On Wed, Feb 13, 2002 at 12:49:16PM +0100, Ali Boudani wrote: > If we have the following figure: > > R1---------R2---------R3---------R4 where R1 to R4 are unicast routers. > R1 is sending packets to R4. If R4 goes down, when R1 will detect this?? > Before sending packets or after? >
This is very dependent on the routing protocols in use. With only static routing, R1 will *never* detect this at all. With proper routing protocols, R3 will detect that R4 is down and send updates to the other routers that it talks routing protocols with, that "I can no longer send data to R4 and what's beyond that". How long it takes for it to discover that, depends on what routing protocols are in place and how it's configured. But if R1 is in process of sending packets to R4 when R4 goes down, it will definitely take some time before it's discovered, i.e. packets will get sent that ends up not reaching its destination. It's not the responsibility of the router to discover this, it's the responsibility of the end systems. In case of TCP, it's built into the protocol, for UDP and other non-connection- based protocols, things are a bit harder. -- - Vegard Engen, member of the first RFC1149 implementation team.
