Re: [tcpm] Thank you for the QUIC session in tcpm
Jana Iyengar <jri.ietf@gmail.com> Thu, 15 November 2018 04:17 UTC
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From: Jana Iyengar <jri.ietf@gmail.com>
Date: Thu, 15 Nov 2018 09:46:47 +0530
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To: Praveen Balasubramanian <pravb@microsoft.com>
Cc: Yuchung Cheng <ycheng@google.com>, Bob Briscoe <ietf@bobbriscoe.net>, Ian Swett <ianswett@google.com>, tcpm@ietf.org
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Subject: Re: [tcpm] Thank you for the QUIC session in tcpm
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Praveen, I think it's a good idea to require that cwnd be reduced in the absence of pacing. I've opened #2007 <https://github.com/quicwg/base-drafts/issues/2007> to track this. - jana On Wed, Nov 14, 2018 at 1:27 AM Praveen Balasubramanian <pravb@microsoft.com> wrote: > That’s all great assuming that RTT increase was the reason for the > prolonged silence. It should be possible to measure both incidences of > spurious RTOs and measured RTT increases post spurious RTOs, and that > should tell us which side we should err on. > > > > For safety reasons, should we put in the text for reducing the cwnd in > absence of pacing to keep consistency with TCP behavior, at least until > data shows otherwise? I worry that most app developers without transport > background will not have the expertise to realize the issues caused by > burst after idle. The other option is to make pacing mandatory in the > draft. > > > > *From:* Jana Iyengar <jri.ietf@gmail.com> > *Sent:* Monday, November 12, 2018 11:13 PM > *To:* Praveen Balasubramanian <pravb@microsoft.com> > *Cc:* Yuchung Cheng <ycheng@google.com>; Bob Briscoe <ietf@bobbriscoe.net>; > Ian Swett <ianswett@google.com>; tcpm@ietf.org > *Subject:* Re: [tcpm] Thank you for the QUIC session in tcpm > > > > 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. > > > > 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> 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> > Sent: Monday, November 12, 2018 8:37 AM > To: Jana Iyengar <jri.ietf@gmail.com> > Cc: Praveen Balasubramanian <pravb@microsoft.com>; Bob Briscoe < > ietf@bobbriscoe.net>; Ian Swett <ianswett@google.com>; tcpm@ietf.org > Extensions <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> 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> 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> On Behalf Of Yuchung Cheng > >> Sent: Thursday, November 8, 2018 4:38 PM > >> To: Bob Briscoe <ietf@bobbriscoe.net> > >> Cc: Ian Swett <ianswett@google.com>; tcpm IETF list <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> > 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 > <https://na01.safelinks.protection.outlook.com/?url=http%3A%2F%2Fbriscoe.net&data=02%7C01%7Cpravb%40microsoft.com%7C7c540bc234b8405b454008d649378123%7C72f988bf86f141af91ab2d7cd011db47%7C1%7C0%7C636776900092531035&sdata=rtr1fEZGcEqbSqZZIhG6HPdmfb5LzGGAFCMtoLNyTWo%3D&reserved=0> > %2F&data=02%7C01%7Cpravb%40microsoft.com > <https://na01.safelinks.protection.outlook.com/?url=http%3A%2F%2F40microsoft.com&data=02%7C01%7Cpravb%40microsoft.com%7C7c540bc234b8405b454008d649378123%7C72f988bf86f141af91ab2d7cd011db47%7C1%7C0%7C636776900092531035&sdata=SQQWIZy8UCRqj5w5I4dg3sgjvTkrygTXBci%2BOJETVE0%3D&reserved=0> > %7C3c4d22866 > >> > 8444691c16808d648bd2ce9%7C72f988bf86f141af91ab2d7cd011db47%7C1%7C0% > >> > 7C636776374678321324&sdata=T%2BWTEc42VC%2Bz%2BsmMFyjlm37hmAwfee > >> > buPcuMYlVhgDY%3D&reserved=0 > >> > > >> > _______________________________________________ > >> > tcpm mailing list > >> > tcpm@ietf.org > >> > https://na01.safelinks.protection.outlook.com/?url=https%3A%2F%2Fww > >> > w.i > >> > etf.org > <https://na01.safelinks.protection.outlook.com/?url=http%3A%2F%2Fetf.org&data=02%7C01%7Cpravb%40microsoft.com%7C7c540bc234b8405b454008d649378123%7C72f988bf86f141af91ab2d7cd011db47%7C1%7C0%7C636776900092541035&sdata=Qrq0JPmfc48NwQ6SU9IMYEvA8x3N6OphFTxX2TpcKuk%3D&reserved=0> > %2Fmailman%2Flistinfo%2Ftcpm&data=02%7C01%7Cpravb%40micr > >> > oso > >> > ft.com > <https://na01.safelinks.protection.outlook.com/?url=http%3A%2F%2Fft.com&data=02%7C01%7Cpravb%40microsoft.com%7C7c540bc234b8405b454008d649378123%7C72f988bf86f141af91ab2d7cd011db47%7C1%7C0%7C636776900092541035&sdata=Bd6AndbLA9pocmFsCvKAGMJw5nsMV4oiydwlT67wNHo%3D&reserved=0> > %7Cf0911eeb74d7446f424508d645dbb779%7C72f988bf86f141af91ab2d7 > >> > cd0 > >> > 11db47%7C1%7C0%7C636773207316711149&sdata=K667a3IQG4rarQ%2FOfAl > >> > yhK > >> > QQ05Cea421rgb64DlEMvs%3D&reserved=0 > >> > >> _______________________________________________ > >> tcpm mailing list > >> tcpm@ietf.org > >> > >> https://na01.safelinks.protection.outlook.com/?url=https%3A%2F%2Fwww. > >> ietf.org > <https://na01.safelinks.protection.outlook.com/?url=http%3A%2F%2Fietf.org&data=02%7C01%7Cpravb%40microsoft.com%7C7c540bc234b8405b454008d649378123%7C72f988bf86f141af91ab2d7cd011db47%7C1%7C0%7C636776900092551049&sdata=vPzeaG8Dy352omyuWVE%2BC6bQIV48o%2FgmiYzkoKP7T1s%3D&reserved=0> > %2Fmailman%2Flistinfo%2Ftcpm&data=02%7C01%7Cpravb%40micro > >> soft.com > <https://na01.safelinks.protection.outlook.com/?url=http%3A%2F%2Fsoft.com&data=02%7C01%7Cpravb%40microsoft.com%7C7c540bc234b8405b454008d649378123%7C72f988bf86f141af91ab2d7cd011db47%7C1%7C0%7C636776900092551049&sdata=KqVybgAgN%2Bl1Ktquxl5hybWgQg05Oz9NQuUD2%2BPqQD4%3D&reserved=0> > %7C3c4d228668444691c16808d648bd2ce9%7C72f988bf86f141af91ab2d7 > >> cd011db47%7C1%7C0%7C636776374678321324&sdata=33WxKh5c6qL4ln5jerpx > >> Rhytfrj1iTK284pEqv5fWKY%3D&reserved=0 > >> > >> _______________________________________________ > >> tcpm mailing list > >> tcpm@ietf.org > >> https://na01.safelinks.protection.outlook.com/?url=https%3A%2F%2Fwww. > >> ietf.org > <https://na01.safelinks.protection.outlook.com/?url=http%3A%2F%2Fietf.org&data=02%7C01%7Cpravb%40microsoft.com%7C7c540bc234b8405b454008d649378123%7C72f988bf86f141af91ab2d7cd011db47%7C1%7C0%7C636776900092561053&sdata=EQblZZj1sns102aGdnHO%2FvXPcRVg1cjEiUMYK%2BaJPSk%3D&reserved=0> > %2Fmailman%2Flistinfo%2Ftcpm&data=02%7C01%7Cpravb%40micro > >> soft.com > <https://na01.safelinks.protection.outlook.com/?url=http%3A%2F%2Fsoft.com&data=02%7C01%7Cpravb%40microsoft.com%7C7c540bc234b8405b454008d649378123%7C72f988bf86f141af91ab2d7cd011db47%7C1%7C0%7C636776900092561053&sdata=XIL2TBtfeQfYNRnzCzxKQhh%2F9GdyWD9lDttd1w1waHM%3D&reserved=0> > %7C3c4d228668444691c16808d648bd2ce9%7C72f988bf86f141af91ab2d7 > >> cd011db47%7C1%7C0%7C636776374678321324&sdata=33WxKh5c6qL4ln5jerpx > >> Rhytfrj1iTK284pEqv5fWKY%3D&reserved=0 > >
- [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