[Cake] [Codel] [aqm] codel with low shape rates

[Jonathan Morton]

> On 19 Jan, 2016, at 15:43, Agarwal, Anil <Anil.Agarwal at viasat.com> wrote:
> 
> (I would like very much to have an aqm that was knobless at these speeds. Most of the work on trying to improve matters at these rates is in the multitude of "cake" variants)

Codel is pretty close to being self-tuning in terms of link capacity, as long as it’s on the bottleneck and only needs to deal with one flow per instance.  The main trouble is when the inherent serialisation delay becomes comparable to the time constants assumed for the total estimated RTT.  On very slow links, the a-priori RTT estimate needs to be increased to account for that.

Cake has an advantage here in knowing the link capacity, via its built-in shaper.  This is how it knows what fine-tuning to apply.  It’s hard for a “pure” AQM qdisc to gather that information automatically.

With that self-tuning, Cake works usably well down to 64Kbps.  At that level, typical modern TCPs (especially CUBIC) saturate the link continuously from the outset, so Codel ends up constantly signalling them to slow down.  With ECN on, this is harmless.

 - Jonathan Morton