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[lwip-devel] [bug #27783] Silly window avoidance for small window sizes


From: Simon Goldschmidt
Subject: [lwip-devel] [bug #27783] Silly window avoidance for small window sizes
Date: Thu, 29 Oct 2009 17:22:45 +0000
User-agent: Mozilla/5.0 (Macintosh; U; Intel Mac OS X 10.4; de; rv:1.9.1.4) Gecko/20091016 Firefox/3.5.4

Follow-up Comment #5, bug #27783 (project lwip):

I just discovered another problem with our silliy-window-avoidance algorithm:
with TCP_WND <= ((TCP_MSS*4)-4) (i.e. window smaller than 4 segments), the
algorithm effectively disables the delayed ACK mechanism, i.e. an ACK is sent
for every (full-MSS-sized) segment received.

As a fix, I guess the definition of TCP_WND_UPDATE_THRESHOLD in opt.h must be
changed:

#ifndef TCP_WND_UPDATE_THRESHOLD
#if (TCP_WND < (TCP_MSS * 4)) && (TCP_WND >= (TCP_MSS * 2))
#define TCP_WND_UPDATE_THRESHOLD        (TCP_MSS * 2)
#else
#define TCP_WND_UPDATE_THRESHOLD        (TCP_WND / 4)
#endif
#endif

This both fixes the delayed-ACK mechanism for windows between 2*MSS and 4*MSS
as well as effectively disables that mechanism for windows < 2*MSS.

Any thoughts? Kieran?

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