网路层通讯协定.ppt

Routing Protocol 網路層通訊協定 Why is a routing protocol needed? Early requirements to exchanges data between computers over interconnected networks. Routing entities had to make a judgement on which path to route traffic to destination. Routing Routing: 尋找一條適當的路徑，將封包從傳送端傳遞到接收端。 如圖: A, B, C, D, E, F為router，由實體連線互相連結。每條實體連線都會因為傳輸延遲、壅塞的情況賦予一個cost值。 Good Path: 一般是指cost累加最小的路徑，但是也有可能因為實際需求的不同而有不同的定義。 Routing Algorithm classification Global or decentralized information? Global: all routers have complete topology, link cost info “link state” algorithms Decentralized: router knows physically-connected neighbors, link costs to neighbors iterative process of computation, exchange of info with neighbors “distance vector” algorithms Static or dynamic? Static: routes change slowly over time Dynamic: routes change more quickly periodic update in response to link cost changes Routing in the Internet The Global Internet consists of Autonomous Systems (AS) interconnected with each other: Stub AS: small corporation Multihomed AS: large corporation (no transit) Transit AS: provider Two-level routing: Intra-AS: administrator is responsible for choice Inter-AS: unique standard Internet AS Hierarchy Intra-AS and Inter-AS routing Intra-AS Routing Also known as Interior Gateway Protocols (IGP) Most common Intra-AS routing protocols: RIP: Routing Information Protocol OSPF: Open Shortest Path First IGRP: Interior Gateway Routing Protocol (Cisco proprietary) Background to RIP RIP dates back to 1969, the early networking days and ARPNET when Xerox and Berkley’s Unix implemented it broadly similar protocols. RFC 1058(version 1), RFC 1723(version 2) Included in BSD-UNIX Distribution in 1982 RIP uses a single class of routing algorithm known as distance vector - based on a simple hop count algorithm (derived from Bellman’s equation). Although superseded by more complex algorithms, its simplicity means is still found widely in smaller autonomous systems. RIP advertisements Distance metric: