`
`Telecommunications
`
`Essentials
`
`The Complete Global Source
`for Communications Fundamentals,
`Data Networking and the Internet,
`and Next-Generation Networks
`
`Lillian Goleniewski
`
`v‘v Addison-Wesley
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`Boston - San Francisco - Nev'v York - Toronto - Montreal
`London - Munich - Paris - Madrid
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`Capetown - Sydney - Tokyo - Singapore - Mexico City
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`Many of the designations used by manufacturers and sellers to distinguish their products are claimed as trade-
`marks. Where those designations appear in this book, and Addison—W'esley, Inc. was aware of a trademark claim,
`the designations have been printed with initial capital letters or in all capitals.
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`Lido Telecommunications Essentials® is the registered trademark of The Lido Organization, Inc.
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`The author and publisher have taken care in the preparation of this book, but make no expressed or implied ware
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`Library oj'Cortgress Ctttaloging—in—Publication Data
`
`Coleniewski, Lillian.
`Telecommunications essentials : the complete global source for communications
`fundamentals, data networking and the internet, and next-generation networks / Lillian Goleniewski.
`p. cm.
`Includes bibliographical references and index.
`ISBN 0201—7603270
`1. Telecommunication. I. Title.
`
`TKBIOI 6598 2002
`621.382rdc21
`
`Copyright © 2002 by Pearson Education, Inc.
`
`2001053752
`
`All rights reserved. No part of this publication maybe reproduced, stored in a retrieval system, or transmitted, in
`any form, 'or by any means, electronic, mechanical. photocopying, recording, or otherwise, Without the prior con
`sent of the publisher. Printed in the United States of America. Published simultaneously in Canada.
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`ISBN 07201—76032—0
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`Text printed on recycled paper
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`1 2 3 4 5 6 7' 8 9 IO—CRS—0504030201
`First printing, December 2001
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`The Internet: Infrastructure and
`Service Providers .
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`This chapter explores the Intern
`structure of the Internet in terms 0
`organization and characteristics of the growin
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`et, including how the Internet actually works, the
`f the various levels of service providers, and the
`g variety of service providers.
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` w Internet Basics
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`hat speaks to the pace of Internet development.
`Figure 9.1 is an astounding graph t
`k a number of technologies to reach 50 million
`It shows the number of years it too
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`years for the telephone to
`users worldwide. As you can see, whereas it took 74
`
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`reach 50 million users, it took the World Wide Web only 4.
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`What forces are propelling our interest in the Internet? One main force is that
`usage is increasing dramatically; today some 250 million people worldwide have
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`Internet access, and that number is growing by leaps and bounds. The Internet is
`her of people in the developed
`very useful and easy to use, and for a growing num
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`world, it is now the first place to look for information. As one colleague recently
`told me, in the past week, besides the numerous times he had used the Internet to
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`get information for my work, he’d used the Internet to look up hotels for a week—
`end break, to determine what concerts are on in Dublin, to check the specification
`
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`of a car, to transfer funds between bank accounts, to find the address of an old
`
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`friend, and to obtain sheet music. Electronic commerce (e-commerce) is also grow-
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`ing, in both the business-to-consumer and business—to-business sectors. Another
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`Telephone
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`Radio
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`PC
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`Technology
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`Figure 9.1
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`Internet pace: Years to reach 50 million users worldwt e
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`contributor is the major shift toward the use of advanced applications, including
`pervasive computing, which introduces a wide range of intelligent appliances that
`are ready to communicate through the Internet, as well as applications that include
`the more captivating visual and sensory streams. Finally, the availability of broad-
`band, or high-speed access technologies, further drives our interest in and our abil—
`ity to interact with Web sites that involve the use of these advanced applications
`and offer e-commerce capabilities.
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`A Brief History of the Internet
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`Chapter 9 I The Internet: Infrastructure and Service Providers
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`To help understand the factors that contributed to the creation of the Internet, let’s
`look very briefly at the history of the Internet. In 1969 the Advanced Research
`Projects Agency (ARPA) of the US. Department of Defense initiated a project to
`develop a distributed network. There were several reasons for doing this. First, the
`project was launched during the Cold War era, when there was an interest in build-
`ing a network that had no single point of failure, and that could sustain an attack
`yet continue to function. Second, four supercomputer centers were located in four
`universities throughout the United States, and we wanted to connect them together
`so that we could engage in some more intensive processing feats. So, the Internet
`started as a wide area, packet-switching network called the ARPANET.
