netdev - empty ack packets

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Message-ID: <AANLkTimkvDsqY==1JQRhH54+P7yAzFX4omYN0r9+_mi=@mail.gmail.com>
Date:	Fri, 10 Sep 2010 01:12:44 -0400
From:	MK <stardust496@...il.com>
To:	netdev@...r.kernel.org
Subject: empty ack packets

Hello list,

I am looking at a tcpdump and I see that very very frequently, after
receiving a segment, my tcp is sending an empty ack back in a matter
of several (around 20 - 50) microseconds. And then after several more
microseconds, my tcp is sending some valid outgoing data. I am trying
to understand why it decided to send an empty ack back when that ack
could potentially have been delayed by microseconds and get
piggybacked on the outgoing data.

>From the code, it appears that the delayed ack timeout is 40 millisecs
so it is likely not the delack timer that is causing this. (And I do
not have the quickack option)

This is RHEL5 (2.6.18) kernel.

Does anybody have an idea as to what is happening?

Thanks a lot!!
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