Subject: Simple question

From: Ali Boudani <[email-masked]>
Date: Wed, 13 Feb 2002 12:49:16 +0100

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?

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From: [email-masked] (Vegard Engen)
Date: Wed, 13 Feb 2002 13:52:14 +0100

> 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.

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From: "Stephen Sprunk" <[email-masked]>
Date: Wed, 13 Feb 2002 10:54:16 -0600

Thus spake "Ali Boudani" <[email-masked]>
> Before sending packets or after?

Your question presupposes that R1 is somehow aware of R4.  If you can answer
how that is, you will know if/when R1 will detect a subsequent failure.

S

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From: Christopher Evans <[email-masked]>
Date: Thu, 14 Feb 2002 10:37:58 -0800

When R1 sends pkts at and they are timeout on reponse?  It'll have to walk
its tables to find an alternative
party to crash.