FreeBSD Window updates

[Venkat Venkatsubra]

Hi Andre,

When delayed Ack is set the window update is not sent.
Does this mean when odd number of packets are received and later read,
a window update won't go out either till the next segment arrives or
200 msecs delayed ack timer ? Can this reduced window block the sender from
sending the next segment that we are waiting for to open up the window ?

What's the purpose of the 2 MSS check by the way ? 

Venkat  




________________________________
From: Andre Oppermann <andre at freebsd.org>
To: David Malone <dwmalone at maths.tcd.ie>
Cc: Rui Paulo <rpaulo at fnop.net>; freebsd-net at freebsd.org; Venkat Venkatsubra <venkatvenkatsubra at yahoo.com>; Kevin Oberman <oberman at es.net>
Sent: Sunday, November 30, 2008 5:18:22 PM
Subject: Re: FreeBSD Window updates

Andre Oppermann wrote:
> David Malone wrote:
>> 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.
> 
> The main problem of the pure window update test in tcp_output() is
> its complete ignorance of delayed ACKs.  Second is the strict 4.4BSD
> adherence to sending an update for every window increase of >= 2*MSS.
> The third issue of sending a slew of window updates after having
> received a FIN (telling us the other end won't ever send more data)
> I have already fixed some moons ago.
> 
> In my new-tcp work I've come across the window update logic some time
> ago and backchecked with relevant RFCs and other implementations.
> Attached is a compiling but otherwise untested backport of the new logic.

Slightly improved version attached.

-- 
Andre