Re: [tsvwg] L4S vs SCE

Sebastian Moeller <> Fri, 22 November 2019 07:14 UTC

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Date: Fri, 22 Nov 2019 08:14:47 +0100
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To:, Greg White <>, Pete Heist <>
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From: Sebastian Moeller <>
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Subject: Re: [tsvwg] L4S vs SCE
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Dear Greg,

On November 22, 2019 5:02:05 AM GMT+01:00, Greg White <> wrote:
>I've seen several statements being made to the effect that SCE can
>possibly work in a single queue or dual queue bottleneck, but I've not
>seen any data to suggest that it is true.
>If the SCE team can come up with such an approach, and provide test
>results (preferably to a similar extent to those provided by the L4S
>team, not just the simple flent scenarios), it would be interesting to

[SM] I do note that the simple flent scenarios, as you call them, revealed and illustrated instantaneously at least three issues with the dual queue coupled AQM and TCP Prague. IMHO this calls into question the extent of the L4S teams testing, at the very least it indicates that testing should be performed with the actual implementation one wants to standardize. Also as far as I can none of the dctcp tests over the DualQ aqm have so far been really testing realistic corner cases.
So I applaud and support the call for diligent testing, but I very include the L4S implementations into that call.

Best Regards

>On 11/21/19, 8:58 PM, "Pete Heist" <> wrote:
>> On Nov 21, 2019, at 7:14 AM, Greg White <>
>    > 
>> Where I think SCE fails is that it is not available to any link that
>doesn't implement FQ, whether that is by choice or by necessity.  I
>don't believe that the IETF should use the last IP codepoint for a
>signaling mechanism that can *only* work in FQ.
>The thoughts are appreciated. What I think needs to be clarified is
>that SCE doesn’t necessarily require full FQ using the traditional many
>queues approach. It only requires some level of help from the network,
>when fairness between SCE and non-SCE flows is required. Options
>- Changing the SCE marking ramp, trading off some of the advantages of
>SCE for improved fairness. This was first described at IETF 105 in
>- Using CNQ, which is implemented but we didn’t have time to present
>today. That provides a minimal level of improved fairness that can
>actually favor SCE flows early in their lifetime.
>- Using LFQ, which like CNQ, is in draft form with a crude discrete
>time simulation, but doesn’t yet have an implementation. This provides
>closer to full FQ but with a lighter weight implementation.
>This is still an active area of research with many options available to
>us, and we feel it’s a tractable problem. We just want to make clear
>that “SCE requires FQ” isn’t very accurate, and needs more
>clarification as to the current and future options available.

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