Thanks again for all of the additional input. I owe you this follow-up because I had some things wrong last night. Here, I hope, is much better information:
On the drive home last night, it began to bother me that Ethereal was only showing 4 bytes of options in my outgoing syns (1 byte MSS, 2 bytes NOP, and 1 byte SACK Permitted). It seemed that the developers would have had to have gone to great trouble to strip out the RFC1323 options in the capture and still have the checksum pass. Why bother? Hmmm... I recaptured a download test from pcpitstop. Sure enough, the web server was having my machine report its value for MaxTcpWindowSize so that the web server could turn right back around and plug it into the "results" being displayed on my machine. Alas, I was not actually opening an rcv window above 65535. It was all smoke and mirrors. I had read several times that RFC1323 window scaling was enabled by default in W2k and newer OSes. Turns out, it will accept offers by default, but not make any of its own. That is, until you create the DWORD value Tcp1323Opts under the \tcpip\parameters key and set it appropriately (url to follow). Furthermore, TcpWindowSize is still intended to be mainly used as a per-interface parameter. GlobalMaxTcpWindowSize enforces a global limit that no interface can exceed. So, setting the two at the same value under \tcpip\parameters, or just TcpWindowSize alone under the same, along with Tcp1323Opts set appropriately, actually enables window scaling. Sure enough, my Ethereal capture now reflects 28 bytes of options, including an offer to window scale. The scale value offered tracks exactly what I would expect based on my various TcpWindowSize experiments. So, although Ethereal doesn't "support" window scaling in that it still reflects the 16-bit value in the "normal" (non-handshake) data segments, you can track back to the original "syn, syn-ack, ack" handshake in the trace file and do the math yourself (assuming, of course, that the distant-end accepted the offer to scale!). And here is more about the MS TCP stack for W2k than you ever wanted to know (Appendix A has all the relevant DWORDS and their valid ranges): http://www.microsoft.com/windows2000/docs/tcpip2000.doc You can even enable MTU discovery on your W2k box?! Regards all, Scott Message Posted at: http://www.groupstudy.com/form/read.php?f=7&i=59487&t=59487 -------------------------------------------------- FAQ, list archives, and subscription info: http://www.groupstudy.com/list/cisco.html Report misconduct and Nondisclosure violations to [EMAIL PROTECTED]
