tcptrace "Default Send Window" in Windows XP/2000

From: Bickerdike, Philip W (Philip.W.Bickerdike@team.telstra.com)
Date: 04/06/04 

Subject: tcptrace "Default Send Window" in Windows XP/2000
Date: Wed, 7 Apr 2004 09:31:43 +1000
Message-ID: <07394BF21D90D311B61E00508B08F0270CE718EA@ntmsg0098.corpmail.telstra.com.au>
From: "Bickerdike, Philip W" <Philip.W.Bickerdike@team.telstra.com>

Hi all,

I'm not sure if this is the correct forum to raise this query however I was hoping someone may be able to help or direct me to a more appropriate mailing list. I have discovered some strange behaviour doing upload tests with Windows XP. Firstly, there seems to be a limit on the packets in flight imposed by what I have discovered to be a registry entry the details of which are:

        DefaultSendWindow
        Value Type: REG_DWORD
        Default: 8192
        Description: Similar to DefaultReceiveWindow, but for the send side of connections.

The result is a link that is dramatically underutilised particularly in situations with a high delay-bandwidth product. To my understanding this violates tcp congestion avoidance which should increase the congestion window until the limit imposed by the receiver's advertised window. Has anyone come across this before? Any ideas as to the justification for this approach in the implementation?

Also, in XP, there seems to be no MSS defined or adhered to. In slow-start segment sizes vary greatly from ~500 bytes to ~1500 bytes and when the connection reaches 'steady state' the sender sends fixed 4096 byte segments - ensuring no more than 2 segments (8192 bytes) are in flight at any time.

Thanks for your help

Philip W. Bickerdike

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