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List:       netbsd-tech-net
Subject:    PMTU blackholes and problems
From:       Martin Husemann <martin () duskware ! de>
Date:       2002-01-05 10:05:15
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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 217.0.157.12

on ftp.netbsd.org, I get: 

36 bytes from 217.5.113.169: 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.


Martin
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