Subject: PMTU blackholes and problems
To: None <tech-net@NetBSD.ORG>
From: Martin Husemann <>
List: tech-net
Date: 01/05/2002 11:05:15
Could someone please check the patch itojun send in PR 12790 and maybe
commit it? It adds PMTUD blackhole discovery.

On a side note, Jared McNeil recently discovered he can not reach He said he'd open a PR on it, but I haven't seen one yet.
The problem is: he is connected via PPPoE (MTU 1492) but the router he
is connected to does not tell the right MTU - it sends 0 instead.

Example: when I do 

   /sbin/ping -D -s 1472

on, I get: 

36 bytes from frag needed and DF set.  Next MTU=1492 for icmp_seq=0

He get's "Next MTU=0" there.

Bill Sommerfeld said some RFC would specify a table of likely MTUs and suggest
to downgrade to the next smaller MTU in this table when this happens.