Subject: Re: pppoe problem
To: David Laight <>
From: Rick Byers <>
List: tech-net
Date: 12/16/2001 19:49:27
On Sun, 16 Dec 2001, David Laight wrote:

> > Perhaps. :-)  But it's not restricted to Webservers.  I know of at
> > least one person who's had trouble sending me large emails because the
> > relevant outgoing mailhost has the won't-frag disease, and my logs show
> > identical symptoms from many other hosts that have't been important
> > enough for me to verify that the problem is won't-frag.
> I wonder if it possible to locate the 'black hole' by sending large
> ICMP echo packets with 'dont fragment' and a small 'hop count',
> comparing the result to the usual 'traceroute' sequence.

I don't think so.  The problem is that the largest IP datagram that can
get from point A to point B with the DF bit set is one that is <= the path
MTU.  So an ICMP echo reply to a max-size echo request will never need
fragmenting, unless the return path is different from the source path.