On Tue, 15 Jul 2008, Thomas Jarosch wrote:

quoted

Luckily I'm able to reproduce the problem locally using an ADSL line from
the same provider, so I'll now bisect the kernel from 2.6.23.17 to 2.6.24.

After bisecting for hours, l only had ten revisions left to test.
There was this commit that caught my eye:

------------------------------
commit c96fd3d461fa495400df24be3b3b66f0e0b152f9
Author: Ilpo Järvinen [off-list ref]
Date:   Thu Sep 20 11:36:37 2007 -0700

    [TCP]: Enable SACK enhanced FRTO (RFC4138) by default
------------------------------

This change sets the value of "tcp_frto" to 2 by default.
If I reset it to zero, the connection works immediately.
@Dâniel Fraga: Does disabling tcp_frto work for you, too?

Disabling tcp_sack makes no difference. To summarize the situation,
I had two different cases of stalling TCP connections, both connecting
to busy SMTP relays servers which probably drop some packets here and there.

I can easily reproduce the problem, so how do we go from here?

FRTO in 2.6.24.y is broken, I recently fixed couple of things in FRTO, 
late 2.6.25.y or 2.6.26 should be used to have all the fixes. If you can 
reproce with either one, please tcpdump it (I just returned, was couple of 
weeks away, so I'm slowly catching up what has happened in between here). 
...I guess somebody had dumped at least 2.6.24.y but that's not 
interesting due to known (and fixed) bugs with FRTO.


-- 
 i.