Re: [aqm] AQM schemes: Queue length vs. delay based

[Michael Welzl <[e-mail]>]

Hi,

Interesting discussion!

I would like to say that in my opinion the real value of what Naeem has presented is not so much in pushing ARED, but in:

- reminding people that there was AQM stuff done between RED and CoDel, and some of this is worth looking at. Rong, we're aware of at least some of your previous work, and I'm personally a fan of CHOKe. However I do understand that CHOKe is probably not appropriate for the problem we're trying to solve here ... but you know, there were lots and lots of others made (e.g. AVQ, just to toss another name of an AQM that seemed to be well done to me when I read about it all these years ago)

- hopefully helping set the bar a bit higher regarding evaluations. I'm not saying we did enough (e.g. we didn't get to look at anything else than bulk TCP transfers yet), but I think we've done more than I have seen documented elsewhere so far (apologies if we missed a study; we tried to cover them in the table in our tech. rep.).


I just personally found it a bit pathetic to first see CoDel and PIE presented with comparisons against only themselves and RED, and then see that the more-than-a-decade-old-ARED very often performs better than both of them. It makes me wonder how many other 5-10 year old AQMs are at least as good as CoDel and PIE.


About this:


> RP: If low latencies are achieved at the cost of losing quite a bit of throughput, it is a no-starter for AQM design. 

That's rather obvious  :-)    but it's about the relationship between the two. e.g., the evaluations in the original CoDel paper essentially showed that CoDel gave higher throughput than RED at the expense of more latency. That's not exactly convincing either, given that latency reduction is the main goal we're pursuing now. And for ARED, we're only talking about a subset of scenarios (and perhaps not even the most relevant ones -  e.g. do we really care that much about the throughput of one single TCP connection?). Let's not forget that in many scenarios that we looked at (in fact, I'd say this was the majority of cases) it had better throughput AND lower delay than CoDel and PIE. I would also venture to guess that the problem with low multiplexing cases could be addressed relatively easily.

Naeem and Preethi have agreed that some significant design changes would be necessary to make use of ARED as a new delay-based AQM. I agree, but mainly I'd expect these significant changes to deal with ECN (where we made the point that ALL AQMs should be changed in fact (*), but that's another story) and with WLAN scenarios, where ARED performed quite badly. We'll see - mainly we conclude that it's probably worth playing around with it.

Cheers,
Michael