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

Jonathan Morton <> Fri, 26 March 2021 17:00 UTC

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From: Jonathan Morton <>
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Date: Fri, 26 Mar 2021 19:00:13 +0200
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To: Gorry Fairhurst <>
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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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> On 26 Mar, 2021, at 6:54 pm, Gorry Fairhurst <> 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.  "

What that means is "relatively short compared to a dumb FIFO".  Whereas what I mean is "relatively deep compared to the threshold for SCE marking".  I'm not putting absolute numbers on this, just following phrasing that I think is also used by L4S drafts.

 - Jonathan Morton