ICMP & PathMTU

Fri Oct 27 12:05:49 UTC 2006

* Jim Popovitch:

> Two questions for everybody...(any and all responses appreciated, even
> if the reply mentions botnets or hammers ;-) )
>
> 1) What value is ICMP if everybody pretty much considers it's accuracy
> suspect?

The problem with ICMP-based traceroutes is that it doesn't necessarily
test the path you are interested in.  Use tcptraceroute or traceproto
instead.

Of course, this doesn't solve the problem that TTL Exceeded messages
might be generated with very low priority, which is in generally a
very good idea.

> 2) How does ICMP's suspect nature affect Path MTU?

In this case, you're interested in the ICMP payload, not the fact
whether an ICMP packet goes through or not.  (You lose if someone
filters ICMP, though.)