X-Recipient: archive-cygwin@delorie.com X-Spam-Check-By: sourceware.org Date: Thu, 15 Dec 2011 15:15:48 +0100 From: Corinna Vinschen To: cygwin@cygwin.com Subject: Re: Issues with TCP Window Scaling Factor and Cygwin Daemons Message-ID: <20111215141548.GE30302@calimero.vinschen.de> Reply-To: cygwin@cygwin.com Mail-Followup-To: cygwin@cygwin.com References: <4EE98816.3020305@gmail.com> MIME-Version: 1.0 Content-Type: text/plain; charset=utf-8 Content-Disposition: inline In-Reply-To: <4EE98816.3020305@gmail.com> User-Agent: Mutt/1.5.21 (2010-09-15) Mailing-List: contact cygwin-help@cygwin.com; run by ezmlm Precedence: bulk List-Id: List-Unsubscribe: List-Subscribe: List-Archive: List-Post: List-Help: , Sender: cygwin-owner@cygwin.com Mail-Followup-To: cygwin@cygwin.com Delivered-To: mailing list cygwin@cygwin.com 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 -- Problem reports: http://cygwin.com/problems.html FAQ: http://cygwin.com/faq/ Documentation: http://cygwin.com/docs.html Unsubscribe info: http://cygwin.com/ml/#unsubscribe-simple