Re: [tcpPrague] [aqm] L4S status update

"Bless, Roland (TM)" <> Mon, 21 November 2016 14:27 UTC

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To: Bob Briscoe <>, tsvwg IETF list <>, TCP Prague List <>, AQM IETF list <>, tcpm IETF list <>
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From: "Bless, Roland (TM)" <>
Organization: Institute of Telematics, Karlsruhe Institute of Technology
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Date: Mon, 21 Nov 2016 15:27:57 +0100
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Subject: Re: [tcpPrague] [aqm] L4S status update
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Hi Bob and all,

see below.

Am 01.11.2016 um 01:02 schrieb Bob Briscoe:
> A few people have been working away to specify and document all the
> aspects of the new Low Latency, Low Loss, Scalable throughput (L4S)
> service, which held a successful BoF in Berlin. As the decision was to
> try to work across multiple WGs, I thought it would be useful to give ...

Thanks for putting this together.

>   * Dual Queue Coupled AQM
>       o With Curvy RED for Linux (access available shortly)
>       o With PI2 for Linux <> [*UPDATED*]

I'll repeat my concerns that I already expressed at the L4S BOF in Berlin:

While I agree that we probably need to separate low-delay congestion
control schemes from traditional "queue-filling" congestion schemes,
I strongly suggest to avoid putting a congestion control-specific
coupling scheme into the network (a classic case for applying the
"end-to-end arguments in system design").
The current Dual queue coupled AQM proposal has got a coupling based on
a congestion control specific dropping law p_C=(p_L/2)². So if
congestion control schemes change then this coupling needs to be
adapted. For example, the currently proposed scheme may fail if that
vast majority of TCP traffic is using BBR other some other forthcoming
CC scheme instead of Cubic, Reno, Compound etc. The same applies to
draft-briscoe-tsvwg-ecn-l4s-id, section 2.5, where the dropping
likelihood is defined.