TAILIEUCHUNG - Internetworking with TCP/IP- P31

Internetworking with TCP/IP- P30: TCP/IP has accommodated change well. The basic technology has survived nearly two decades of exponential growth and the associated increases in traffic. The protocols have worked over new high-speed network technologies, and the design has handled applications that could not be imagined in the original design. Of course, the entire protocol suite has not remained static. New protocols have been deployed, and new techniques have been developed to adapt existing protocols to new network technologies | Sec. Core Routers 259 To avoid the inefficiencies default routes cause Internet designers arranged for all core routers to exchange routing information so that each would have complete information about optimal routes to all possible destinations. Because each core router knew routes to all possible destinations it did not need a default route. If the destination address on a datagram was not in a core router s routing table the router would generate an ICMP destination unreachable message and drop the datagram. In essence the core design avoided inefficiency by eliminating default routes. Figure depicts the conceptual basis of a core routing architecture. The figure shows a central core system consisting of one or more core routers and a set of outlying routers at local sites. Outlying routers keep information about local destinations and use a default route that sends datagrams destined for other sites to the core. Figure 143 The routing architecture of a simplistic core system showing default routes. Core routers do not use default routes outlying routers labeled L each have a default route that points to the core. Although the simplistic core architecture illustrated in Figure is easy to understand it became impractical for three reasons. First the Internet outgrew a single centrally managed long-haul backbone. The topology became complex and the protocols needed to maintain consistency among core routers became nontrivial. Second not every site could have a core router connected to the backbone so additional routing structure and protocols were needed. Third because core routers all interacted to ensure consistent routing information the core architecture did not scale to arbitrary size. We will return to this last problem in Chapter 15 after we examine the protocols that the core system used to exchange routing information. 260 Routing Cores Peers And Algorithms Chap. 14 Beyond The Core Architecture To Peer Backbones The introduction of .

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