[Ecn-sane] rtt-fairness question

Thu Apr 14 17:25:26 EDT 2022

Just indulge me here for a few crazy ideas ;)

> On Apr 14, 2022, at 18:54, David P. Reed <dpreed at deepplum.com> wrote:
> 
> Am I to assume, then, that routers need not pay any attention to RTT to achieve RTT-fairness?

Part of RTT-bias seems caused by the simple fact that tight control loops work better than sloppy ones ;)

There seem to be three ways to try to remedy that to some degree:
1) the daft one:
	define a reference RTT (larger than typically encountered) and have all TCPs respond as if encountering that delay -> until the path RTT exceeds that reference TCP things should be reasonably fair

2) the flows communicate with the bottleneck honestly:
	if flows would communicate their RTT to the bottleneck the bottleneck could partition its resources such that signaling (mark/drop) and puffer size is bespoke per-flow. In theory that can work, but relies on either the RTT information being non-gameably linked to the protocol's operation* or everybody being fully veridical and honest
*) think a protocol that will only work if the best estimate of the RTT is communicated between the two sides continuously

3) the router being verbose:
	If routers communicate the fill-state of their queue (global or per-flow does not matter all that much) flows in theory can do a better job at not putting way too much data in flight remedying the cost of drops/marks that affects high RTT flows more than the shorter ones. (The router has little incentive to lie here, if it wanted to punish a flow it would be easier to simply drop its packets and be done with). 

IMHO 3, while theoretically the least effective of the three is the only one that has a reasonable chance of being employed... or rather is already deployed in the form of ECN (with mild effects).

> How does a server or client (at the endpoint) adjust RTT so that it is fair?

	See 1) above, but who in their right mind would actually implement something like that (TCP Prague did that, but IMHO never in earnest but just to "address" the L4S bullet point RTT-bias reduction). 

> 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.
>  
> 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.
>  
> 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.

	If posed like this, I guess the better question is, what can/should routers be expected to do here: either equitably share their queues or share queue inequitably such that throughput is equitable. From a pure router point of the view the first seems "fairest", but as fq_codel and cake show, within reason equitable capacity sharing is possible (so not perfectly and not for every possible RTT spread).

>  
> 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.

	Theoretically nothing, in reality on a home link FQ+competent AQM goes a long way in that direction.

>  
> 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).

	The typical applies, be suspicious of too round numbers.... 100ms is in no way magic and also not "correct" it is however a decent description of reaction times in a number of perceptul tasks that can be mis-interpreted as showing things like the brain runs at 10Hz or similar...

>  
> 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).
>  
> 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.
>  
> 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.
>  
> So at the very highest level, what is RTT-fairness's objective function optimizing, and how can it work?
>  
> Can it be done without any change to routers?

	Well the goal here seems to undo the RTT-dependence of throughput so a router can equalize per flow throughput and thereby (from its own vantage point) enforce RTT independence, within the amount of memory available. And that already works today for all identifiable flows, but apparently at a computational cost that larger routers do not want to pay. But you knew all that

>  
>  
>  
>  
> On Tuesday, April 12, 2022 3:07pm, "Michael Welzl" <michawe at ifi.uio.no> said:
> 
> 
> 
> 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?
> The spec says it’s optional:  https://www.rfc-editor.org/rfc/rfc9000.html#name-latency-spin-bit
> Otherwise endpoints might just game this if faking their RTT at a router yields an advantage...
> 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. 
>  
> 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
> 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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