kern/116335: Excessive TCP window updates

[Kevin Oberman]

>Number:         116335
>Category:       kern
>Synopsis:       Excessive TCP window updates
>Confidential:   no
>Severity:       serious
>Priority:       high
>Responsible:    freebsd-bugs
>State:          open
>Quarter:        
>Keywords:       
>Date-Required:
>Class:          sw-bug
>Submitter-Id:   current-users
>Arrival-Date:   Thu Sep 13 20:20:06 GMT 2007
>Closed-Date:
>Last-Modified:
>Originator:     Kevin Oberman
>Release:        FreeBSD 6.2-STABLE i386
>Organization:
ESnet--The Energy Sciences Network
>Environment:
System: FreeBSD ptavv.es.net 6.2-STABLE FreeBSD 6.2-STABLE #11: Thu Aug 16 17:18:58 PDT 2007 root at ptavv.es.net:/usr/obj/usr/src/sys/PTAVV i386

>Description:
	Testing over a trans-continental 10GE between two boxes with
mxge cards, at a point about 2.5 seconds into the tansfer, the receive
node starts updating the window size as fast as it can process the
data. The result is that it is sending updates at intervals of between
0 and 4 microseconds. This can result in several hundred window
updates between "real" packets and, I suspect, is causing performance
problems.

I see an old message at:
http://lists.freebsd.org/pipermail/freebsd-net/2005-January/006141.html
that may be the source of the problem, though I have not yet figured
out exactly how this code works.

>How-To-Repeat:
	Send a TCP stream between to hosts with a ~100 ms. RTT between
them at speeds exceeding 3 Gbps. 
>Fix:
Unknown.
>Release-Note:
>Audit-Trail:
>Unformatted: