[Ecn-sane] rtt-fairness question

Tue Apr 12 11:51:21 EDT 2022

I strongly object to congestion control *in the network* attempting to measure RTT (which is an end-to-end comparative metric). Unless the current RTT is passed in each packet a router cannot enforce fairness. Period. 

Today, by packet drops and fair marking, information is passed to the sending nodes (eventually) about congestion. But the router can't know RTT today.

The result of *requiring* RTT fairness would be to put the random bottleneck router (chosen because it is the slowest forwarder on a contended path) become the endpoint controller.

That's the opposite of an "end-to-end resource sharing protocol".

Now, I'm not saying it is impossible - what I'm saying it is asking all endpoints to register with an "Internet-wide" RTT real-time tracking and control service.

This would be the technical equivalent of an ITU central control point.

So, either someone will invent something I cannot imagine (a distributed, rapid-convergence algortithm that rellects to *every potential user* of a shared router along the current path the RTT's of ALL other users (and potential users).

IMHO, the wish for RTT fairness is like saying that the entire solar system's gravitational pull should be equalized so that all planets and asteroids have fair access to 1G gravity.

On Friday, April 8, 2022 2:03pm, "Michael Welzl" <michawe at ifi.uio.no> said:

Hi,
FWIW, we have done some analysis of fairness and convergence of DCTCP in:
Peyman Teymoori, David Hayes, Michael Welzl, Stein Gjessing: "Estimating an Additive Path Cost with Explicit Congestion Notification", IEEE Transactions on Control of Network Systems, 8(2), pp. 859-871, June 2021. DOI 10.1109/TCNS.2021.3053179
Technical report (longer version):
[ https://folk.universitetetioslo.no/michawe/research/publications/NUM-ECN_report_2019.pdf ]( https://folk.universitetetioslo.no/michawe/research/publications/NUM-ECN_report_2019.pdf )
and there’s also some in this paper, which first introduced our LGC mechanism:
[ https://ieeexplore.ieee.org/document/7796757 ]( https://ieeexplore.ieee.org/document/7796757 )
See the technical report on page 9, section D: a simple trick can improve DCTCP’s fairness  (if that’s really the mechanism to stay with…   I’m getting quite happy with the results we get with our LGC scheme   :-)   )

Cheers,
Michael

On Apr 8, 2022, at 6:33 PM, Dave Taht <[ dave.taht at gmail.com ]( mailto:dave.taht at gmail.com )> wrote:

I have managed to drop most of my state regarding the state of variousdctcp-like solutions. At one level it's good to have not been keepingup, washing my brain clean, as it were. For some reason or another Iwent back to the original paper last week, and have been poundingthrough this one again:Analysis of DCTCP: Stability, Convergence, and Fairness"Instead, we propose subtracting α/2 from the window size for each marked ACK,resulting in the following simple window update equation:One result of which I was most proud recently was of demonstratingperfect rtt fairness in a range of 20ms to 260ms with fq_codel[ https://forum.mikrotik.com/viewtopic.php?t=179307 ]( https://forum.mikrotik.com/viewtopic.php?t=179307 ) )- and I'm prettyinterested in 2-260ms, but haven't got around to it.Now, one early result from the sce vs l4s testing I recall was severelatecomer convergence problems - something like 40s to come into flowbalance - but I can't remember what presentation, paper, or rtt thatwas from. ?Another one has been various claims towards some level of rttunfairness being ok, but not the actual ratio, nor (going up to thepaper's proposal above) whether that method had been tried.My opinion has long been that any form of marking should look moreclosely at the observed RTT than any fixed rate reduction method, andcompensate the paced rate to suit. But that's presently just reducedto an opinion, not having kept up with progress on prague, dctcp-sce,or bbrv2. As one example of ignorance, are 2 packets still paced backto back? DRR++ + early marking seems to lead to one packet beingconsistently unmarked and the other marked.-- I tried to build a better future, a few times:[ https://wayforward.archive.org/?site=https%3A%2F%2Fwww.icei.org ]( https://wayforward.archive.org/?site=https%3A%2F%2Fwww.icei.org )Dave Täht CEO, TekLibre, LLC_______________________________________________Ecn-sane mailing listEcn-sane at lists.bufferbloat.nethttps://lists.bufferbloat.net/listinfo/ecn-sane
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