CITS3002 Computer Networks  
← prev
next → CITS3002 help3002 CITS3002 schedule  

The Count-To-Infinity Problem

There is a simple, though serious, problem with the distance vector routing algorithms that requires them all to be modified in networks where the reliability is uncertain (links and routers may crash). Consider the following [Tan 5/e, Fig 5-11].

infinity
In part (a) we assume that router A is initially down - all other routers record an infinite distance (or time) to router A.

When router A returns, each other router eventually hears of its return, one router at a time - good news travels fast.

However, consider the dual problem in (b). Router A is initially up. All other routers know the distance to A. Now, router A crashes - router B first senses that A is now an infinite distance away. However, B is excited to learn that router C is a distance of 2 to A, and so B chooses to now send traffic to A via C (a distance of 3).

At the next exchange, C notices that its 2 neighbours each have a path to A of length 3. C now records its path to A, via B, with a distance of 4. At the next exchange, B has direct path to A (of infinite distance), or one via C with a distance of 4. B now records its path to A is via C with a distance of 5.....

CITS3002 Computer Networks, Lecture 5, The Network Layer, p14, 25th March 2024.