Re: [aqm] TCP ACK Suppression

Greg White <g.white@CableLabs.com> Wed, 07 October 2015 16:48 UTC

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From: Greg White <g.white@CableLabs.com>
To: Richard Scheffenegger <rs.ietf@gmx.at>, Mikael Abrahamsson <swmike@swm.pp.se>, "Bless, Roland (TM)" <roland.bless@kit.edu>
Thread-Topic: [aqm] TCP ACK Suppression
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Date: Wed, 7 Oct 2015 16:48:32 +0000
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Cc: "LAUTENSCHLAEGER, Wolfram \(Wolfram\)" <wolfram.lautenschlaeger@alcatel-lucent.com>, "aqm@ietf.org" <aqm@ietf.org>
Subject: Re: [aqm] TCP ACK Suppression
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I'm not sure that would be the result in this case.  Here the CM has a
choice of sending (e.g.) 16 back-to-back ACKs, each for 2 segments, or one
ACK for 32 segments.  In either case, 32 segments worth of
acknowledgements will arrive at the sender at once.

-Greg

On 10/7/15, 9:57 AM, "Richard Scheffenegger" <rs.ietf@gmx.at> wrote:

>[as individual]
>
>Hi Mikael,
>
>ACK thinning will interfere with the ACK clock of the sender; a good
>recipie 
>to require much more buffering at the bottlneck hop of that session
>(since 
>the sender will start bursting out TCP data packets at it's line rate,
>rather than the bottleneck link rate).
>
>So, if your goal is to have the sender send out TCP data segments at the
>bottleneck rate, you'd not want to ACK too many segments at once...
>
>Of course, the drawback is your observation, that TCP (over Ethernet)
>typically requires 64 bytes of ACK per 2x 1448 bytes, or about 1:45
>uplink/downlink bandwidth).
>
>Others have already pointed out some relevant RFCs in this thread too...
>
>Best regards,
>  Richard
>
>----- Original Message -----
>From: "Mikael Abrahamsson" <swmike@swm.pp.se>
>To: "Bless, Roland (TM)" <roland.bless@kit.edu>
>Cc: "LAUTENSCHLAEGER, Wolfram (Wolfram)"
><wolfram.lautenschlaeger@alcatel-lucent.com>t.com>; "Greg White"
><g.white@CableLabs.com>s.com>; <aqm@ietf.org>
>Sent: Wednesday, October 07, 2015 12:51 PM
>Subject: Re: [aqm] TCP ACK Suppression
>
>
>> On Wed, 7 Oct 2015, Bless, Roland (TM) wrote:
>>
>>> Oh, I hope that this is an exception. Such kind of optimizations may
>>> cause a lot of trouble since a link layer device is interfering with
>>> transport layer semantics. We all know that exactly these kinds of
>>> interference eventually end up in problems with end-to-end
>>>transparency 
>>> and deployment of new protocol options. At least it interferes with
>>>the 
>>> ACK clocking expectation of some congestion control algorithms...
>>
>> Personally, I think you're going to see more and more of this. There
>>are 
>> mulitple shared access medium where you're allowed to send only part of
>> the time, and it's someone else who tells you when you may send.
>>
>> So if you're sitting there with 100 ACK packets all nicely ACKing
>> increasing values indicating no packet loss, and now you're allowed to
>> send, why not just kill 99 of those ACK packets?
>>
>> While I agree with you on principle, these kinds of mechanisms cut the
>> amount of ACK traffic down by factor 15-30, meaning I can download at
>>250 
>> megabit/s and only have a few hundred kilobit/s of upstream ACKs,
>>instead 
>> of 5-10 megabit/s of ACKs. That's a huge opimization for any kind of
>> asymmetric and/or time slot access medium such as ADSL, LTE, DOCSIS,
>>PON 
>> and I'm sure there are others.
>>
>> Btw, what is the reason for TCP doing ACKs for every 2 packets in
>> steady-state, even with window sizes in the hundreds of kilobytes or
>>even 
>> megabytes? I would prefer if the TCP host stack sent fewer ACKs instead
>>of 
>> trying to optimize this at the access routing device.
>>
>> -- 
>> Mikael Abrahamsson    email: swmike@swm.pp.se
>>
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