[Bloat] [iccrg] Musings on the future of Internet Congestion Control

[Michael Welzl]

Hi Sebastian,

Neither our paper nor me are advocating one particular solution - we point at a problem and suggest that research on ways to solve the under-utilization problem might be worthwhile.
Jumping from this to discussing the pro’s and con’s of a potential concrete solution is quite a leap…

More below:


> On Jul 10, 2022, at 11:29 PM, Sebastian Moeller <moeller0 at gmx.de> wrote:
> 
> Hi Michael,
> 
> 
>> On Jul 10, 2022, at 22:01, Michael Welzl <michawe at ifi.uio.no> wrote:
>> 
>> Hi !
>> 
>> 
>>> On Jul 10, 2022, at 7:27 PM, Sebastian Moeller <moeller0 at gmx.de> wrote:
>>> 
>>> Hi Michael,
>>> 
>>> so I reread your paper and stewed a bit on it.
>> 
>> Many thanks for doing that! :)
>> 
>> 
>>> I believe that I do not buy some of your premises.
>> 
>> you say so, but I don’t really see much disagreement here. Let’s see:
>> 
>> 
>>> e.g. you write:
>>> 
>>> "We will now examine two factors that make the the present situation particularly worrisome. First, the way the infrastructure has been evolving gives TCP an increasingly large operational space in which it does not see any feedback at all. Second, most TCP connections are extremely short. As a result, it is quite rare for a TCP connection to even see a single congestion notification during its lifetime."
>>> 
>>> And seem to see a problem that flows might be able to finish their data transfer business while still in slow start. I see the same data, but see no problem. Unless we have an oracle that tells each sender (over a shared bottleneck) exactly how much to send at any given time point, different control loops will interact on those intermediary nodes.
>> 
>> You really say that you don’t see the solution. The problem is that capacities are underutilized, which means that flows take longer (sometimes, much longer!) to finish than they theoretically could, if we had a better solution.
> 
> 	[SM] No IMHO the underutilization is the direct consequence of requiring a gradual filling of the "pipes" to probe he available capacity. I see no way how this could be done differently with the traffic sources/sinks being uncoordinated entities at the edge, and I see no way of coordinating all end points and handle all paths. In other words, we can fine tune a parameters to tweak the probing a bit, make it more or less aggressive/fast, but the fact that we need to probe capacity somehow means underutilization can not be avoided unless we find a way of coordinating all of the sinks and sources. But being sufficiently dumb, all I can come up with is an all-knowing oracle or faster than light communication, and neither strikes me to be realistic ;)

There’s quite a spectrum of possibilities between an oracle or “coordinating all of the sinks and sources” on one hand, and quite “blindly” probing from a constant IW on the other. The “fine tuning” that you mention is interesting research, IMO!


>>> I might be limited in my depth of thought here, but having each flow probing for capacity seems exactly the right approach... and doubling CWND or rate every RTT is pretty aggressive already (making slow start shorter by reaching capacity faster within the slow-start framework requires either to start with a higher initial value (what increasing IW tries to achieve?) or use a larger increase factor than 2 per RTT). I consider increased IW a milder approach than the alternative. And once one accepts that a gradual rate increasing is the way forward it falls out logically that some flows will finish before they reach steady state capacity especially if that flows available capacity is large. So what exactly is the problem with short flows not reaching capacity and what alternative exists that does not lead to carnage if more-aggressive start-up phases drive the bottleneck load into emergency drop territory?
>> 
>> There are various ways to do this

[snip: a couple of concrete suggestions from me, and answers about what problems they might have, with requests for references from you]

I’m sorry, but I wasn’t really going to have a discussion about these particular possibilities. My point was only that many possible directions exist - being completely “blind” isn’t the only possible approach.
Instead of answering your comments to my suggestions, let me give you one single concrete piece here: our reference 6, as one example of the kind of resesarch that we consider worthwhile for the future:

"X. Nie, Y. Zhao, Z. Li, G. Chen, K. Sui, J. Zhang, Z. Ye, and D. Pei, “Dynamic TCP initial windows and congestion control schemes through reinforcement learning,” IEEE JSAC, vol. 37, no. 6, 2019.”
https://1989chenguo.github.io/Publications/TCP-RL-JSAC19.pdf

This work learns a useful value of IW over time, rather than using a constant. One author works at Baidu, the paper uses data from Baidu, and it says:
"TCP-RL has been deployed in one of the top global search engines for more than a year. Our online and testbed experiments show that for short flow transmission, compared with the common practice of IW = 10, TCP-RL can reduce the average transmission time by 23% to 29%.”

- so it’s probably fair to assume that this was (and perhaps still is) active in Baidu.


