Transcript Lecture 34
Telecommunications Networking II Lecture 34 Routing Algorithms Ref: Tanenbaum pp345-374; 424-431 Copyright 1999, S.D. Personick. All Rights Reserved. Routing Algorithms • An IP network consists of a number (possibly a large number) of interconnected routers • As discussed previously, routers contain routing tables…which are used to determine which outgoing port each incoming IP datagram should be directed to Copyright 1999, S.D. Personick. All Rights Reserved. Routing Algorithms • The question then arises: how are these routing tables populated and maintained? • Alternatives: -A central entity computes and disseminates a set of routing tables -Each router tries to “discover” what the network topology is, and creates/maintains its own routing tables Copyright 1999, S.D. Personick. All Rights Reserved. Routing Algorithms here there Getting from “here” to “there” Copyright 1999, S.D. Personick. All Rights Reserved. 144.128.X.X Routing Algorithms • Issues -Efficient routing (finding the “best” path) -How fast can the routing tables be updated in the event that something about the network has changed (e.g., a router is added, a link “goes down”, ... Copyright 1999, S.D. Personick. All Rights Reserved. Routing Algorithms • “Least cost” routing -Pick routes that have the least number of hops, or the least delay, or the least expense, or some other measure of least “cost” -Algorithms exist for calculating least cost routes on a global network basis -Iterative methods are used for individual routers to calculate least cost routes based on routing costs advertised by their neighbors Copyright 1999, S.D. Personick. All Rights Reserved. Routing Algorithms • Discovering one’s neighbors -send out an “HELLO” message on each output port…wait for a response -send out “ECHO” packets to measure delays to neighboring routers -query neighboring routers regarding the destinations they can reach, and the corresponding costs of reaching those destinations Copyright 1999, S.D. Personick. All Rights Reserved. Routing Protocols • Interior Gateway Routing Protocol -used within an “autonomous system”, e.g., Drexel’s collection of routers -current IETF standard is OSPF: Open Shortest Path First Copyright 1999, S.D. Personick. All Rights Reserved. OSPF (overview) • In the public domain (“open”) • Supports a variety of “distance” metrics: physical distance, delay, … • Dynamic: adapts to changes in network topology (lost routers, added, routers…) • Supports classes of service • Supports load balancing (splitting loads among multiple paths) Copyright 1999, S.D. Personick. All Rights Reserved. OSPF (overview) • Supports hierarchical routing (no one router needs to know the whole network topology) • Has some security capabilities (e.g., to prevent hackers from changing routing tables) • Some details are provided in Tanenbaum, but a more in-depth reference on routing protocols is needed to really understand this subject Copyright 1999, S.D. Personick. All Rights Reserved. Border Gateway Routing Protocol (BGP) • Allows autonomous systems (AS’s) to: - enforce certain rules…such as not accepting “transit” packets; or carrying transit packets only from certain foreign AS’s to other foreign AS’s - control routing to avoid certain AS’s • BGP routers inform their neighbors about the complete paths they use to get to various destinations Copyright 1999, S.D. Personick. All Rights Reserved.