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class5 - CSCI 233 Internet Protocols Class 5 David C....

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1 CSCI 233 Internet Protocols Class 5 David C. Roberts
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2 Agenda Cores, peers and algorithms Exterior gateway protocols Autonomous systems Routing in an autonomous system
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3 Origin of Routing Tables Routers interconnect networks Datagram travels from router to router until it  reaches a router attached to the same network  as the final destination Routing table has network id, address of next  machine for that network id How does the routing table get its information? How is the routing table maintained?
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4 Routers and Hosts Host may have only 2 routes Route for local network Default route to local router Interstate road sign analogy—successive signs  on 270 headed south toward Potomac  Right to Gaithersburg Right to Rockville  Right to Potomac Interstate road signs don’t list all destinations Still give complete directions Signs analogous to routing tables
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5 Road Analogy Possible configurations: Star configuration of roads, single central  intersection—say, at town square. Signs at  square must show routes to all destinations. Roads a richly connected network, signs at  every intersection to all destinations. Two towns, separated by bridge over river.   Neither trusts the other, so signs on West for  destinations on Western side, signs on East for  destinations on Eastern side of river
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6 Cores Original Internet architecture Use central core routers with complete  information, outer routers with partial  information Job of local administrators is simplified Potential for inconsistency, which can  make some outlying destinations  unreachable
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7 Core Routers Early Internet used Arpanet as backbone. Every site had to advertise Internet addresses All nonlocal traffic was passed to the closest core router.
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8 Core Router Problem This approach doesn’t work if core routers have incomplete information. If destination is unknown, default route is to core router. Then core router default path can pass through all other routers if core router doesn’t know the route to it . Potential for large number of router hops for a datagram that takes the default path.
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9 Core Router Problem…Solved Core routers exchanged routing information so that each had complete information about optimal routes to all destinations What’s wrong with this picture: Internet outgrew any possible central backbone Not every site had could have a core router Core router interaction limits ultimate scalability
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Peer Backbones—More Complexity NFSNet backbone, when it became a peer backbone, introduced more complexity. Very complex for core routers to know optimal routes to all
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This note was uploaded on 09/03/2010 for the course CS 233 taught by Professor Davidc.roberts during the Fall '10 term at GWU.

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class5 - CSCI 233 Internet Protocols Class 5 David C....

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