Re: [aqm] [Bloat] TCP BBR paper is now generally available

Jonathan Morton <chromatix99@gmail.com> Sat, 03 December 2016 21:38 UTC

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From: Jonathan Morton <chromatix99@gmail.com>
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Date: Sat, 3 Dec 2016 23:38:18 +0200
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To: Eric Dumazet <eric.dumazet@gmail.com>
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Cc: "Steinar H. Gunderson" <sgunderson@bigfoot.com>, Neal Cardwell <ncardwell@google.com>, "aqm@ietf.org" <aqm@ietf.org>, bloat <bloat@lists.bufferbloat.net>
Subject: Re: [aqm] [Bloat] TCP BBR paper is now generally available
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> On 3 Dec, 2016, at 23:07, Eric Dumazet <eric.dumazet@gmail.com> wrote:
> 
> I do not attend IETF meetings so maybe my words are not exact.
> 
> What I meant was that we receive ACKS in bursts, with huge gaps between
> them.
> 
> Look at tsval/tsecr, and that all ACKS are received in a 20 usec time
> window, covering data that was sent in a 5 ms window.

Nevertheless, there are no acks obviously *missing*.  Decimation would be aggregation of acks such that all but the last (over some interval) is dropped.  Here, they are merely delivered in rapid succession.

It could be that the receiver has poor receive-interrupt latency, or that it is aggregating the reverse send path too heavily.  The variable spacing within the ack burst (in particular, in both examples the gap between first and second is larger than the rest) favours the former explanation.

 - Jonathan Morton