netdev - Re: empty ack packets

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Date:	Thu, 9 Sep 2010 22:29:15 -0700
From:	Mitchell Erblich <erblichs@...thlink.net>
To:	MK <stardust496@...il.com>
Cc:	netdev@...r.kernel.org
Subject: Re: empty ack packets

MK,


	The 1st packet is probably generating window update information.

	If the TCP flow is in slow start, it should not be doing any delacks
	to keep as many normal acks in the flow.

	Mitchell Erblich

	
On Sep 9, 2010, at 10:12 PM, MK wrote:

> 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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