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Date: Wed, 14 Dec 2011 17:58:54 +0000
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Subject: Issues with TCP Window Scaling Factor and Cygwin Daemons
From: David Groves <dave AT fibrecat DOT org>
To: cygwin AT cygwin DOT com
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I am having some issues with cygwin applications (specifically sshd)
and TCP Window Scaling Factors. I am using OpenSSH client on either a
Debian Linux or FreeBSD machine to connect to sshd on a Windows 2k8 R2
server.

My SYN from the client has a non-zero WSF (16 for Debian, 8 for
FreeBSD), but the SYNACK response from the windows box has 0 as the
WSF. This means I will have a maximum of 64Kbytes of traffic allowed
to be unacknowledged in the TCP window. This is causing me
considerable performance problems in transfers from Europe to the US
West Coast and Australia.

To try and decide if cygwin was a factor in this, I tried to replicate
this with Apache2 and pure-ftpd on cygwin, and in both cases, the WSF
was 0. I then tried a non-cygwin Apache2 install, and it returned a
non-zero WSF.

TCP settings is clearly a function of the OS, but I'm a little unsure
where the line is drawn between the OS with cygwin, and it is
interesting I can reproduce this with multiple cygwin apps, and I
cannot reproduce it with non-cygwin apps, I'm thinking cygwin is at
least somewhat involved as part of the problem.

Anyone have any ideas/pointers ?

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