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Date: Thu, 15 Dec 2011 15:15:48 +0100
From: Corinna Vinschen <corinna-cygwin AT cygwin DOT com>
To: cygwin AT cygwin DOT com
Subject: Re: Issues with TCP Window Scaling Factor and Cygwin Daemons
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On Dec 15 05:39, Dave Korn wrote:
> On 15/12/2011 05:23, Lee wrote:
> > 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.
> 
> > 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
> 
>   1.  MS completely replaced the networking stack in Vista/2k8 with their
> so-called "Next Generation TCP/IP stack", so you can't extrapolate from XP to 2k8.
> 
>   2.  On XP, window scaling is enabled by a registry setting, and when it is
> in effect, Windows decides what scale setting to use by considering the
> SO_RCVBUF size used when the connection is initiated.  In that light, the fact
> that Cygwin applications show a scale of zero is probably not unrelated to
> this snippet of code from net.cc#fdsock():
> 
> >   /* Raise default buffer sizes (instead of WinSock default 8K).
> > 
> >      64K appear to have the best size/performance ratio for a default
> >      value.  Tested with ssh/scp on Vista over Gigabit LAN.
> > 
> >      NOTE.  If the SO_RCVBUF size exceeds 65535(*), and if the socket is
> >      connected to a remote machine, then calling WSADuplicateSocket on
> >      fork/exec fails with WinSock error 10022, WSAEINVAL.  Fortunately
> >      we don't use WSADuplicateSocket anymore, rather we just utilize
> >      handle inheritance.  An explanation for this weird behaviour would
> >      be nice, though.
> > 
> >      (*) Maximum normal TCP window size.  Coincidence?  */
> >   ((fhandler_socket *) fd)->rmem () = 65535;
> >   ((fhandler_socket *) fd)->wmem () = 65535;
> >   if (::setsockopt (soc, SOL_SOCKET, SO_RCVBUF,
> > 		    (char *) &((fhandler_socket *) fd)->rmem (), sizeof (int)))
> >     {
> 	[ ... snip error handling ... ]
> >     }
> 
>   I would guess that a >65536 default would cause Windows to send a non-zero
> scale in the SYN packet, but as the comment says, that created some other sort
> of problem somehow.
> 
>   3.  On 2k8 (also Vista and 7), window scaling is no longer controlled by the
> registry but enabled automatically, as part of a new feature called "Receive
> Window Auto-Tuning".  I could not infer from any documentation that I could
> find in ten minutes googling whether or not it still pays attention to the
> initial SO_RCVBUF setting.  It might be worth trying a custom build of the
> Cygwin DLL with a 64k or 128k setting in place of 65535.  Then again, it seems
> there may yet be problems in this functionality, e.g. the known bug described
> at http://support.microsoft.com/kb/983528; perhaps if the custom Cygwin DLL
> doesn't help, that hot-fix might.

Alternatively we could only set the send/recv buffer sizes if we're
running under pre-Vista.  However, I have no idea what implications
this has in terms of network performance.  "Somebody" would have to
test it.


Corinna

-- 
Corinna Vinschen                  Please, send mails regarding Cygwin to
Cygwin Project Co-Leader          cygwin AT cygwin DOT com
Red Hat

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