Re: [tcpPrague] A Congestion Control Independent L4S Scheduler - TCP Prague preliminary results

["De Schepper, Koen (Nokia - BE/Antwerp)" <[e-mail]>]

Hi Szilveszter,

Interesting work, thanks for sharing. There is indeed a very big difference between how DCTCP behaves today in recent kernels, than what we used in the original L4S work testing (Linux kernel 3.19). That original version was a "clean" DCTCP version that matched very well the theoretical equations: r=2/(p.RTT) for uniform stable random and r=2/(p².RTT) on/off marking. Due to many interactions, integer range constraints, pacing/GSO interactions and bugs, the performance really degraded in the recent Linux versions. These issues were fixed within the Linux Prague version, so that is why Prague should perform "better" in respect to matching the equations. It would be interesting to get an idea of the deviation from the r=2/(p.RTT) equation in your tests (relevant in DualPI2 as the coupling gives a smooth probability). Collecting the average RTT (base + queuing latency), marking probability, and rate would make it possible to evaluate this.

Main reasons why we saw deviations is when the probability varies a lot in time. As you might know DCTCP's (and Prague's) equation becomes r=2/(p².RTT) when it receives on/off marking episodes (on episodes in the order of 1 RTT, off in the order of multiple RTTs). Any not so stable marking probability results in a rate between those 2 boundaries. This is the theory, looking forward on how your practice matches this.

>From a first quick look at the results it might not deviate that much, I guess. The more aggressive part is probably due to the RTT unfairness: 1 to 5 rate ratio (with a 5ms base RTT, Classic gets a queue of 5ms+20ms target, so about 25ms and 5 times less throughput). Did you try to use the RTT independent version? If you set it to the f=max(15, RTT) mode, the minimum effective RTT becomes 15ms, so the rate ratio is 3 to 5 in that case.

Thanks and Regards,
Koen.

From: tcpPrague <[e-mail]> On Behalf Of Szilveszter Nadas
Sent: Tuesday, November 3, 2020 6:30 PM
To: [e-mail]
Cc: Ferenc Fejes <[e-mail]>; Gombos Gergo <[e-mail]>; LAKI Sandor <[e-mail]>
Subject: [tcpPrague] A Congestion Control Independent L4S Scheduler - TCP Prague preliminary results

Hi all,

During the review of our article "A Congestion Control Independent L4S Scheduler" we received comments from one of the reviewers, on why we did not also evaluate TCP Prague. So now we rerun the test cases with replacing DCTCP to TCP Prague.

The results are at the below link. We are somewhat surprised by much increased aggressiveness of TCP Prague. Can you comment on it? The settings and setup we used are described in the pdf (and in the original article).
Results: http://ppv.elte.hu/tcp-prague/
Original article (including YouTube presentation): http://ppv.elte.hu/cc-independent-l4s/

Can you comment on the results and the settings we used?

Cheers,
Szilveszter