>>> And as an aside, a PEP (performance enhancing proxy) that does not enhance performance is useless at best and likely harmful (rather a PDP, performance degrading proxy).
>> 
>> You’ve made it sound worse by changing the term, for whatever that’s worth. If they never help, why has anyone ever called them PEPs in the first place?
> 
> 	[SM] I would guess because "marketing" was unhappy with "engineering" emphasizing the side-effects/potential problems and focussed in the best-case scenario? ;)

It appears that you want to just ill-talk PEPs. There are plenty of useful things that they can do and yes, I personally think they’re the way of the future - but **not** in their current form, where they must “lie” to TCP, cause ossification, etc.  PEPs have never been considered as part of the congestion control design - when they came on the scene, in the IETF, they were despised for breaking the architecture, and then all the trouble with how they need to play tricks was discovered (spoofing IP addresses, making assumptions about header fields, and whatnot). That doesn’t mean that a very different kind of PEP - one which is authenticated and speaks an agreed-upon protocol - couldn’t be a good solution.

You’re bound to ask me for concrete things next, and if I give you something concrete (e.g., a paper on PEPs), you’ll find something bad about it - but this is not a constructive direction of this conversation. Please note that I’m not saying “PEPs are always good”: I only say that, in my personal opinion, they’re a worthwhile direction of future research. That’s a very different statement.


>> Why do people buy these boxes?
> 
> 	[SM] Because e.g. for GEO links, latency is in a range where default unadulterated TCP will likely choke on itself, and when faced with requiring customers to change/tune TCPs or having "PEP" fudge it, ease of use of fudging won the day. That is a generous explanation (as this fudging is beneficial to both the operator and most end-users), I can come up with less charitable theories if you want ;) .
> 
>>> The network so far has been doing reasonably well with putting more protocol smarts at the ends than in the parts in between.
>> 
>> Truth is, PEPs are used a lot: at cellular edges, at satellite links… because the network is *not* always doing reasonably well without them.
> 
> 	[SM] Fair enough, I accept that there are use cases for those, but again, only if the actually enhance the "experience" will users be happy to accept them.

… and that’s the only reason to deploy them, given that (as the name suggests) they’re meant to increase performance. I’d be happy to learn more about why you appear to hate them so much (even just anecdotal).


> The goals of the operators and the paying customers are not always aligned here, a PEP might be advantageous more to the operator than the end-user (theoretically also the other direction, but since operators pay for PEPs they are unlikely to deploy those) think mandatory image recompression or forced video quality downscaling.... (and sure these are not as clear as I pitched them, if after an emergency a PEP allows most/all users in a cell to still send somewhat degraded images that is better than the network choking itself with a few high quality images, assuming images from the emergency are somewhat useful).

What is this, are you inventing a (too me, frankly, strange) scenario where PEPs do some evil for customers yet help operators, or is there an anecdote here?


>>> I have witnessed the arguments in the "L4S wars" about how little processing one can ask the more central network nodes perform, e.g. flow queueing which would solve a lot of the issues (e.g. a hyper aggressive slow-start flow would mostly hurt itself if it overshoots its capacity) seems to be a complete no-go.
>> 
>> That’s to do with scalability, which depends on how close to the network’s edge one is.
> 
> 	[SM] I have heard the alternative that it has to do with what operators of core-links request from their vendors and what features they are willing to pay for... but this is very anecdotal as I have little insight into big-iron vendors or core-link operators. 
> 
>>> I personally think what we should do is have the network supply more information to the end points to control their behavior better. E.g. if we would mandate a max_queue-fill-percentage field in a protocol header and have each node write max(current_value_of_the_field, queue-filling_percentage_of_the_current_node) in every packet, end points could estimate how close to congestion the path is (e.g. by looking at the rate of %queueing changes) and tailor their growth/shrinkage rates accordingly, both during slow-start and during congestion avoidance.
>> 
>> That could well be one way to go. Nice if we provoked you to think!
> 
> 	[SM] You mostly made me realize what the recent increases in IW actually aim to accomplish ;)

That’s fine!  Increasing IW is surely a part of the solution space - though I advocate doing something else (as in the example above) than just to increase the constant in a worldwide standard.


> and that current slow start seems actually better than its reputation; it solves a hard problem surprisingly well.

Actually, given that the large majority of flows end somewhere in slow start, what makes you say that it solves it “well”?


> The max(pat_queue%) idea has been kicking around in my head ever since reading a paper about storing queue occupancy into packets to help CC along (sorry, do not recall the authors or the title right now) so that is not even my own original idea, but simply something I borrowed from smarter engineers simply because I found the data convincing and the theory sane. (Also because I grudgingly accept that latency increases measured over the internet are a tad too noisy to be easily useful* and too noisy for a meaningful controller based on the latency rate of change**)
> 
>>> But alas we seem to go the path of a relative dumb 1 bit signal giving us an under-defined queue filling state instead and to estimate relative queue filling dynamics from that we need many samples (so literally too little too late, or L3T2), but I digress.
>> 
>> Yeah you do :-)
> 
> 	[SM] Less than you let on ;). If L4S gets ratified

[snip]

I’m really not interested in an L4S debate.

Cheers,
Michael