CS519: Computer NetworksLecture 3, Part 2: Feb 16, 2004IP Forwarding TableCS519Best-match rules revisited| Select matching FIB entry with longest prefix| If multiple matching FIB entries have the same prefix size, then any may be usedz Even simultaneously---path splitting for load balancingz But try to maintain source affinity (i.e. send different flows along different paths, but don’t split a given flow)CS519Paths to multi-homed site XISP AInternet (other ISPs)ISP BX20.1.2/2420.1/1620.2/1620.1.2/2420.1.1/2420.1.2/2420.1.1/24Y20.1/1620.1.1/24CS519Paths to Site X after X-B link failureISP AInternet (other ISPs)ISP BX20.1.2/2420.1/1620.2/1620.1.2/2420.1.1/2420.1.2/2420.1.1/24YX20.1/1620.1.1/24CS519Better load balance (without increasing FIB size)ISP AInternet (other ISPs)ISP BX20.1.2/2420.1/1620.1.2/2420.2/1620.1.2/2420.1.1/2420.1.2/2420.1.1/24Y20.1/1620.1.1/24CS519Paths to Site YISP AInternet (other ISPs)ISP BX20.1.2/2420.1/1620.2/1620.1.2/2420.1.1/2420.1.2/2420.1.1/24Y20.1.1/2420.1/16CS519Paths “to” Site Y after Y-B link failureISP AInternet (other ISPs)ISP BX20.1.2/2420.1/1620.2/1620.1.2/2420.1.1/2420.1.2/2420.1.1/24YXX20.1/1620.1.1/24CS519Implementing the forwarding table| First-match style ok for small forwarding tablesz Scales poorly with the number of entries| Hash structures work for flat addresses, but not hierarchical (masked) addressesz “Bridged Ethernets”| High-end routers implement forwarding table in hardwarez CAM-based (Content Addressable Memory)| Otherwise, some kind of tree-like data structure is typically usedz We’ll look at this later in the courseCS519Other types of forwarding| What we looked at so far is hop-by-hop forwarding with hierarchical addresses| Hop-by-hop means that every switch in the path makes an “independent” forwarding decision| But we can also have source routingz The entire path is listed in the packetz IP has a (never used) option for thisCS519Hop-by-hop versus source routing| Source routing is (kindof) what you do when you print out directions from mapquestz I.e., you carry you path with you| Hop-by-hop routing is often (kindof) how you find your way around Wal-Martz “where is kids clothing?”, “where are socks?”CS519Hop-by-hop versus source routing| Hop-by-hop is what is used in the Internetz Though many people have proposed source routing| With the exception of routing through a switch fabric within a routerz But we’ll look at router/switcharchitecture
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