On Thursday, 17. July 2008 15:55:25 Ilpo Järvinen wrote:

It would quite interesting to know more details about the mail server and
why the duplicate ACKs are not generated or don't ever reach the sender
but I guess the details are out of reach?

It will be quite difficult to get more details as it's the SMTP relay sever
of Germany's biggest ISP. There's a comment about them
in Patrick's blog from 2008-06-23 if you are curious ;-)

We see the same issue with a MX server from "United Internet".
Normally they are pretty accurate about standards (they run GMX),
so I guess this must be a problem of a router in between.

This is also supported by the fact that 935 of our boxes already updated
to kernel 2.6.24.7, yet the problem occured only at three sites and I guess
there are more people out there using that SMTP relay server.

Could you somehow "probe" the servers to see if they normally
send duplicated ACKs by faking/forcing a retransmission?
Though I guess this would invole writing some TCP "test" code.

One option would be to disable reentry to FRTO when some progress was
made... Please try with the patch below...

Thanks for the patch. It seemed to help a bit. Here are two more traces:
http://www.intra2net.com/de/download/tcpdump/tcp_frto_with_patch.tar.bz2

The first connection somehow made it after 400 seconds,
the second one stalled and timed out :-(
Hope they dumps are useful to you.

Chers,
Thomas
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