<html><head><meta http-equiv="Content-Type" content="text/html; charset=utf-8"></head><body style="word-wrap: break-word; -webkit-nbsp-mode: space; line-break: after-white-space;" class="">Question: is QUIC actually using the spin bit as an essential part of the protocol? Otherwise endpoints might just game this if faking their RTT at a router yields an advantage...<br><br>This is why pping's use of tcp timestamps is elegant, little incentive for the endpoints to fudge....<br><br>Regards<br> Sebastian<br><br><br><div class="gmail_quote">On 12 April 2022 18:00:15 CEST, Michael Welzl <michawe@ifi.uio.no> wrote:<blockquote class="gmail_quote" style="margin: 0pt 0pt 0pt 0.8ex; border-left: 1px solid rgb(204, 204, 204); padding-left: 1ex;">
Hi,<div class=""><br class=""></div><div class="">Who or what are you objecting against? At least nothing that I described does what you suggest.</div><div class=""><br class=""></div><div class="">BTW, just as a side point, for QUIC, routers can know the RTT today - using the spin bit, which was designed for that specific purpose.</div><div class=""><br class=""></div><div class="">Cheers,</div><div class="">Michael</div><div class=""><br class=""><div><br class=""><blockquote type="cite" class=""><div class="">On Apr 12, 2022, at 5:51 PM, David P. Reed <<a href="mailto:dpreed@deepplum.com" class="">dpreed@deepplum.com</a>> wrote:</div><br class="Apple-interchange-newline"><div class=""><font face="arial" size="2" class=""><div style="margin: 0px; padding: 0px; font-family: arial; font-size: 10pt; overflow-wrap: break-word;" class="">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. </div><p style="margin:0;padding:0;font-family: arial; font-size: 10pt; overflow-wrap: break-word;" class=""> </p><div style="margin: 0px; padding: 0px; font-family: arial; font-size: 10pt; overflow-wrap: break-word;" class="">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.</div><p style="margin:0;padding:0;font-family: arial; font-size: 10pt; overflow-wrap: break-word;" class=""> </p><div style="margin: 0px; padding: 0px; font-family: arial; font-size: 10pt; overflow-wrap: break-word;" class="">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.</div><p style="margin:0;padding:0;font-family: arial; font-size: 10pt; overflow-wrap: break-word;" class=""> </p><div style="margin: 0px; padding: 0px; font-family: arial; font-size: 10pt; overflow-wrap: break-word;" class="">That's the opposite of an "end-to-end resource sharing protocol".</div><p style="margin:0;padding:0;font-family: arial; font-size: 10pt; overflow-wrap: break-word;" class=""> </p><div style="margin: 0px; padding: 0px; font-family: arial; font-size: 10pt; overflow-wrap: break-word;" class="">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.</div><p style="margin:0;padding:0;font-family: arial; font-size: 10pt; overflow-wrap: break-word;" class=""> </p><div style="margin: 0px; padding: 0px; font-family: arial; font-size: 10pt; overflow-wrap: break-word;" class="">This would be the technical equivalent of an ITU central control point.</div><p style="margin:0;padding:0;font-family: arial; font-size: 10pt; overflow-wrap: break-word;" class=""> </p><div style="margin: 0px; padding: 0px; font-family: arial; font-size: 10pt; overflow-wrap: break-word;" class="">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).</div><p style="margin:0;padding:0;font-family: arial; font-size: 10pt; overflow-wrap: break-word;" class=""> </p><div style="margin: 0px; padding: 0px; font-family: arial; font-size: 10pt; overflow-wrap: break-word;" class="">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.</div><p style="margin:0;padding:0;font-family: arial; font-size: 10pt; overflow-wrap: break-word;" class=""> </p><p style="margin:0;padding:0;font-family: arial; font-size: 10pt; overflow-wrap: break-word;" class=""> </p><div style="margin: 0px; padding: 0px; font-family: arial; font-size: 10pt; overflow-wrap: break-word;" class="">On Friday, April 8, 2022 2:03pm, "Michael Welzl" <<a href="mailto:michawe@ifi.uio.no" class="">michawe@ifi.uio.no</a>> said:<br class=""><br class=""></div>
<div id="SafeStyles1649778161" class="">Hi,
<div class="">FWIW, we have done some analysis of fairness and convergence of DCTCP in:</div>
<div class="">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</div>
<div class="">Technical report (longer version):</div>
<div class=""><a class="" href="https://folk.universitetetioslo.no/michawe/research/publications/NUM-ECN_report_2019.pdf">https://folk.universitetetioslo.no/michawe/research/publications/NUM-ECN_report_2019.pdf</a></div>
<div class="">and there’s also some in this paper, which first introduced our LGC mechanism:</div>
<div class=""><a class="" href="https://ieeexplore.ieee.org/document/7796757">https://ieeexplore.ieee.org/document/7796757</a></div>
<div class="">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 :-) )</div>
<div class=""><br class="">
<div class="">Cheers,</div>
<div class="">Michael</div>
<div class=""><br class="">
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<div class="">On Apr 8, 2022, at 6:33 PM, Dave Taht <<a class="" href="mailto:dave.taht@gmail.com">dave.taht@gmail.com</a>> wrote:</div>
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<div class="">I have managed to drop most of my state regarding the state of various<br class="">dctcp-like solutions. At one level it's good to have not been keeping<br class="">up, washing my brain clean, as it were. For some reason or another I<br class="">went back to the original paper last week, and have been pounding<br class="">through this one again:<br class=""><br class="">Analysis of DCTCP: Stability, Convergence, and Fairness<br class=""><br class="">"Instead, we propose subtracting α/2 from the window size for each marked ACK,<br class="">resulting in the following simple window update equation:<br class=""><br class="">One result of which I was most proud recently was of demonstrating<br class="">perfect rtt fairness in a range of 20ms to 260ms with fq_codel<br class=""><a class="" href="https://forum.mikrotik.com/viewtopic.php?t=179307">https://forum.mikrotik.com/viewtopic.php?t=179307</a> )- and I'm pretty<br class="">interested in 2-260ms, but haven't got around to it.<br class=""><br class="">Now, one early result from the sce vs l4s testing I recall was severe<br class="">latecomer convergence problems - something like 40s to come into flow<br class="">balance - but I can't remember what presentation, paper, or rtt that<br class="">was from. ?<br class=""><br class="">Another one has been various claims towards some level of rtt<br class="">unfairness being ok, but not the actual ratio, nor (going up to the<br class="">paper's proposal above) whether that method had been tried.<br class=""><br class="">My opinion has long been that any form of marking should look more<br class="">closely at the observed RTT than any fixed rate reduction method, and<br class="">compensate the paced rate to suit. But that's presently just reduced<br class="">to an opinion, not having kept up with progress on prague, dctcp-sce,<br class="">or bbrv2. As one example of ignorance, are 2 packets still paced back<br class="">to back? DRR++ + early marking seems to lead to one packet being<br class="">consistently unmarked and the other marked.<br class=""><br class="">-- <br class="">I tried to build a better future, a few times:<br class=""><a class="" href="https://wayforward.archive.org/?site=https%3A%2F%2Fwww.icei.org">https://wayforward.archive.org/?site=https%3A%2F%2Fwww.icei.org</a><br class=""><br class="">Dave Täht CEO, TekLibre, LLC<br class="">_______________________________________________<br class="">Ecn-sane mailing list<br class=""><a href="mailto:Ecn-sane@lists.bufferbloat.net" class="">Ecn-sane@lists.bufferbloat.net</a><br class="">https://lists.bufferbloat.net/listinfo/ecn-sane</div>
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