Re: [aqm] TCP ACK Suppression

Yuchung Cheng <ycheng@google.com> Thu, 08 October 2015 21:52 UTC

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From: Yuchung Cheng <ycheng@google.com>
Date: Thu, 8 Oct 2015 14:52:05 -0700
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To: David Lang <david@lang.hm>
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Cc: "LAUTENSCHLAEGER, Wolfram \(Wolfram\)" <wolfram.lautenschlaeger@alcatel-lucent.com>, Greg White <g.white@cablelabs.com>, "aqm@ietf.org" <aqm@ietf.org>, Joe Touch <touch@isi.edu>
Subject: Re: [aqm] TCP ACK Suppression
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On Thu, Oct 8, 2015 at 2:31 PM, David Lang <david@lang.hm> wrote:

> On Thu, 8 Oct 2015, Joe Touch wrote:
>
> On 10/7/2015 12:42 AM, LAUTENSCHLAEGER, Wolfram (Wolfram) wrote:
>> ...
>>
>>> Is this topic addressed in some RFC already?
>>>
>>
>> It's a direct violation of RFC793, which expects one ACK for every two
>> segments:
>>
>> 4.2 Generating Acknowledgments
>>
>>   The delayed ACK algorithm specified in [Bra89] SHOULD be used by a
>>   TCP receiver.  When used, a TCP receiver MUST NOT excessively delay
>>   acknowledgments.  Specifically, an ACK SHOULD be generated for at
>>   least every second full-sized segment, and MUST be generated within
>>   500 ms of the arrival of the first unacknowledged packet.
>>
>
> actually, this is only a violation of the SHOULD section, not the MUST
> section.
>
> And if the Ack packets are going to arrive at wire-speed anyway (due to
> other causes), is there really an advantage to having 32 ack packets
> arriving one after the other instead of making it so that the first ack
> packet (which arrives at the same time) can ack everything?


I would like to add that GRO also cause stretched ACKs (acking up to 64KB),
and GRO is crucial to reduce cycles for +10Gbps transfers on some
architectures.


> And if there is such an advantage, does it outweight the disadvantages
> that the extra ack packets cause by causing highly asymmetric links to be
> overloaded and drop packets?
>
> David Lang
>
>
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