[Ecn-sane] rtt-fairness question

Thu Apr 14 13:08:21 EDT 2022

I guess to try and clarify, the objective function here is fq_codel on
the bottleneck link, which,
starting with exceeding the 5ms target threshold consistently, hammers
down on drops starting after 100ms on an invsqrt interval until it
finds the right drop rate for the rtt to fill the pipe and not the
queue.

It is NOT aware of the actual RTT, there are no bits in the header
inspected, the flow IS isolated by the 5 tuple. Very long thread with
a ton of graphs (if you login) here:

https://forum.mikrotik.com/viewtopic.php?t=179307

On Thu, Apr 14, 2022 at 9:54 AM David P. Reed <dpreed at deepplum.com> wrote:
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> Am I to assume, then, that routers need not pay any attention to RTT to achieve RTT-fairness?
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> How does a server or client (at the endpoint) adjust RTT so that it is fair?
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> Now RTT, technically, is just the sum of the instantaneous queue lengths in bytes along the path and the reverse path, plus a fixed wire-level delay. And routers along any path do not have correlated queue sizes.
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> It seems to me that RTT adjustment requires collective real-time cooperation among all-or-most future users of that path.  The path is partially shared by many servers and many users, none of whom directly speak to each other.
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> And routers have very limited memory compared to their throughput-RTdelay product. So calculating the RTT using spin bits and UIDs for packets seems a bit much to expect all routers to do.

I don't have much of an opinion of the spin bit. I was originally
describing achieving near-perfect
RTT fairness using cake in the real world on the string of tests on
the link above, in a range of 20ms to 260ms. So we are having two
different conversations here.

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> So, what process measures the cross-interactions among all the users of all the paths, and what control-loop (presumably stable and TCP-compatible) actually converges to RTT fairness IRL.

I am temped to fork this in light of the sqm-autorate work, but let's
stick to two confusing conflations at a time. I was talking about
fq_codels interactions with tcp's control loops. Michael, quic.

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> Today, the basis of congestion control in the Internet is that each router is a controller of all endpoint flows that share a link, and each router is free to do whatever it takes to reduce its queue length to near zero as an average on all timescales larger than about 1/10 of a second (a magic number that is directly derived from measured human brain time resolution).

Actually I thought the 100ms came from the coast-to-coast of the USA
in the 70s, much like ATM had cell sizes of 48 bytes (USA) vs 32 bytes
(europe) that were optimal for typical distances here, in order to
fill a BDP for a single reno-style flow.

As this starting limit is disconcerting for videoconfernecing and
voice traffic I've often thought that starting to actively manage a
queue at 60ms would be better more generally, and in fact a cap on
fifos of that much due to the stanford thinking of sqrt flows bdp. I
don't care all that much about single flow utilization, but about
interactive voice/video conversations.

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> So, for any two machines separated by less than 1/10 of a light-second in distance, the total queueing delay has to stabilize in about 1/10 of a second. (I'm using a light-second in a fiber medium, not free-space, as the speed of light in fiber is a lot slower than the speed of light on microwaves, as Wall Street has recently started recoginizing and investing in).
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> I don't see how RTT-fairness can be achieved by some set of bits in the IP header. You can't shorten RTT below about 2/10 of a second in that desired system state. You can only "lengthen" RTT by delaying packets in source or endpoint buffers, because it's unreasonable to manage all the routers.
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> And the endpoints that share a path can't talk to each other and reach a decision in on the order of 2/10 of a second.
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> So at the very highest level, what is RTT-fairness's objective function optimizing, and how can it work?
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> Can it be done without any change to routers?
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> On Tuesday, April 12, 2022 3:07pm, "Michael Welzl" <michawe at ifi.uio.no> said:
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> On Apr 12, 2022, at 8:52 PM, Sebastian Moeller <moeller0 at gmx.de> wrote:
> Question: is QUIC actually using the spin bit as an essential part of the protocol?
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> The spec says it’s optional:  https://www.rfc-editor.org/rfc/rfc9000.html#name-latency-spin-bit
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> Otherwise endpoints might just game this if faking their RTT at a router yields an advantage...
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> This was certainly discussed in the QUIC WG. Probably perceived as an unclear incentive, but I didn’t really follow this.
> Cheers,
> Michael
>
> This is why pping's use of tcp timestamps is elegant, little incentive for the endpoints to fudge....
>
> Regards
> Sebastian
>
>
> On 12 April 2022 18:00:15 CEST, Michael Welzl <michawe at ifi.uio.no> wrote:
>>
>> Hi,
>> Who or what are you objecting against?   At least nothing that I described does what you suggest.
>> 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.
>> Cheers,
>> Michael
>>
>>
>> On Apr 12, 2022, at 5:51 PM, David P. Reed <dpreed at deepplum.com> wrote:
>> 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.
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>>
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>> 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.
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>> 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.
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>> That's the opposite of an "end-to-end resource sharing protocol".
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>> 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.
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>> This would be the technical equivalent of an ITU central control point.
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>> 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).
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>> 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.
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>> 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
>> and there’s also some in this paper, which first introduced our LGC mechanism:
>> 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> wrote:
>> I have managed to drop most of my state regarding the state of various
>> dctcp-like solutions. At one level it's good to have not been keeping
>> up, washing my brain clean, as it were. For some reason or another I
>> went back to the original paper last week, and have been pounding
>> through 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 demonstrating
>> perfect rtt fairness in a range of 20ms to 260ms with fq_codel
>> https://forum.mikrotik.com/viewtopic.php?t=179307 )- and I'm pretty
>> interested in 2-260ms, but haven't got around to it.
>>
>> Now, one early result from the sce vs l4s testing I recall was severe
>> latecomer convergence problems - something like 40s to come into flow
>> balance - but I can't remember what presentation, paper, or rtt that
>> was from. ?
>>
>> Another one has been various claims towards some level of rtt
>> unfairness being ok, but not the actual ratio, nor (going up to the
>> paper's proposal above) whether that method had been tried.
>>
>> My opinion has long been that any form of marking should look more
>> closely at the observed RTT than any fixed rate reduction method, and
>> compensate the paced rate to suit. But that's presently just reduced
>> to an opinion, not having kept up with progress on prague, dctcp-sce,
>> or bbrv2. As one example of ignorance, are 2 packets still paced back
>> to back? DRR++ + early marking seems to lead to one packet being
>> consistently 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
>>
>> Dave Täht CEO, TekLibre, LLC
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>
> --
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>
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-- 
I tried to build a better future, a few times:
https://wayforward.archive.org/?site=https%3A%2F%2Fwww.icei.org

Dave Täht CEO, TekLibre, LLC