W7 L2 - Distance Vector Routing

IPv6 Motivation

Initial motivation:

• 32 bit IPv4 address space will eventually be allocated

IPv6 vs IPv4 datagram format

IPv6…

• Has no checksum (to speed processing at routers)

• Has no fragmentation / reassembly

• No options

Distance Vector Algorithm

• Every node (router) keeps a table (vector) of the best known didstance to every other node

• Initially, each node only knows distance to its direct neighbours

• Periodically, each node sends its table to its neighbours

• On receiving a neighbours table, a node updates its own table if it finds a shorter path through that neighbour

DVA - Simpler, Cheaper, Slower and Riskier

Dijkstras - Smarter, Faster, Needs more memory and CPU

DVA Problem - Count to Infinity

If a route breaks, nodes can incorrectly think the destination is still reachable, leading to incrementing slowly to infinity

DVA Solution - Poisoned Reverse

To stop the endless infinity loop, Poisoned Reverse allows nodes to advertise a route to a neighbour with infinity cost, if the route would have to go through that neighbour.

Eg: If B can reach A through C, then when B tells C about A, it says the cost is infinity — so C doesn't wrongly think it can reach A via B.