[nwcrg] Fwd: [iccrg] Very fundamental congestion control questions...
Marie-Jose Montpetit <marie@mjmontpetit.com> Mon, 18 February 2019 13:54 UTC
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From: Marie-Jose Montpetit <marie@mjmontpetit.com>
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Date: Mon, 18 Feb 2019 08:54:13 -0500
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Subject: [nwcrg] Fwd: [iccrg] Very fundamental congestion control questions...
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FYI. This is a discussion a lot of us have had in the past. mjm Marie-Jose Montpetit, Ph.D. mariejo@mit.edu marie@mjmontpetit.com +1-781-526-2661 @SocialTVMIT > Begin forwarded message: > > From: "Bless, Roland (TM)" <roland.bless@kit.edu> > Subject: [iccrg] Very fundamental congestion control questions... > Date: February 18, 2019 at 8:50:22 AM EST > To: "iccrg@irtf.org" <iccrg@irtf.org> > > Hi all, > > I think I can second Keith Winstein's statement > "we lack a real understanding and common language about > some of the fundamental concepts and hard parts..." > (https://mailarchive.ietf.org/arch/msg/iccrg/LHRAWIjbFktaqLC5hFqgG37M9zk) > and want to open a > new discussion thread for this. > > One of the problems that we have is (IMHO), that there > is currently no commonly agreed definition of what > _congestion_ is actually. So every congestion control > developing people should at least have clear answers > for themselves to these questions: > > 1) what is congestion (or congestion situation)? > > 2) how to detect a congestion situation > (or even severity levels of congestion)? > > 3) how to react on a detected congestion situation? > > Letting the related fairness discussion aside, I think that even > RFCs do not give a clear definition of what congestion is and > the views, Internet usage, and requirements have considerably > changed over time. > > So in former times, the term congestion was mainly > driven by the congestion collapse experience. > Consequently, most RFCs refer to the prevention of > that congestion collapse when discussion congestion control > (or congestion avoidance), e.g., RFC 7567: > "It is primarily these TCP congestion avoidance > algorithms that prevent the congestion collapse of today's Internet." > > Nowadays, the interest has probably shifted towards > systematically reducing queueing delay and congestion could > be seen as being related to the level of waiting time in > bottleneck queues (this would also allow for the definition > of different levels of congestion). > > While the analogy to ordinary car traffic scenarios is not always > right, in this case I find it useful: you do not consider the > waiting time in front of traffic lights as congestion > unless you are waiting more than x min in addition > to what you normally experience during non-congested > times (e.g., waiting two traffic light phases is probably > considered being okay, but more than that may be already > viewed as being traffic congestion). > However, both traffic broadcast and navigation systems > nowadays announce congestion by indication of expected > additional waiting/queueing time (which is more useful than > just mentioning the length of the congestion in kilometers). > In this regard, I would heavily recommend to reread > Kathie's and Van's "Controlling Queue Delay" Paper: > https://queue.acm.org/detail.cfm?id=2209336 > and the introduction of good/bad queues, standing queues, > and sojourn times, which could be quite helpful in the > congestion definition discussion. > > Similarly, the answer to 2) should not be "packet loss" > alone anymore, but probably trying to distinguish > between congestion induced packet loss and short-term (burst) > loss that is not caused by a real congestion situation. > So in lack of ECN or other explicit congestion signals, > multiple measured parameters may be combined for better > congestion detection than relying on packet loss alone. > > I think it could be useful to discuss a bit along > the above listed questions in order to move CC research > forward by probably achieving a better understanding and > common language about the fundamental concepts... :-) > > Regards > Roland > > _______________________________________________ > iccrg mailing list > iccrg@irtf.org > https://www.irtf.org/mailman/listinfo/iccrg
- [nwcrg] Fwd: [iccrg] Very fundamental congestion … Marie-Jose Montpetit