On Wed, Dec 27, 2006 at 04:16:23PM +0000, Mike M. Volokhov wrote:
> Peter Eisch <peter@boku.net> wrote:
> [snip]
> > 20:59:10.146020 IP (tos 0x0, ttl 115, id 57089, offset 0, flags [DF], \
> >      length: 1431) SVR.80 > CLIENT.40033: FP [tcp sum ok] 519:1910(1391) \
> >     ack 420 win 65116
> > 20:59:10.146067 IP (tos 0x0, ttl 255, id 46179, offset 0, flags [none], \
> >      length: 56) FW > SVR: icmp 36: CLIENT unreachable - need to frag for \
> >     IP  (tos 0x0, ttl 114, id 57089, offset 0, flags [DF], length: 1431, \
> >     bad cksum 99ca (->9aca)!) SVR.80 > CLIENT.40033: [|tcp]
> 
> This looks like a sort of problem - wrong checksumming leading to
> ICMP message rejected by <SVR>, and as result broken PMTUD.

I don't think that is the problem. "tcpdump" will always report incorrect
checksums on outgoing packets if checksum offload is enabled.

	Kind regards

-- 
Matthias Scheler                                  http://zhadum.org.uk/