`Toward the mid-19705, ARPA was renamed the Defense Advanced Research
`Projects Agency (DARPA), and while it was working on the distributed, or packet-
`switched, network, it was also working on local area networks (LANS), paging net—
`works, and satellite networks. DARPA recognized that there was a need for some
`form of internetworking protocol that would allow open communications between
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`l d
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`isparate networks. So, Internet Protocol (IP) was created to support an open-
`architecture network that could link multiple disparate networks via gateways;
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`Internet Basics
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`what we today refer to as routers.
`In 1980, Transmission Control Protocol/Internet Protocol (TCPfIP) began to
`be implemented on an experimental basis, and by 1983, it was required in order for
`a subnetwork to participate in the larger virtual Internet.
`The original Internet model was not based on the telephone network model. It
`involved distributed control rather than centralized control, and it relied on coop-
`eration among its users, which initially were largely academicians and researchers.
`With the original Internet, there‘s no regulation, no monopoly, and no universal
`service mandate (although these issues are being considered seriously now).
`Today, no one agency is in charge of the Internet, although the Internet Society
`(ISOC) is a nonprofit, nongovernmental, international organization that focuses on
`Internet standards, education, and policy issues. ISOC is an organization for Inter-
`net professionals that serves as the organizational home of the Internet Engineer-
`ing Task Force (IETF), which oversees various organizational and coordinating
`tasks. ISOC is composed of a board of trustees, the Internet Architecture Board, the
`IETF, the Internet Research Task Force, the Internet Engineering Steering Group,
`and the Internet Research Steering Group.
`The IETF is an international community of network designers, operators, venr
`dors, and researchers, whose job is to evolve the Internet and smooth its operation
`by creating technical standards through consensus. Other organizations that are
`critical to the functioning of the Internet include American Registry for Internet
`Numbers (ARIN) in the United States, Asia Pacific Network Information Center
`
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`To understand the Internet, it’s important to first understand the concept of a com-
`puter network (see Figure 9.2). A network is formed by interconnecting a set of
`computers, typically referred to as hosts, in such a way that they can interoperate
`with one another. Connecting these hosts involves two major components: hard-
`ware (that is, the physical connections) and software. The software can be run on
`the same or dissimilar host operating systems, and it is based on standards that
`define its operation. These standards, referred to as protocols, provide the formats
`for passing packets of data, specify the details of the packet formats, and describe
`how to handle error conditions. The protocols hide the details of network hard-
`ware and permit computers of different hardware types, connected by different
`physical connections, to communicate, despite their differences. (Protocols are dis—
`cussed in detail later in this chapter.)
`In the strictest sense, the Internet is an internetwork composed of a worldwide
`collection of networks, routers, gateways, servers, and clients, that use a common
`set of telecommunications protocols—the IP family—to link them together (see
`Figure 9.3). The term client is often used to refer to a computer on the network
`
`(APNIC) in Asia-Pacific, and RIPE NCC (Reseaux IP Europeens Network Coordi—
`nation Center) in Europe. These organizations manage and sell IP addresses and
`autonomous system numbers. IANA manages and assigns protocol and port num-
`ber, and ICANN (formed in 1998) is responsible for managing top-level domain
`names and the root name servers. ICANN also delegates control for domain name
`registry below the top—level domains. (Domain names and the role of IP addresses
`are discussed later in this chapter.)
`
`What the Internet Is and How It Works
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`Internet Basics
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`;In.teliigen;t '
`a.
`_-
`" * hub . . vi
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`Imelfigenr .
`#1an
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`M‘-
`'r"
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`Figure 9.3 An internetwork
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`ered by a server. It also refers to a user run-
`that takes advantage of the services off
`lieation. The term server describes either a
`11ng the client side of a client/server app
`ervices to network users or
`computer or a softwarebased process that provides 5
`Web services to Internet users.
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`L245
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`Networks connect servers and clients, allowing the sharing of information and
`computing resources. Network equipment includes cable and wire, network adapt-
`ers, hubs, switches, and various other physical connectors. In order for the net-
`work to be connected to the Internet, the network must send and retrieve data by
`using TCP/IP and related protocols. Networks can also be connected to form their
`own internets: Site-to-site connections are know as intranets, internal networks
`that are generally composed of LANs interconnected by a WAN that uses IP. Con-
`nections between partnering organizations, using IP, are known as extranets.
