Subject: PMTU blackholes and problems
To: None <tech-net@NetBSD.ORG>
From: Martin Husemann <email@example.com>
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
ftp.netbsd.org. 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 126.96.36.199
on ftp.netbsd.org, I get:
36 bytes from 188.8.131.52: 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.