Re: [aqm] TCP ACK Suppression

Yuchung Cheng <ycheng@google.com> Fri, 09 October 2015 17:05 UTC

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From: Yuchung Cheng <ycheng@google.com>
Date: Fri, 9 Oct 2015 10:04:19 -0700
Message-ID: <CAK6E8=c_h2dMstJSSO9ATh1XUgriWOyeVixk4DHi+PAjO6LSMg@mail.gmail.com>
To: David Lang <david@lang.hm>
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Cc: "LAUTENSCHLAEGER, Wolfram \(Wolfram\)" <wolfram.lautenschlaeger@alcatel-lucent.com>, Christian Huitema <huitema@microsoft.com>, Joe Touch <touch@isi.edu>, "aqm@ietf.org" <aqm@ietf.org>, Greg White <g.white@cablelabs.com>
Subject: Re: [aqm] TCP ACK Suppression
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On Thu, Oct 8, 2015 at 9:03 PM, David Lang <david@lang.hm> wrote:

> On Fri, 9 Oct 2015, Christian Huitema wrote:
>
> On Thursday, October 8, 2015 5:43 PM, David Lang wrote:
>>
>>> ...
>>> For example, in the fq_codel/cake development, we're finding that there
>>> are
>>> some transports that bundle very large numbers of packets together to
>>> send
>>> at one time in order to maximize the transport bandwidth. (for example,
>>> 4x4
>>> wifi sends a LOT of data in one transmit timeslot). Treating that large
>>> aggregate as a single packet seriously hurts fairness and latency on the
>>> next
>>> hop. So 'pulling apart' this aggregate into the individual
>>> packets/streams and
>>> making decisions based on the pieces ends up being a serious win in
>>> fairness
>>> and latency.
>>>
>>
>> Define "bundle" please. If they are making sure that several IP packets
>> are sent back to back in a single Wi-Fi slot, then it is of course
>> perfectly fine for AQM to handle the IP packets one by one. Does 4x4 Wi-Fi
>> do something else?
>>
>
> I don't remember the details from the discussion, but the combined bundle
> required extra steps to pull apart to get at the individual packets. IIRC,
> not doing so ended up with multi-MB chunks of data to be delivered, which
> blocked all other traffic while it was being delivered.
>
> Suggesting that the queues that build up produce a special enough case to
>>> consider thinning out the duplicate acks is a far cry from 'making a
>>> recommendation that breaks other recommendations'
>>>
>>
>> That definitely contradicts the TCP specs. So it is very much in "don't
>> go there" territory...
>>
>
> By 'not going there' you are crippling people's networks for the sake of
> following a spec. Rather than following the letter of the old spec, we
> should be looking at the reasons for it, and reasons to make exceptions.
> There is a long history of introducing new things that break the old way of
> doing things, from breaking "classful" network routing to Anycast, there
> are lots of things that "broke" the old way of doing things.
>
> In this case there is more than a decade of people doing exactly what
> shouldn't even be considered.
>
> I'll ask yet again, if acks have already been delayed so that they will be
> delivered at the same time as later acks, how much value do they actually
> provide? We need to compare whatever value this is against the cost of the
> misinformation that they provide, and the impact on other traffic.

Does DOCSIS suppress/filter ACK with SACK blocks and/or ACKs with
timestamps?


>
>
> David Lang
>
>
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