Re: [tcpm] Thank you for the QUIC session in tcpm
Jana Iyengar <jri.ietf@gmail.com> Thu, 15 November 2018 04:18 UTC
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From: Jana Iyengar <jri.ietf@gmail.com>
Date: Thu, 15 Nov 2018 09:47:47 +0530
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To: eric.dumazet@gmail.com
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Subject: Re: [tcpm] Thank you for the QUIC session in tcpm
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Thanks for the clarification, Eric. I'm a bit surprised to see srtt be used by both Cubic and BBR, but that discussion's not for this thread. On Wed, Nov 14, 2018 at 9:44 PM Eric Dumazet <eric.dumazet@gmail.com> wrote: > > > On 11/12/2018 11:13 PM, Jana Iyengar wrote: > > On what to do with an unconfirmed RTO: So, my thought is that if you've > hit an RTO, it's been a REALLY LONG time, because we've gone through TLP(s) > by now and failed. (This might change if we decide to combine TLP and RTO, > but that's for a later discussion.) It's not fully clear to me what the > right responses here are, but some thoughts follow. > > > > 1/ If this was an actual increase in the RTT, it would naturally roll > into the SRTT and RTTVAR. We might even consider resetting the RTT > estimator, but we already know that the EWMA estimator we love takes just a > while to catch on, and I don't want to litigate changing the estimator > fundamentally here. > > > > 2/ Linux fq uses min_rtt IIRC, which shouldn't change as a result of an > increase in RTT. This *might* be a good time to kick the min_rtt estimator > to pace more conservatively. > > linux fq does not use min_rtt, but the pacing rate set by TCP. > > pacing rate for Cubic is using SRTT ( > https://elixir.bootlin.com/linux/latest/source/net/ipv4/tcp_input.c#L806 ) > > For BBR, pacing rate depends on various factors and phases, but SRTT is > used, not min_rtt > > https://elixir.bootlin.com/linux/latest/source/net/ipv4/tcp_bbr.c#L231 > > > > > > 3/ I'm not convinced that it makes sense to change the cwnd. It seems > clear that an RTT increase with the same BW should increase the cwnd if > anything, so it seems reasonable to continue with the same cwnd at least. > If the BW changed, hopefully that'll be reflected in a congestion signal, > but in the absence of a congestion signal, I think we should keep the cwnd > as is. > > > > > > On Tue, Nov 13, 2018 at 12:35 AM Praveen Balasubramanian < > pravb@microsoft.com <mailto:pravb@microsoft.com>> wrote: > > > > Yuchung this might be worth experimenting with and getting more data > on. Maybe there should be some reaction to an unconfirmed RTO even if it is > not reduction all the way to LW? I like Jana's suggestion that the > implementation SHOULD drop the cwnd even for unconfirmed RTO if not pacing. > > > > I checked the latest draft and pacing is a RECOMMENDED. I am > noticing that transport algorithms in Linux and in QUIC are evolving > assuming pacing which is a function usually implemented and configured > outside of the transport.. So the transport is potentially making an > assumption which may result in safety issues. For example BBR congestion > control may not work very well if pacing was disabled or misconfigured? I > don’t know of any robust solution to this other than build pacing into the > transport itself. Pacing is also very challenging for low latency flows > because of lack of support for fine grained timers. > > > > -----Original Message----- > > From: Yuchung Cheng <ycheng@google.com <mailto:ycheng@google.com>> > > Sent: Monday, November 12, 2018 8:37 AM > > To: Jana Iyengar <jri.ietf@gmail.com <mailto:jri.ietf@gmail.com>> > > Cc: Praveen Balasubramanian <pravb@microsoft.com <mailto: > pravb@microsoft.com>>; Bob Briscoe <ietf@bobbriscoe.net <mailto: > ietf@bobbriscoe.net>>; Ian Swett <ianswett@google.com <mailto: > ianswett@google.com>>; tcpm@ietf.org <mailto:tcpm@ietf.org> Extensions < > tcpm@ietf.org <mailto:tcpm@ietf.org>> > > Subject: Re: [tcpm] Thank you for the QUIC session in tcpm > > > > On Mon, Nov 12, 2018 at 4:13 AM, Jana Iyengar <jri.ietf@gmail.com > <mailto:jri.ietf@gmail.com>> wrote: > > > Praveen, > > > > > > The point you're raising -- that we've lost the ack clock after an > RTO > > > -- is a reasonable point. My argument is that with pacing, cwnd > > > reduction is unwarranted because in the extreme case, this > collapses > > > to restart after idle, where sending at cwnd with pacing is > reasonable > > I agree w/ Jana as well - the most generic way to treat burst due to > prior_inflight << cwnd is pacing. With QUIC's approach, when RTO fires, > cwnd remains unchanged until the ACK of the first retransmission (i.e. a > probe packet) comes back. Therefore the delay is one RTT and the potential > damage is an additional cwnd-worth of burst. > > > > Yes the worst case is more aggressive, and it can be too much for DC > incast case if pacing isn't supported - one idea is to selective enable > that if RTT variance is very large vs RTT. > > > > > > > > The draft does not say that a sender should reduce the cwnd if it > is > > > not pacing, which we should add. Does that make sense? > > > > > > - jana > > > > > > On Fri, Nov 9, 2018 at 8:34 AM Praveen Balasubramanian > > > <pravb=40microsoft.com@dmarc.ietf.org <mailto: > 40microsoft.com@dmarc.ietf.org>> wrote: > > >> > > >> Yuchung I brought that difference up in an email to the quic wg > > >> earlier this week. > > >> > > >> In app send limited case, inflight could be very small compared > to cwnd. > > >> So in QUIC there is potential to send a burst out after a long > idle > > >> period (with outstanding data) where TCP wouldn't. The draft > claims > > >> this is okay to do because RTO may have been a result of RTT > increase > > >> instead of loss. Is there data to suggest on which side we should > > >> err? i.e. data on what are the chances that an RTO is due to an > RTT increase versus loss. > > >> > > >> Do you see any safety concerns with delayed reduction of cwnd in > case > > >> where the RTO is not spurious? > > >> > > >> -----Original Message----- > > >> From: tcpm <tcpm-bounces@ietf.org <mailto:tcpm-bounces@ietf.org>> > On Behalf Of Yuchung Cheng > > >> Sent: Thursday, November 8, 2018 4:38 PM > > >> To: Bob Briscoe <ietf@bobbriscoe.net <mailto:ietf@bobbriscoe.net > >> > > >> Cc: Ian Swett <ianswett@google.com <mailto:ianswett@google.com>>; > tcpm IETF list <tcpm@ietf.org <mailto:tcpm@ietf.org>> > > >> Subject: Re: [tcpm] Thank you for the QUIC session in tcpm > > >> > > >> On Thu, Nov 8, 2018 at 3:14 AM, Bob Briscoe <ietf@bobbriscoe.net > <mailto:ietf@bobbriscoe.net>> wrote: > > >> > > > >> > I just wanted to thank Jana for explaining QUIC loss recovery > to us > > >> > (and QUIC CC as far as it goes). > > >> > And thank you Jana, Ian, the chairs of both WGs (and anyone else > > >> > involved) for setting it up. > > >> > > > >> > If one is not full-time on QUIC, it's very difficult to keep up > > >> > with all the changes. But now we have a checkpoint to start > from, I > > >> > feel I will not be wasting people's time if I try to get > involved - > > >> > at least I only might say something un-QUIC occasionally, rather > > >> > than nearly always. This has allowed people who understand how > TCP > > >> > cold be improved to help with QUIC, when working on QUIC isn't > their day job. > > >> > > > >> > Again, Thank you. > > >> I like particularly that QUIC only reduces cwnd to 1 after the > loss > > >> is confirmed not upon RTO fires. It should be very feasible for > TCP > > >> (at least > > >> Linux) w/ TCP timestamps. It'll save a lot of spurious cwnd > reductions! > > >> > > >> Also IMHO