[Techtalk] Can't get to http://linuxchix.org

[Conor Daly]

On Wed, Nov 10, 2004 at 08:05:04AM -0800 or so it is rumoured hereabouts, 
Carla Schroder thought:
> 
> Dancer posted this on Volunteers:
> 
> "If you are using a late-model 2.6 kernel, you will want to put:
> net/ipv4/tcp_default_win_scale=0
> in your /etc/sysctl.conf (or equivalent).
> 
> 2.6 introduces a new tcp_default_win_scale that tickles many routing 
> bugs in TCP streams in assorted hardware. Some of that hardware is 

This is detailed at: http://lwn.net/Articles/92727/ .  The problem affects
kernels from 2.6.7 upwards and seems not to affect FC2's stock 2.6.5
kernel.  We ran up against this at work and a series of tests revealed
that values of tcp_default_win_scale>=7 caused a problem but that all was
fine with tcp_default_win_scale<=6 .

The tcp_default_win_scale=0 is a workaround rather than a fix.  We need to
complain to ISPs about this so that the routers get fixed since the window
scaling thing is RFC compliant.

If anyone has a >= 2.6.7 kernel to test with (I've only a stock 2.6.5 at
present), they could try these tests (Dancer permitting of course!):

#>echo 0 > /proc/sys/net/ipv4/tcp_default_window_scale

#>telnet mail.linuxchix.org 25
Trying 203.7.155.11...
Connected to mailman.linuxchix.org.
Escape character is '^]'.
220 www.linuxchix.org ESMTP Postfix (Debian/GNU)
^]

#>echo 7 > /proc/sys/net/ipv4/tcp_default_window_scale

#>telnet mail.linuxchix.org 25
Trying 203.7.155.11...
Connected to mailman.linuxchix.org.
Escape character is '^]'.
^]

#>echo 6 > /proc/sys/net/ipv4/tcp_default_window_scale

#>telnet mail.linuxchix.org 25
Trying 203.7.155.11...
Connected to mailman.linuxchix.org.
Escape character is '^]'.
220 www.linuxchix.org ESMTP Postfix (Debian/GNU)
^]

This was the behaviour we saw.  When the win_scale value was 7 or higher,
things would hang.  With win_scale 6 or less, all was fine...

Akkana said:
> Addendum to that: a few weeks ago we were having the same
> problem getting to linuxchix.org from here, and we discovered that
> we only saw the problem on machines running the 2.6.8.1 kernel.
> Machines running 2.6.7 or 2.6.9 or 2.4.* could get to
> linuxchix.org, but 2.6.8.1 couldn't.  Perhaps 2.6.9 changed the
> tcp_default_win_scale, or its handling of it, to be more tolerant?

What is the value of /proc/sys/net/ipv4/tcp_default_window_scale with the
2.6.9 kernels?  Is it less than 7?

Conor
-- 
Conor Daly <conor.daly at oceanfree.net>

Domestic Sysadmin :-)
---------------------
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