Re: [aqm] ACK Suppression

David Lang <david@lang.hm> Wed, 07 October 2015 21:13 UTC

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Date: Wed, 7 Oct 2015 14:13:09 -0700 (PDT)
From: David Lang <david@lang.hm>
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To: Jonathan Morton <chromatix99@gmail.com>
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Cc: "dpreed@reed.com" <dpreed@reed.com>, "Agarwal, Anil" <Anil.Agarwal@viasat.com>, "aqm@ietf.org" <aqm@ietf.org>, Mikael Abrahamsson <swmike@swm.pp.se>
Subject: Re: [aqm] ACK Suppression
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On Wed, 7 Oct 2015, Jonathan Morton wrote:

>> On 7 Oct, 2015, at 23:40, Agarwal, Anil <Anil.Agarwal@viasat.com> wrote:
>> 
>>> Since the cable modem link will lead to clumped ACKs the difference between 
>>> sending 100 ACKs vs. 1 ACK is probably not that big... (except w.r.t. 
>>> reliability).
>> 
>> The difference may not be big in the spacing of new packets that a sender 
>> will send, unless the sender implements some sort of pacing or if the return 
>> link is very thin.
>
>> But with ABC, there will be a difference in the amount of cwnd increase at 
>> the sender.
>
> There is also a potential difference for detecting packet loss in the forward 
> direction.  It’s entirely possible that thinning would cause a DupAck 
> condition to be recognised only after three MAC grants in the reverse 
> direction have elapsed, rather than one.  Receivers are REQUIRED to send an 
> ack for every received packet under these conditions, but that would be 
> subverted by the modem.  AckCC would not induce this effect, because the 
> receiver would still produce the extra acks as required.
>
> Packet loss causes head-of-line blocking at the application level, which is 
> perceived as latency and jerkiness by the end-user, until the lost packet is 
> retransmitted and actually arrives.  Hence the addition of two MAC grant 
> delays (60ms?) may make the difference between an imperceptible problem and a 
> noticeable one.

and excessive ack traffic causes congestion and results in packet loss on 
real-world hightly asymmetric links.

So things that reduce the flow of acks can result in very real benefits to 
users.

David Lang