"don't overcommit" includes the common practice of setting very large limits on the client side, where in aggregate the case of server being flow control limited is effectively non-existent.
I am curious to hear clarification of the precise definition of MP-HoL blocking here. is it not flow control, but rather path aliasing where distinct paths are actually sharing some physical link(s)? On Fri, Jul 16, 2021 at 12:13 PM Roberto Peon <[email protected]> wrote: > I too am curious! > There are only two ways to handle flow control—overcommit, or don’t > overcommit. > > The “don’t overcommit” choice leads to blocking, since any of that > resource allocated to one path can’t be used by the other. > > The “overcommit” choice either leads to OOM, or throwing out some > successfully transmitted and received data. > > > Underlying this is a fun question: Which inefficiency is worse? Not using > resources that should be used (i.e. from choosing to not overcommit), or > sometimes redundantly using a resource (from choosing to overcommit)? > I’m curious too about what implementation strategies we end up doing in > general around this, and.. if enough implementations are choosing > overcommit, if we need some different protocol mechanisms to bound the > redundancy? > -=R > > > > *From: *QUIC <[email protected]> on behalf of Mirja Kuehlewind > <[email protected]> > *Date: *Friday, July 16, 2021 at 6:15 AM > *To: *"Ma, Yunfei" <[email protected]>, Robin > MARX <[email protected]>, Yanmei Liu <[email protected]> > *Cc: *"matt.joras" <[email protected]>, 李振宇 <[email protected]>, > Christian Huitema <[email protected]>, "lucaspardue.24.7" < > [email protected]>, quic <[email protected]>, Qing An < > [email protected]> > *Subject: *Re: Multi-path QUIC Extension Experiments > > > > Hi Yunfei, > > > > thanks as well for you sharing your results! Can you explain even a bit > more what you mean by MP-HoL Blocking? Is this because of the flow control > limits? If so wouldn’t it make sense to reserve a certain “space” for each > path? > > > > Mirja > > > > > > *From: *QUIC <[email protected]> on behalf of "Ma, Yunfei" <yunfei.ma= > [email protected]> > *Date: *Thursday, 15. July 2021 at 04:18 > *To: *Robin MARX <[email protected]>, Yanmei Liu < > [email protected]> > *Cc: *"matt.joras" <[email protected]>, 李振宇 <[email protected]>, > Christian Huitema <[email protected]>, "lucaspardue.24.7" < > [email protected]>, quic <[email protected]>, Qing An < > [email protected]> > *Subject: *Re: Re: Multi-path QUIC Extension Experiments > > > > Hi Robin, > > > > Thanks so much for your questions! > > > > First, the head of line > blocking discussed here is called multi-path head-of-line blocking or MP-HoL > blocking, and its root cause is quite different from the stream HoL blocking > usually discussed in > QUICv1. The MP-HoL blocking happens when one path blocks the other path, not > when one stream blocks the other stream. Please note that we indeed > use multiple streams, for example, different video requests are carried in > different QUIC streams. QUIC’s stream multiplexing ability and its benefits > still hold in this scenario. > > > > Second, regarding packet scheduling mode, > right now, in our Taobao A/B test, we transmit packets on multiple paths > simultaneously. However, you can definitely use > traffic switching only and choose to switch when one path could not meet > your bandwidth requirement. Basically, if you use multiple paths > simultaneously, you get the most elasticity from a resource pooling > perspective. > It really comes down on what your application needs. We will also update the > packet scheduling section > soon in a newer version of the > draft, in which we plan to include more discussions on the packet scheduling > policy. > > > > Third, regarding the benefits of more bandwith versus the "downsides". > Whether you want more bandwidth depends on your application. For videos, yes, > more bandwidth is > extremely helpful in improving the long tail QoE, which is an important > target for Taobao. We find multi-path QUIC helps us improve two important > metrics, rebuffer rate and video