[Bloat] RED against bufferbloat

[Michael Welzl]

> On 25 Feb 2015, at 11:24, toke at toke.dk wrote:
> 
> Michael Welzl <michawe at ifi.uio.no> writes:
> 
>> but that's FQ (or FQ_CoDel's changed FQ variant), much more than the
>> AQM mechanism in use (as we have also seen presented by Toke at the
>> last ICCRG meeting).
> 
> Well, actually, that presentation did also include an evaluation of the
> AQMs in an asymmetrical scenario. And that shows that while generally
> ARED does perform fairly well, it tends to be a bit on the aggressive
> side. In the asymmetrical case this results in too many drops on the
> slow side of the asymmetrical link (typically upload), hurting throughput
> in the other direction due to lost ACKs.
> 
> There's also some other issues in there, with PIE and CoDel in
> particular, most notably their reactions when conditions change: it can
> take tens of seconds for the algorithms to get queueing latency under
> control in this case.
> 
> Slides for the IETF presentation available here:
> http://www.ietf.org/proceedings/91/slides/slides-91-iccrg-4.pdf
> 
> There's also a longer version of the talk (from the Stanford Netseminar)
> available on Youtube: https://www.youtube.com/watch?v=kePhqfKA3SM
> 
>> But this discussion is about AQM mechanisms, not (changed)FQ.
> 
> While the academic side of me enjoys studying AQMs (and I'm still far
> from anything resembling a thorough understanding of them), the
> practical "I just want my network to work" side of me has become
> increasingly convinced (in part by doing the experiments in the above
> mentioned talk) that FQ is more important than AQM in many scenarios.

+1, certainly it has a big influence. This has been well known for many years though, and documented broadly, perhaps most notably by Jim Roberts.


> As
> such, I think that excluding FQ from the conversation is mostly of, well,
> academic interest ;)

Here I disagree, for two reasons:
1) The AQM part kicks in per flow. So, whenever you have one flow, the behavior of FQ_AQM and AQM will be the same. Investigating what an AQM mechanism does to one flow is then worthwhile.
2) Not everyone will always want FQ everywhere. There are potential disadvantanges (e.g. the often mentioned with-a-VPN-I'm-only-1-flow problem). What's necessary is to quantify them - to see how the effect of FQ (or FQ_CoDel's changed FQ) plays out, and you've done a great start there in my opinion.

Cheers,
Michael