TCP w/ quality timestamps are almost as good as QUIC > pkt-ids. > > >> Google internally uses usec. We wish we could upstream it but RFC > > >> needs to be updated. > > >> > > >> > > > >> > > > >> > Bob > > >> > > > >> > > > >> > -- > > >> > ________________________________________________________________ > > >> > Bob Briscoe > > >> > > https://na01.safelinks.protection.outlook.com/?url=http%3A%2F%2Fbob > > >> > briscoe.net <http://briscoe.net>%2F&data=02%7C01%7Cpravb% > 40microsoft.com <http://40microsoft.com>%7C3c4d22866 > > >> > > 8444691c16808d648bd2ce9%7C72f988bf86f141af91ab2d7cd011db47%7C1%7C0% > > >> > > 7C636776374678321324&sdata=T%2BWTEc42VC%2Bz%2BsmMFyjlm37hmAwfee > > >> > buPcuMYlVhgDY%3D&reserved=0 > > >> > > > >> > _______________________________________________ > > >> > tcpm mailing list > > >> > tcpm@ietf.org <mailto:tcpm@ietf.org> > > >> > > https://na01.safelinks.protection.outlook.com/?url=https%3A%2F%2Fww > > >> > w.i > > >> > etf.org <http://etf.org > >%2Fmailman%2Flistinfo%2Ftcpm&data=02%7C01%7Cpravb%40micr > > >> > oso > > >> > ft.com <http://ft.com > >%7Cf0911eeb74d7446f424508d645dbb779%7C72f988bf86f141af91ab2d7 > > >> > cd0 > > >> > > 11db47%7C1%7C0%7C636773207316711149&sdata=K667a3IQG4rarQ%2FOfAl > > >> > yhK > > >> > QQ05Cea421rgb64DlEMvs%3D&reserved=0 > > >> > > >> _______________________________________________ > > >> tcpm mailing list > > >> tcpm@ietf.org <mailto:tcpm@ietf.org> > > >> > > >> > https://na01.safelinks.protection.outlook.com/?url=https%3A%2F%2Fwww. > > >> ietf.org <http://ietf.org > >%2Fmailman%2Flistinfo%2Ftcpm&data=02%7C01%7Cpravb%40micro > > >> soft.com <http://soft.com > >%7C3c4d228668444691c16808d648bd2ce9%7C72f988bf86f141af91ab2d7 > > >> > cd011db47%7C1%7C0%7C636776374678321324&sdata=33WxKh5c6qL4ln5jerpx > > >> Rhytfrj1iTK284pEqv5fWKY%3D&reserved=0 > > >> > > >> _______________________________________________ > > >> tcpm mailing list > > >> tcpm@ietf.org <mailto:tcpm@ietf.org> > > >> > https://na01.safelinks.protection.outlook.com/?url=https%3A%2F%2Fwww. > > >> ietf.org <http://ietf.org > >%2Fmailman%2Flistinfo%2Ftcpm&data=02%7C01%7Cpravb%40micro > > >> soft.com <http://soft.com > >%7C3c4d228668444691c16808d648bd2ce9%7C72f988bf86f141af91ab2d7 > > >> > cd011db47%7C1%7C0%7C636776374678321324&sdata=33WxKh5c6qL4ln5jerpx > > >> Rhytfrj1iTK284pEqv5fWKY%3D&reserved=0 > > > > > > > > _______________________________________________ > > tcpm mailing list > > tcpm@ietf.org > > https://www.ietf.org/mailman/listinfo/tcpm > > > > _______________________________________________ > tcpm mailing list > tcpm@ietf.org > https://www.ietf.org/mailman/listinfo/tcpm >
- [tcpm] Thank you for the QUIC session in tcpm Bob Briscoe
- Re: [tcpm] Thank you for the QUIC session in tcpm Yuchung Cheng
- Re: [tcpm] Thank you for the QUIC session in tcpm Praveen Balasubramanian
- Re: [tcpm] Thank you for the QUIC session in tcpm Mark Nottingham
- Re: [tcpm] Thank you for the QUIC session in tcpm Yoshifumi Nishida
- Re: [tcpm] Thank you for the QUIC session in tcpm Jana Iyengar
- Re: [tcpm] Thank you for the QUIC session in tcpm Jana Iyengar
- Re: [tcpm] Thank you for the QUIC session in tcpm Yuchung Cheng
- Re: [tcpm] Thank you for the QUIC session in tcpm Praveen Balasubramanian
- Re: [tcpm] Thank you for the QUIC session in tcpm Jana Iyengar
- Re: [tcpm] Thank you for the QUIC session in tcpm Jana Iyengar
- Re: [tcpm] Thank you for the QUIC session in tcpm Mike Bishop
- Re: [tcpm] Thank you for the QUIC session in tcpm Praveen Balasubramanian
- Re: [tcpm] Thank you for the QUIC session in tcpm Eric Dumazet
- Re: [tcpm] Thank you for the QUIC session in tcpm Jana Iyengar
- Re: [tcpm] Thank you for the QUIC session in tcpm Jana Iyengar
- Re: [tcpm] Thank you for the QUIC session in tcpm Neal Cardwell
- Re: [tcpm] Thank you for the QUIC session in tcpm Jana Iyengar