Re: [tcpm] Thank you for the QUIC session in tcpm

Jana Iyengar <jri.ietf@gmail.com> Mon, 12 November 2018 12:13 UTC

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From: Jana Iyengar <jri.ietf@gmail.com>
Date: Mon, 12 Nov 2018 19:13:32 +0700
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To: Praveen Balasubramanian <pravb=40microsoft.com@dmarc.ietf.org>
Cc: ycheng=40google.com@dmarc.ietf.org, 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,

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.

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>om>; 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
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