, using 4B/5B encoding, you can w achieve a 100 megabit per second (Mbps) 10BaseX transmission rate with a clock speed of only The designations 10Base2, 10Base5, 125 megahertz (MHz). 10BaseF, and 10BaseT refer to various In contrast, the Manchester signal- types of baseband Ethernet networks. encoding method, which is used in Ethernet and other types of networks, is only 50 per- 10Base2 cent efﬁcient. For example, to achieve a 100 10Base2 uses thin coaxial cable. This ver- Mbps rate with Manchester encoding, you sion can operate at up to 10 megabits per need a 200 MHz clock speed. second (Mbps) and can support cable seg- w ments of up to 185 meters (607 feet). It is 5B/6B Encoding also known as thin Ethernet, ThinNet, or A data-translation scheme that serves CheaperNet, because thin coaxial cable is as a preliminary to signal encoding in considerably less expensive than the thick 100BaseVG networks. In 5B/6B, every coaxial cable used in 10Base5 networks. group of ﬁve bits is represented as a six-bit 10Base5 symbol. This symbol is associated with a bit pattern that is then encoded using a stan- 10Base5 uses thick coaxial cable. This ver- dard signal-encoding method, such as NRZ sion is the original Ethernet. It can operate (non-return to zero). at up to 10 Mbps and support cable seg- ments of up to 500 meters (1,640 feet). It is w 8B/10B Encoding also known as thick Ethernet or ThickNet. A data-translation scheme related to 4B/5B 10BaseF encoding that recodes eight-bit patterns into 10BaseF is a baseband 802.3-based Ethernet 10-bit symbols. 8B/10B encoding is used, for network that uses ﬁber-optic cable. This example, in IBM’s SNA (Systems Network version can operate at up to 10 Mbps. Architecture) networks. Standards for the following special- w purpose versions of 10BaseF are being 9-Track Tape formulated by the IEEE 802.3: A tape storage format that uses nine parallel 10BaseFP (ﬁber passive): For desktops tracks on 1/2-inch, reel-to-reel magnetic tape. Eight tracks are used for data, and one 10BaseFL (ﬁber link): For intermediate track is used for parity information. These hubs and workgroups tapes are often used as backup systems on 10BaseFB (ﬁber backbone): For central minicomputer and mainframe systems; digi- facility lines between buildings tal audio tapes (DATs) are more common on networks.
66-Type Punch-Down Block 5 10BaseT cables for each direction, so that each cable needs only an 18 MHz bandwidth. 10BaseT is a baseband 802.3-based Ethernet network that uses unshielded twisted-pair B RO A D E R C A T E G O R I E S (UTP) cable and a star topology. This ver- Ethernet; Network, Broadband sion can operate at up to 10 Mbps. It is also SEE ALSO known as twisted-pair Ethernet or UTP 1Base5; 10BaseX Ethernet. w B RO A D E R C A T E G O R Y 56K Line Ethernet A digital telephone circuit with a 64 Kbps SEE ALSO bandwidth, but with a bandwidth of only 1Base5; 10Broad36; 100BaseT 56 Kbps data, with the other 8 Kbps being used for signaling. Also known as an ADN w (Advanced Digital Network) or a DDS 10Broad36 (Dataphone Digital Service) line. 10Broad36 is a broadband, 802.3-based, Ethernet network that uses 75-ohm coaxial w (CATV) cable and a bus or tree topology. 64K Line This version can operate at up to 10 mega- A digital telephone circuit with a 64 Kbps bits per second (Mbps) and support cable bandwidth. Also known as a DS0 (digital segments of up to 1,800 meters (about signal, level 0) line. When the entire 64 Kbps 6,000 feet). are allocated for the data, the circuit is A 10Broad36 network uses differential known as a clear channel. This is in contrast phase shift keying (DPSK) to convert the to a circuit in which 8 Kbps are used for data to analog form for transmission. signaling, leaving only 56 Kbps for data. Because of the encoding details, a w 10Broad36 network actually needs 66-Type Punch-Down Block 18 megahertz (MHz) for each channel: 14 MHz to encode the 10 Mbps signal and A device for terminating wires, with the 4 MHz more for collision detection and possibility of connecting input and output reporting capabilities. wires. This type of punch-down block can In a 10Broad36 network, throughput is handle wires with up to 25 twisted pairs. 10 Mbps in each direction—that is, a total The 66-type have generally been superseded bandwidth of 36 MHz is needed. This band- by 110-type punch-down blocks. width can be provided in a single cable or in SEE ALSO two separate cables. A split-cable approach Punch-Down Block uses half the cable for each direction, which means the cable must have a 36 MHz band- width. A dual-cable approach uses separate
6 100BaseFX w The main differences between fast (100 100BaseFX Mbps) Ethernet and standard (10 Mbps) A 100BaseT basal type variant that runs Ethernet are: over multimode ﬁber-optic cable. Nodes on s A 100BaseT Ethernet allows a much a 100BaseFX network can be up to 2 kilo- shorter gap between signals. meters apart. This variant is also written 100Base-FX. s A 100BaseT Ethernet requires either higher-grade cable or more wire pairs. SEE It can run at 100 Mbps speeds on 100BaseT Category 3 or 4 cable—provided four C O M P A RE pairs are available; Category 5 cable 100BaseT4; 100BaseTX requires only two pairs. s Currently, a 100BaseT Ethernet can w 100BaseT support a network that is only about a tenth of the length allowed for an The general name for any of three 100 Mbps ordinary Ethernet network. For net- Ethernet variants that have just been made a works that use copper (as opposed to standard by an IEEE 802.3 subcommittee ﬁber-optic) cabling: Two nodes of a (802.3u). 100BaseT Ethernet is one of the 100BaseT4 network can be no further candidates trying to become the standard apart than 205 meters—regardless of 100 Mbps Ethernet. This version was devel- whether the nodes are next to each oped and proposed originally by Grand other. Junction, in collaboration with several other corporations. The following variants of 100BaseT The term fast Ethernet is often used for Ethernet have been deﬁned: this version. This is unfortunate, since that 100BaseFX: Runs over multimode ﬁber- term is also used to refer to any Ethernet optic cable. Nodes on a 100BaseFX implementation that supports speeds faster network can be up to two kilometers than the ofﬁcial 10 Mbps standard. To add apart. to the confusing terminology, a software product (no longer available) was also 100BaseTX: Uses two wire pairs, named fastEthernet. but requires Category 5 unshielded 100BaseT Ethernet retains Ethernet’s or shielded twisted pair (UTP or CSMA/CD (Carrier Sense Multiple Access/ STP) wire. Collision Detect) media access method—in 100BaseT4: Can use category 3, 4, or 5 contrast to the 100BaseVG variant (now UTP cable. The T4 in the name comes ofﬁcially, IEEE 802.12)—which is the other from the fact that four wire pairs are major 100 Mbps Ethernet available. needed: two for sending and two for receiving.
