TCP IP Illustrated

This is often a special purpose hardware box for

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Unformatted text preview: ing about routers is that they provide connections to many different types of physical networks: Ethernet, token ring, point-to-point links, FDDI (Fiber Distributed Data Interface), and so on. These boxes are also called IP routers, but we'll use the term router. Historically these boxes were called gateways, and this term is used throughout much of the TCP/IP literature. Today the term gateway is used for an application gateway: a process that connects two different protocol suites file:///D|/Documents%20and%20Settings/bigini/Docu...homenet2run/tcpip/tcp-ip-illustrated/introduc.htm (3 of 20) [12/09/2001 14.46.31] Chapter 1. Introduction (say, TCP/IP and IBM's SNA) for one particular application (often electronic mail or file transfer). Figure 1.3 shows an internet consisting of two networks: an Ethernet and a token ring, connected with a router. Although we show only two hosts communicating, with the router connecting the two networks, any host on the Ethernet can communicate with any host on the token ring. In Figure 1.3 we can differentiate between an end system (the two hosts on either side) and an intermediate system (the router in the middle). The application layer and the transport layer use end-to-end protocols. In our picture these two layers are needed only on the end systems. The network layer, however, provides a hop-by-hop protocol and is used on the two end systems and every intermediate system. Figure 1.3 Two networks connected with a router. In the TCP/IP protocol suite the network layer, IP, provides an unreliable service. That is, it does its best job of moving a packet from its source to its final destination, but there are no guarantees. TCP, on the other hand, provides a reliable transport layer using the unreliable service of IP To provide this service, TCP performs timeout and retransmission, sends and receives end-to-end acknowledgments, and so on. The transport layer and the network layer have distinct responsibilities. A router, by definition, has two or more network interface layers (since it connects t...
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