Re: [tsvwg] SCReAM (RFC8298) with CoDel-ECN and L4S

Ingemar Johansson S <ingemar.s.johansson@ericsson.com> Tue, 10 March 2020 09:59 UTC

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From: Ingemar Johansson S <ingemar.s.johansson@ericsson.com>
To: Jonathan Morton <chromatix99@gmail.com>, Ingemar Johansson S <ingemar.s.johansson=40ericsson.com@dmarc.ietf.org>
CC: "tsvwg@ietf.org" <tsvwg@ietf.org>, "iccrg@irtf.org" <iccrg@irtf.org>, Ingemar Johansson S <ingemar.s.johansson@ericsson.com>
Thread-Topic: [tsvwg] SCReAM (RFC8298) with CoDel-ECN and L4S
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Date: Tue, 10 Mar 2020 09:59:05 +0000
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Subject: Re: [tsvwg] SCReAM (RFC8298) with CoDel-ECN and L4S
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Hi

The throughput reduction with  L4S is a natural consequence of the irregular frame size from the video encoder. 
If one look at the congestion window size and inspect the code to see how the pacing rate is computed, one can see that the pacing rate is ~70Mbps  during the first 50s *. This means that SCreAM with L4S adjust to be able to transmit the large frames without causing large queue buildup in the network (or in the RTP queue). This means that the average rate goes down to give the necessary headroom. This is exactly the behavior that I seek for.

/Ingemar

* CWND = 180kByte, RTT = 20ms --> 180000*8/0.02 = 72Mbps

> -----Original Message-----
> From: Jonathan Morton <chromatix99@gmail.com>
> Sent: den 10 mars 2020 10:43
> To: Ingemar Johansson S
> <ingemar.s.johansson=40ericsson.com@dmarc.ietf.org>
> Cc: tsvwg@ietf.org; Ingemar Johansson S
> <ingemar.s.johansson@ericsson.com>; iccrg@irtf.org
> Subject: Re: [tsvwg] SCReAM (RFC8298) with CoDel-ECN and L4S
> 
> > On 10 Mar, 2020, at 10:02 am, Ingemar Johansson S
> <ingemar.s.johansson=40ericsson.com@dmarc.ietf.org> wrote:
> >
> > I recently updated the readme on the SCReAM github with a comparison with
> SCReAM in three different settings
> > 	• No ECN
> > 	• CoDel ECN
> > 	• L4S
> > https://github.com/EricssonResearch/scream#ecn-explicit-congestion-
> notification
> >
> > Even though it is more than a magnitude difference in queue delay between
> CoDel-ECN and L4S, it is fair to say that these simple simulations should of
> course be seen as just a snapshot.
> > We hope to present some more simulations with 5G access, and not just
> simple bottlenecks with one flow, after the summer.
> 
> Looking at the throughput numbers, I also notice the L4S case has less than half
> the throughput of the Codel ECN case.  That seems like a significant tradeoff for
> the lower delay numbers.  In fact, even though the throughput *before* the
> capacity reduction is less than the capacity *after* it, the throughput is still
> halved when the capacity reduces.
> 
> This seems like something you might still want to look into.
> 
>  - Jonathan Morton