Distance Vector Routing 박주호

Introduction.  Modern computer Network generally use Dynamic routing algorithms rather than The static one.  Two dynamic algorithm –Distance vector routing –Link state routing

Distance vector routing  This algorithms operate by –Having each router maintain a table –The table(i.e vector) give  the best known distance to each destination and  which line to use to get there. Updated by exchanging information with the neighbors.

History  Sometimes called by other names …  Distributed Bellman-Ford routing algorithm(1957)  Ford-Fulkerson algorithm(1962)  It was original ARPANET routing algorithm  And was used –In the Internet(RIP) –In early versions of DECnet and Novell ’ s IPX  AppleTalk and Cisco routers use improved distributed vector protocols.

The Routing table  Each router maintains a routing table –Contain one entry. –Two part of one entry  The preferred outgoing line  An estimate of the time or distance  The router is assumed to know the “ distance ” to each of its neighbors. distanceLine ABAB

The Metric of distance  Number of hops –The distance is just one hop  Time delay in milliseconds –The router can measure … –Using special ECHO packets(timestamps)  Total number of packets queued along the path –The router simply examines each queue

Algorithm K K - I H H I I H I A A L K J I H G F E D C B A ToAIHKLine JA delay is 8 JI delay is 10 JH delay is 12 JK delay is 6 Vectors received from J’s four neighbors New routing table for J New estimated dealy from J (a) (b) Receive table from neighbors Distances to each neighbor JA = 8 AB = 12 JB = JA+AB = 20

The Count-to-Infinity Problem  Distance vector routing works in theory –A serious drawback in practice  Reacts rapidly to good news,  But leisurely to bad news. ABCDEABCDE     Initially 1    After 1 exchange 1 2   After 2 exchange  After 3 exchange After 4 exchange Initially After 1 exchange After 2 exchange After 3 exchange After 4 exchange After 5 exchange After 6 exchange  

 In case good news. –If longest path of subnet is N hops, –Everyone will know good news.  In case bad news. –Infinity … –Set infinity to the longest path + 1. –If metric is time delay, no well-defined upper bound. –There is a tradeoff …

The Split Horizon Hack  Many ad hoc solutions to the problem have been proposed … –But, complicate, useless …  One of them will be described(why fail?)  The Split Horizon Hack algorithm ABCDE Initially  After 1 exchange   3 4 After 2 exchange   4 After 3 exchange     After 4 exchange

A B C D Router Fig An example where split horizon fails.