X-Recipient: archive-cygwin@delorie.com X-SWARE-Spam-Status: No, hits=-1.9 required=5.0 tests=AWL,BAYES_00,DKIM_SIGNED,DKIM_VALID,DKIM_VALID_AU,FREEMAIL_ENVFROM_END_DIGIT,FREEMAIL_FROM,T_FRT_STOCK2 X-Spam-Check-By: sourceware.org MIME-Version: 1.0 In-Reply-To: <4EE98816.3020305@gmail.com> References: <4EE98816.3020305@gmail.com> Date: Thu, 15 Dec 2011 03:00:57 -0500 Message-ID: Subject: Re: Issues with TCP Window Scaling Factor and Cygwin Daemons From: Lee To: cygwin@cygwin.com Content-Type: text/plain; charset=ISO-8859-1 X-IsSubscribed: yes 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 12/15/11, 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. Arguh.. I don't know where I got WinXP from. Sorry for the noise. Lee > 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. > > cheers, > DaveK > > > -- > 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 > > -- 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