100BaseVG 7 In some conﬁgurations, fast and ordinary SEE Ethernet nodes can share the same network. 100BaseT Fast Ethernet devices identify themselves as C O M P A RE such by sending a series of FLPs (fast link 100BaseT4; 100BaseFX pulses) at startup. w P R I M A R Y S O U RC E S 100BaseVG IEEE 802.3u committee publications 100BaseVG is a version of Ethernet devel- B RO A D E R C A T E G O R I E S oped by Hewlett-Packard (HP) and AT&T Ethernet Microelectronics, and is currently under C O M P A RE consideration by an IEEE 802.12 committee. It is an extension of 10BaseT Ethernet that 100BaseVG will support transmissions of up to 100 w megabits per second (Mbps) over voice- 100BaseT4 grade (Category 3) twisted-pair wire. The A 100BaseT Ethernet variant that can use VG in the name stands for voice-grade. category 3, 4, or 5 unshielded twisted pair Differences from 10 Mbps Ethernet (UTP) cable. The T4 means that four wire pairs are needed: two for sending and two 100BaseVG Ethernet differs from ordinary for receiving. Two nodes of a 100BaseT4 (10 Mbps) Ethernet in the following ways: network can be no further apart than 205 s Uses demand priority (rather than meters, regardless of whether the nodes are CSMA/CD) as the media access next to each other. This variant is sometimes method. written 100Base-T4. s Can use ordinary (Category 3) SEE unshielded twisted-pair (UTP) cable, 100BaseT provided that the cable has at least C O M P A RE four wire pairs. Ordinary Ethernet 100BaseTX; 100BaseFX needs only two pairs: one to send and one to receive. w 100BaseTX s Uses quartet signaling to provide four A 100BaseT Ethernet variant that uses two transmission channels (wire pairs) wire pairs, but requires Category 5 UTP or instead of just one. All wire pairs are STP wire. Two nodes of a 100BaseTX net- used in the same direction at a given work can be no further apart than 205 time. meters—regardless of whether the nodes are s Uses the more efﬁcient 5B/6B NRZ next to each other. This variant is sometimes signal encoding, as opposed to the written 100Base-TX.
8 100BaseX Manchester encoding scheme used by 100BaseVG/AnyLAN ordinary Ethernet. 100BaseVG/AnyLAN is an extension s For category 3 cable, a VG network of 100BaseVG, developed as a joint effort can be at most 600 meters from end to between Hewlett-Packard and IBM. This end—and only 200 meters if all hubs version also supports the Token Ring archi- in the network are connected in the tecture, and it can be used with either Ether- same wiring closet. These values net or Token Ring cards (but not both at the increase by 50%—that is, to 900 and same time or in the same network). Because 300 meters, respectively—when cate- the demand priority access method can be gory 5 cable is used. For VG using deterministic, the 100BaseVG/AnyLAN ﬁber-optic cable, the most widely sepa- architecture could handle isochronous rated network nodes can be up to data—that is, data (such as voice or video) 5000 meters, or 5 kilometers, apart. that requires a constant transmission rate. Upgrading to 100BaseVG The 100VG-AnyLAN Forum is the advo- cacy group for this Ethernet variant. This 100BaseVG is designed to provide an easy consortium includes over 20 members, upgrade path from 10 Mbps Ethernet. An including Apple, Compaq, and IBM. upgrade requires two new components: 100Base VG/AnyLAN is also known simply as VG or AnyLAN. s A 100BaseVG network interface card (NIC) for each node being upgraded. B RO A D E R C A T E G O R Y This NIC replaces the 10 Mbps version Ethernet in the node. SEE ALSO s A 100BaseVG hub to replace the 10 HSLAN (High-Speed Local-Area Mbps hub. This type of hub is plug- Network) compatible with a 10 Mbps hub, so C O M P A RE that the upgrade requires simply unplugging a node from one hub and 100BaseT plugging it into the 100BaseVG hub. w This can all take place in the wiring 100BaseX closet. 100BaseX (sometimes written as 100 If you are already using twisted-pair Base-X) is a function that translates bet- Ethernet cabling, you may not need any ween the FDDI (Fiber Distributed Data new wiring, provided that the cable has four Interface)-based physical layer and the wire pairs. CSMA/CD-based data-link layer in a 100