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>>t;>; Bob Briscoe <ietf@bobbriscoe.net <mailto:
> ietf@bobbriscoe.net>>t;>; Ian Swett <ianswett@google.com <mailto:
> ianswett@google.com>>t;>; 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
> >     >> >
> >     >> >
> >     >> > --
> >     >> > ________________________________________________________________
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> >     >> >
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