FreeBSD Window updates

Venkat Venkatsubra venkatvenkatsubra at
Sat Nov 29 12:39:51 PST 2008

Hi David,

Assuming each of these 5 window updates are sent after the apps
completed the read that it issued, what's the buffer size the apps is passing
to the read ? If it is small, could that be increased ?

But I agree with you that there is room for optimizing sending of these window updates.

(Since I don't know the window scale factor for the connection, I couldn't figure out
how much each of the window updates was opening up the window.)


From: David Malone <dwmalone at>
To: Venkat Venkatsubra <venkatvenkatsubra at>
Cc: Rui Paulo <rpaulo at>; freebsd-net at; Kevin Oberman <oberman at>
Sent: Saturday, November 29, 2008 11:46:30 AM
Subject: Re: FreeBSD Window updates

I've got an example extract tcpdump of this at the end of the mail
- here 6 ACKs are sent, 5 of which are pure window updates and
several are 2us apart!

I think the easy option is to delete the code that generates explicit
window updates if the window moves by 2*MSS. We then should be doing
something similar to Linux. The other easy alternative would be to
add a sysclt that lets us generate an window update every N*MSS and
by default set it to something big, like 10 or 100. That should
effectively eliminate the updates during bulk data transfer, but
may still generate some window updates after a loss.

Normal ACKing for driving congestion control shouldn't be impacted
by either of these suggested changes.


1227622713.276609 > . ack 144097745 win 40798 <nop,nop,timestamp 5365425 5103763> (DF)
1227622713.276611 > . ack 144097745 win 40830 <nop,nop,timestamp 5365425 5103763> (DF)
1227622713.276613 > . ack 144097745 win 40862 <nop,nop,timestamp 5365425 5103763> (DF)
1227622713.276615 > . ack 144097745 win 40894 <nop,nop,timestamp 5365425 5103763> (DF)
1227622713.276852 > . ack 144097745 win 40926 <nop,nop,timestamp 5365425 5103763> (DF)
1227622713.276855 > . ack 144097745 win 40958 <nop,nop,timestamp 5365425 5103763> (DF)
1227622713.296585 > . ack 144100641 win 40956 <nop,nop,timestamp 5365445 5103766> (DF)
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