`The Internet is a complex, highly redundant network of telecommunications
`circuits connected together with internetworking equipment, including routers,
`bridges, and switches. In an environment consisting of several network segments
`with different protocols and architectures, the network needs a device that not only
`knows the address of each segment but can also determine the best path for send-
`ing data and filtering broadcast traffic to the local segment. The Internet moves
`data by relaying traffic in packets from one computer netw0rk to another. If a par-
`ticular network or computer is down or busy, the network is smart enough to
`reroute the traffic automatically. This requires computers (that is, routers) that are
`able to send packets from one network to another. Routers make decisions about
`how to route the data or packets, they decide which pipe is best, and then they use
`that best pipe. Routers work at the network layer, Layer 3, of the 051 model, which
`allows them to switch and route packets across multiple networks. Routers read
`complex network addressing information in the packet; they can share status and
`routing information with one another and use this information to bypass slow or
`malfunctioning connections.
`Routing is the main process that the Internet host uses to deliver packets. The
`Internet uses a hop—byvhop routing model, which means that each host or router
`that handles a packet examines the destination address in the packets IP header,
`computes the next hop that will bring the packet one step closer to its destination,
`and delivers that packet to the next hop, where the process is repeated. To make
`this happen, routing tables must match destination addresses with next hops, and
`routing protocols must determine the content of these tables. Thus, the Internet
`and the public switched telephone network (PSTN) operate quite differently from
`one another. The Internet uses packet switching, where there’s no dedicated con-
`nection and the data is fragmented into packets. Packets can be delivered via differ-
`ent
`routes over
`the Internet and reassembled at
`the ultimate destination.
`Historically, “back-office” functions such as billing and network management have
`not been associated with Internet. But the Internet emphasizes flexibilityithe
`capability to route packets around congested or failed points.
`Recall from Chapter 5, “The PSTN,” that the PSTN uses circuit switching, so a
`dedicated circuit is set up and taken down for each call. This allows charging based
`on minutes and circuits used, which, in turn, allows chain-of—supply dealings. The
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`major emphasis of the PSTN is on reliability. So, the Internet and the PSTN have dif-
`ferent models and different ways of managing or routing traffic through the network,
`but they share the same physical foundation in terms of the transport infrastructure,
`or the types of communication links they use. {Chapter 4, “Establishing Communi-
`cations Channels," discusses packet switching and circuit switching in detail.)
`
`Internet Protocols
`The Internet is a collection of networks that are interconnected logically as a single,
`large, virtual network. Messages between computers are exchanged by using packet
`switching. Networks can communicate with one another because they all use an
`internetworkirig protocol. Protocols are formal descriptions of messages to be
`exchanged and of rules to be followed in order for two or more systems to exchange
`information in a manner that both parties will understand. The following sections
`examine the Internet’s protocols: TCP/1P, User Datagram Protocol (UDP), Internet
`Control Message Protocol (ICMP), Internet Group Management Protocol (IGMP),
`Address Resolution Protocol (ARP)fReverse Address Resolution Protocol (RARP),
`routing protocols, and network access protocols.
`
`or videor—anything digital that the network can transmit. The sequence numbers
`
`TCP/IF
`The IETF has technical responsibility for TCP/1P, which is the most popular and
`widely used of the internetworking protocols. All information to be transmitted
`over the Internet is divided into packets that contain a destination address and a
`sequence number. Packets are relayed through nodes in a computer network,
`along the best route currently available between the source and destination. Even
`though the packets may travel along different routes and may arrive out of
`sequence,
`the receiving computer is able to reassemble the original message.
`Packet size is kept relatively small—sat 1,500 bytes or lessHso that in the event of
`an error, retransmission is efficient. To manage the traffic routing and packet
`assembly/disassembly, the networks rely on intelligence from the computers and
`software that control delivery.
`TCP/IF, referred to as the TCP/IP suite in Internet standards documents, gets its
`name from its two most important protocols, TCP and 1E which are used for
`interoperability among many different types of computers. A major advantage of
`TCP/IP is that it is a nonproprietary network protocol suite that can connect the
`hardware and operating systems of many different computers.
`
`TCP Network applications present data to TCP. TCP divides the data into pack-
`ets and gives each packet a sequence number that is not unique, but which is nonre-
`peating for a very long time. These packets could represent text, graphics, sound,
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`TCP is the protocol for sequenced and reliable data transfer. It breaks the
`data into pieces and numbers each piece so that the receipt can be verified and
`the data can be put back in the proper order. TCP provides Layer 4 (transport
`layer) functionality, and it is responsible for virtual circuit setup, acknowledg-
`ments, flow control, and retransmission of lost or damaged data. TCP provides
`end-towend, connection-oriented, reliable, virtual circuit service. It uses virtual
`
`ports to make connections; ports are used to indicate where information must be
`delivered in order to reach the appropriate program, and this is how firewalls and
`application gateways can filter and direct the packets.
`
`help to ensure that the packets can be reassembled correctly at the receiving end.
`Thus, each packet consists of content, or data, as well as the protocol header; the
`information that the protocol needs to do its work. TCP uses another piece of
`information to ensure that the data reaches the right application when it arrives at
`a system: the port number; which is within the range 1 to 65,535. Port numbers
`identify running applications on servers, applications that are waiting for incoming
`connections from clients. Port numbers identify one listening application from
`another. Ports 1 to 1,023 are reserved for server applications, although servers can
`use higher port numbers as well. Numbers between 1 and 1,023 are reserved for
`“well-known” applications (for example, Web servers run on port 80, FTP runs on
`port 21). Also, many recent protocols have been assigned well—known port num-
`bers above 1,023. Ports with higher numbers, called “ephemeral” ports, are
`dynamically assigned to client applications as needed. A client obtains a random
`ephemeral port when it opens a connection to a well-known server port.
`Data to be transmitted by TCP/IP has a port from which it is coming and a port
`to which it is going, plus an IP source and a destination address. Firewalls can use
`these addresses to control the flow of information. (Firewalls are discussed in
`
`
`
`IP IP handles packet forwarding and transporting of datagrams across a network.
`With packet forwarding, computers can send a packet on to the next appropriate
`network component, based on the address in the packet’s header. 1P defines the
`basic unit of data transfer, the datagram, also referred to as the packet, and it also
`defines the exact format of all data as it travels across the Internet. 1P works like an
`
`envelope in the postal service, directing information to its proper destination. With
`this arrangement, every computer on the Internet has a unique address. (Address—
`ing is discussed later in this chapter.)
`1P provides software routines to route and to store and forward data among
`hosts on the network. IP functions at Layer 3 (the network layer), and it provides
`several services, including host addressing, error notification, fragmentation and
`reassembly, routing, and packet timeout. TCP presents the data to IP in order to
`provide basic host—to—host communication. 1P then attaches to the packet, in a pro-
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`tocol header, the address from which the data comes and the address of the system
`to which it is going.
`Under the standards, 1P allows a packet size of up to 64,000 bytes, but we don‘t
`transmit packets that large because they would cause session timeouts and big con-
`gestion problems. Therefore, IP packets are segmented into 1,500-byte—maximum
`chunks.
`
`IP always does its best to make the delivery to the requested destination host,
`but if it fails for any reason, it just drops the packet. As such, upperalevel protocols
`should not depend on IP to deliver the packet every time. Because IP provides con—
`nectionless, unreliable service and because packets can get lost or arrive out of
`sequence, or the messages may take more than 1,500 bytes, TCP provides the
`recovery for these problems.
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`UDP
`
`Like TCP, UDP is a Layer 4 protocol that operates over IP. UDP provides end-to-
`end, connectionless, unreliable datagram service.
`It
`is well suited for query-
`response applications, for multicasting, and for use with Voice over IP (VoIP).
`(VoIP is discussed in Chapter 11.) Because UDP does not request retransmissions,
`it minimizes what would otherwise be unmanageable delay; the result is that some-
`times the quality is not very good. For instance, if you encounter losses or errors
`associated with a voice packet, the delays that would be associated with retransmit—
`ting that packet would render the conversation unintelligible. In VoIP, when you
`lose packets, you do not request retransmissions. Instead, you hope that the user
`can recover from the losses by other means. Unlike TCP, UDP does not provide for
`error correction and sequenced packet delivery; it is up to the application itself to
`incorporate error correction if required.
`
`IGMP enables a router to determine which host groups have members on a given
`
`ICMP
`
`ICMP provides error handling and control functions. It is tightly integrated with IP.
