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Date:	Thu, 15 Dec 2011 00:23:01 -0500
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Subject:	Re: Issues with TCP Window Scaling Factor and Cygwin Daemons
From:	Lee <ler762 AT gmail DOT com>
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On 12/14/11, David Groves wrote:
> 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 ?

No pointers, but it does look like a Cygwin problem.

Start up wireshark on a Windows XP machine and ssh (from a cygwin
window) to another machine - tcp syn packet has a window size of 65535
and a widow scale of zero.  Use putty from the dos prompt to ssh to
the same machine and that has a window size of 65535 and a window
scale of 2

Lee

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