Re: [aqm] TCP ACK Suppression

Joe Touch <touch@isi.edu> Thu, 08 October 2015 21:28 UTC

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To: "Francini, Andrea (Andrea)" <andrea.francini@alcatel-lucent.com>, Steve Bauer <bauer@mit.edu>
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From: Joe Touch <touch@isi.edu>
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Cc: Greg White <g.white@cablelabs.com>, "tcpm@ietf.org" <tcpm@ietf.org>, "aqm@ietf.org" <aqm@ietf.org>, touch@isi.edu
Subject: Re: [aqm] TCP ACK Suppression
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CC'ing TCPM on my responses to this thread...

On 10/7/2015 8:50 AM, Francini, Andrea (Andrea) wrote:
> How about the effect of ACK suppression on the growth of the
> congestion window, for TCP sources where the trigger for window growth
> is the arrival of an ACK, not the number of bytes it acknowledges?
> 
> The Appropriate Byte Counting (ABC) option
> (https://tools.ietf.org/html/rfc3465) was addressing a similar issue
> with delayed ACKs, but it looks like it is no longer available in Linux
> (http://www.spinics.net/lists/netdev/msg225164.html).

Even with ABC, killing off some of the ACKs would result in sender
bursts because you're interfering with ACK clocking -- unless ABC were
coupled with some sort of rate pacing.

Note also that when you kill off intermediate ACKs you're increasing the
round-trip delay. The sender can't start sending what you ACK until the
ACK arrives; if you kill off earlier ACKs, then the later ACK will (by
definition) arrive later.

This is bad on many levels.

Joe