Re: [tsvwg] From L4S to SCE+DSCP and RFC-4774 Option 3 (CE-marking on shallow queues)

Gorry Fairhurst <> Fri, 26 March 2021 16:54 UTC

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To: Jonathan Morton <>, tsvwg IETF list <>
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From: Gorry Fairhurst <>
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Date: Fri, 26 Mar 2021 16:54:00 +0000
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Subject: Re: [tsvwg] From L4S to SCE+DSCP and RFC-4774 Option 3 (CE-marking on shallow queues)
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Correcting just one part here, on the expected CE-marking behaviour:

On 26/03/2021 10:56, Jonathan Morton wrote:
> 2: CE marks are applied by both RFC-3168 and SCE AQMs under similar circumstances, at a relatively deep queue threshold.  They are fed back by the receiver to the sender in the normal way, using ECE and CWR in the case of TCP, and senders are expected to respond with a Multiplicative Decrease compliant to RFC-8511.  Not-ECT traffic receives packet drops instead.

RFC 8511 already anticipates, that queues doing CE-marking are shallow. 
As clear from the RFC's abstract:


    Active Queue Management (AQM) mechanisms allow for burst tolerance
    while enforcing short queues to minimise the time that packets spend
    enqueued at a bottleneck.  This can cause noticeable performance
    degradation for TCP connections traversing such a bottleneck,
    especially if there are only a few flows or their bandwidth-delay
    product (BDP) is large.  The reception of a Congestion Experienced
    (CE) Explicit Congestion Notification (ECN) mark indicates that an
    AQM mechanism is used at the bottleneck, and the bottleneck network
    queue is therefore likely to be short.  "