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

Jonathan Morton <chromatix99@gmail.com> Fri, 02 December 2016 20:32 UTC

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From: Jonathan Morton <chromatix99@gmail.com>
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Date: Fri, 2 Dec 2016 22:32:14 +0200
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To: Aaron Wood <woody77@gmail.com>
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Cc: bloat <bloat@lists.bufferbloat.net>, Dave Taht <dave.taht@gmail.com>, "aqm@ietf.org" <aqm@ietf.org>
Subject: Re: [aqm] [Bloat] TCP BBR paper is now generally available
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> On 2 Dec, 2016, at 21:15, Aaron Wood <woody77@gmail.com> wrote:
> 
> So, how is this likely to be playing with our qos_scripts and with cake?

Cake’s deficit-mode shaper behaves fairly closely like an ideal constant-throughput link, which is what BBR is supposedly designed for.  I haven’t read that far in the paper yet, but it shouldn’t trigger any “bucket detection” algorithms, because it doesn’t have a “bucket”.  It is capable of bursting, but only to the minimum extent required to reconcile required throughput with timer resolution and scheduling latency; I’ve tested it with millisecond timers.

The older schemes involving HTB and HFSC *do* have token-bucket behaviour, with an explicitly configured burst size (this excess traffic will collect in downstream buffers).  However, these are shapers, not policers, so they will start delaying packets (leaving them in child qdiscs) when the bucket is empty, not simply dropping them.

The interaction with AQM-related marking and dropping will be interesting to read, though.  It’s not a-priori obvious how much a shaper-AQM combination looks like a policer.

 - Jonathan Morton