`ICMP messages, delivered in IP packets, are used for out-of—band messages related
`to network operation or misoperation. Because ICMP uses 1?, ICMP packet deliv-
`ery is unreliable. ICMP functions include announcing network errors, announcing
`network congestion, assisting in troubleshooting, and announcing timeouts.
`
`lGMP
`
`Another Layer 3 protocol is Internet Group Management Protocol (IGMP), whose
`primary purpose is to allow Internet hosts to participate in multicasting. The IGMP
`standard describes the basics of multicasting 1P traffic, including the format of
`multicast IP addresses, multicast Ethernet encapsulation, and the concept of a host
`group (that is, a set of hosts interested in traffic for a particular multicast address).
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`network segment, but IGMP does not address the exchange of multicast packets
`between routers.
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`ARP and RARP
`
`At Layer 3 you also find ARP/RARP. ARP determines the physical address of a node,
`given that node’s IP address. ARP is the mapping link between IP addresses and the
`underlying physical (MAC) address. RARP enables a host to discover its own IP
`address by broadcasting its physical address. When the broadcast occurs, another
`node on the LAN answers back with the IP address of the requesting node.
`
`Routing Protocols
`
`
`
`Routing protocols are protocols that allow routers to communicate with each other.
`They include Routing Information Protocol
`(RIP), Interior Gateway Protocol
`(IGP), Open Shortest Path First (OSPF), Exterior Gateway Protocol (EGP), and
`Border Gateway Protocol (BGP).
`There are several processes involved in router operation. First, the router cre~
`ates a routing table to gather information from other routers about the optimum
`paths. As discussed in Chapter 8, “Local Area Networking,” the routing tables can
`be static or dynamic; dynamic routing tables are best because they adapt to chang—
`ing network conditions. Next, when data is sent from a network host to a router en
`route to its destination, the router breaks open the data packet and looks at the
`destination address to determine the most efficient path between two endpoints. To
`identify the most efficient path, the router uses algorithms to evaluate a number of
`factors (called metrics), including distance and cost. Routing protocols consider all
`the various metrics involved when computing the best path.
`
`Distance-Vector Versus Link—State Protocols
`cols are involved in making routing decisions:
`
`Two main types of routing proto-
`
`a Distance-vector routing protocols—These routing protocols require that
`each router simply inform its neighbors of its routing table. For each net-
`work path, the receiving router picks the neighbor advertising the lowest
`cost, and then the router adds this into its routing table for readvertisement.
`Common distance-vector routing protocols are RIP, Internetwork Packet
`Exchange (IPX) RIP, AppleTalk Routing Table Management Protocol
`(RTMP), and Cisco’s Interior Gateway Routing Protocol (IGRP).
`
`Link-state routing protocols—Link—state routing protocols require that
`each router maintain at least a partial map of the network. When a network
`link changes stateeup to down or vice versa—a notification is flooded
`throughout the network. All the routers note the change and recompute the
`routes accordingly This method is more reliable, easier to debug, and less
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`bandwidth-intensive than distance—vector routing, but it is also more com-
`plex and more computer~ and memory-intensive. OSPE Intermediate Sys-
`tem to Intermediate System (IS-IS), and Network Link Services Protocol
`(NLSP) are link-state routing protocols.
`
`Interior routing occurs within an autonomous sys-
`Interior and Exterior Routing
`tem, which is a collection of routers under a single administrative authority that
`uses a common interior gateway protocol for routing packets. Most of the common
`routing protocols, such as RIP and OSPF, are interior routing protocols. The auton~
`omous system number is a unique number that identifies an autonomous system in
`the Internet. Autonomous system numbers are managed and assigned by ARIN
`(North America), APNIC (Asia-Pacific), and RIPE NCC (Europe). Exterior routing
`protocols, such as BGP, use autonomous system numbers to uniquely define an
`autonomous system. The basic routabl‘e element is the IP network or subnetwork,
`or the Classless Interdomain Routing (CIDR) prefix for newer protocols. (CIDR is
`discussed a little later in the chapter.)
`OSPF, which is sanctioned by the IETF and supported by TCP, is intended to
`become the Internets preferred interior routing protocol. OSPF is a link-state proto-
`col with a complex set of options and features. Link~state algorithms control the
`routing process and enable routers to respond quickly to changes in the network.