start-up delays. > In the past, if you work on multi-path scheduling that does not collaborate > close enough with applications such as MPTCP, the MP-HoL blocking becomes > the downside that cripples the > performance. However, the user space nature of QUIC provides us the > opportunity to solve this problem, > so now our conclusion is that > you can enjoy the benefits of more bandwidth and more reliable connectivity > from multi-path without much of the “downsides”. > > > > I hope my answer is helpful, but feel free to let me know if you have any > additional comments. > > > > Cheers, > > Yunfei > > > > from Alimail macOS > <https://protect2.fireeye.com/v1/url?k=7cc82aa7-2353138a-7cc86a3c-8692dc8284cb-e08a325a5c75cf95&q=1&e=de295b4f-9105-4e32-980f-779c711eaa62&u=https://mail.alibaba-inc.com/> > > ------------------Original Mail ------------------ > > *Sender:*Robin MARX <[email protected]> > > *Send Date:*Wed Jul 14 07:39:37 2021 > > *Recipients:*Yanmei Liu <[email protected]> > > *CC:*quic <[email protected]>, Ma, Yunfei <[email protected]>, > Christian Huitema <[email protected]>, Qing An < > [email protected]>, 李振宇 <[email protected]>, matt.joras < > [email protected]>, lucaspardue.24.7 <[email protected]> > > *Subject:*Re: Multi-path QUIC Extension Experiments > > Hello Yanmei, > > > > Thanks for the additional results on an interesting topic. I'm looking > forward to reading the SIGCOMM paper. > > > > I was a bit surprised to (apparently) see HOL blocking mentioned as a > major issue, as that's one of the things QUIC aims to be better at than TCP. > > It's a bit difficult to understand from the slides, but it seems like > you're sending packets for a single stream (Stream ID 1 in the diagrams) on > both the slow and fast path, which would indeed induce HOL blocking. > > Consequently, I was wondering what the practical reasons are for you to > multiplex packets for a single stream over multiple paths, as opposed to > for example attaching a single stream to a single path (say: high priority > streams use the fast path for all their packets). > > > > I see this mentioned a bit in the draft under "packet scheduling", where > it talks about switching paths once the cwnd is full for one. That indeed > leads to the behaviour seen in the slides, but that's my question: why > would you take those approaches then? > > Are there so many cases where the additional "bandwidth" from using > multiple path's cwnd for a single stream outweigh the downsides of HOL > blocking? Relatedly: what are the packet loss rates you've observed on > real networks? > > Have you experimented with e.g., tying streams to paths more closely? Does > that work better or worse? Why? > > > > I'm mainly wondering how these tradeoffs evolve depending on the type of > paths available and if it's possible to make a model to drive this logic. > > I assume there is much existing work on this for MPTCP, but I also assume > some of that changes due to QUIC's independent streams / stream > prioritization flexibility. > > > > Thank you in advance and with best regards, > > Robin > > > > > > On Sun, 11 Jul 2021 at 20:48, Yanmei Liu <miaoji.lym= > [email protected]> wrote: > > Hi everyone, > > We have finished some experiments about deploying multi-path quic > extension(https://datatracker.ietf.org/doc/draft-liu-multipath-quic/) in > Alibaba Taobao short-form video streaming, and the experiment results are > concluded in the slides (attached file). > If anyone is interested in the experimental details about multi-path quic, > please let us know. > All the feedbacks and suggestions are appreciated! > > Best regards, > Yanmei > > > > > -- > > > > *dr. Robin Marx* > > Postdoc researcher - Web protocols > > Expertise centre for Digital Media > > > > *Cellphone *+32(0)497 72 86 94 > > > > www.uhasselt.be > <https://protect2.fireeye.com/v1/url?k=37557dd4-68ce44f9-37553d4f-8692dc8284cb-fe608437d16ed9d9&q=1&e=de295b4f-9105-4e32-980f-779c711eaa62&u=http://www.uhasselt.be/> > > Universiteit Hasselt - Campus Diepenbeek > > Agoralaan Gebouw D - B-3590 Diepenbeek > > Kantoor EDM-2.05 > > > > [image: Image removed by sender.] > > > >