`Link-state routing makes use of the Dijkstra algorithm (which determines routes
`based on path length and is used in OSPF) to determine routes based on the number
`of hops, the line speed, the amount of traffic, and the cost. Link—state algorithms are
`more efficient and create less network traffic than do distance-vector algorithms,
`which can be crucial in environments that involve multiple WAN links.
`Exterior routing occurs between autonomous systems and is of concern to ser-
`vice providers and other large or complex networks. Whereas there may be many
`different interior routing schemes, a single exterior routing scheme‘manages the
`global Internet, and it is based on the exterior routing protocol BGP version 4
`(BGP-4). The basic routable element is the autonomous system. Routers determine
`the path for a data packet by calculating the number of hops between internetwork
`segments. Routers build routing tables and use these tables along with routing
`algorithms.
`
`
`
`Network Access Protocols
`Network access protocols operate at Layer 2. They provide the underlying basis for
`the transport of the IP datagrams. The original network access protocol was Ether-
`net, but 1P can be transported transparently over any underlying network, includ-
`ing Token Ring, FDDI, Fibre Channel, Wireless, X25, ISDN, Frame Relay, or ATM.
`Both Serial Line Internet Protocol (SLIP) and Point-to—Point Protocol (PPP) were
`designed specifically for IP over point—to-point connections. PPP provides data—link
`
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`
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`Chapter 9 a The Internet: Infrastructure and Service Providers
`
`layer functionality for IP over dialup/dedicated links. In other words, whenever you
`dial in to your 151?, you negotiate a PPP session, and part of what PPP does is to pro-
`vide a mechanism to identify and authenticate the user that is dialing up.
`
`Internet Addressing
`
`
`
`To make the Internet an open communications system, a globally accepted method
`of identifying computers was needed, and IP acts as the formal addressing mecha—
`nism for all 1nternet messaging.
`Each host on the Internet is assigned a unique 32-bit Internet address, called the
`IP address, which is placed in the 1P header and which is used to route packets to their
`destinations. IP addresses are assigned on a per—interface basis, so a host can have sev-
`eral IP addresses if it has several interfaces (note that a single interface can have multi~
`ple addresses, too). Therefore, an IP address refers to an interface, not to the host. A
`basic concept of IP addressing is that some of the bits of the IP address can be used for
`generalized routing decisions because these bits indicate which network (and possibly
`which subnet) the interface is a member of. Addressing is performed on the basis of
`network/subnet and host; routing is performed based on the network/subnet portion
`of the address only. When a packet reaches its target network, the host portion of the
`address is then examined for final delivery.
`The current generation of IP is called 1? version 4 (IPv4). IP addresses have
`two parts: The first is the network 1D and the second is the host lD. Under va4,
`there are five classes, which differ in how many networks and hosts are supported
`
`(see Figure 9.4):
`
`a Class A—With Class A, there can be a total of 126 networks, and on each
`of those networks there can be 16,777,214 hosts. Class A address space is
`largely exhausted, although there is some address space reserved by IANA.
`
`Class B—Class B addresses provide for 16,384 networks and each of which
`can have 65,534: hosts. Class B space is also largely exhausted, with a few
`still available, albeit at a very high cost.
`
`Class C—Class C allows 2,097,152 networks, each of which can have
`254 hosts.
`
`Class D—Class D belongs to a special aspect of the Internet called the mul-
`ticast backbone (MBONE). Singlecast, or unicast, means going from one
`transmitter to one receiver. Multicast implies moving from one transmitter
`to multiple receivers. Say, for instance, that you are in San Francisco and
`you want to do a videoconferencing session that involves three offices
`located in London. In the unicast mode, you need three separate 1P connec—
`tions to London from the conferencing point in San Francisco. With multi—
`
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`lnternet Basics
`
`Class A
`1—126
`
`Class B
`128.0-1 91 .255
`
`Class C
`192.0-223.255.255
`
`Class D
`multicast
`224—239
`
`Class E
`reserved
`24024?
`
`is, 77721 4
`
`2,0911 52 I
`
`
`
`
`
`Figure 9.4 IPv4 32~bit addressing
`
`cast, however, you need only one IP connection. A multicast router
`(mrouter) would enfold your IP packets in special multicast packets and
`forward those packets on to an rnrouter in London; in London that mrouter
`would remove the IP packets, replicate those packets, and then distribute
`them to the three locations in London. The MBONE system therefore conr
`serves bandwidth over a distance, relieves congestion on transit links, and
`makes it possible to address a large population in a single multicast.
`a