[tcpPrague] Realistic Queue delay targets

"Rodney W. Grimes" <ietf@gndrsh.dnsmgr.net> Sun, 01 November 2020 20:24 UTC

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From: "Rodney W. Grimes" <ietf@gndrsh.dnsmgr.net>
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To: Bob Briscoe <ietf@bobbriscoe.net>
Date: Sun, 1 Nov 2020 12:23:46 -0800 (PST)
CC: Christian Huitema <huitema@huitema.net>, tsvwg IETF list <tsvwg@ietf.org>, iccrg IRTF list <iccrg@irtf.org>, TCP Prague List <tcpPrague@ietf.org>, "De Schepper, Koen (Koen)" <koen.de_schepper@nokia.com>
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Subject: [tcpPrague] Realistic Queue delay targets
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Bob,

> Christian,
> 
> I've changed the subject line given it's no longer appropriate.
> See inline tagged [BB]...

And I am changing it again... as only addressing a statement that
I have great concerns about.

See inline ragged [RWG]

> On 01/11/2020 01:07, Christian Huitema wrote:
... [ much text deleted ] ...

> [BB] Yes, once the IETF assigns the codepoint, BBRv2 will be 'allowed' 
> to send packets over the Internet as ECT(1). Then, yes, an L4S DualQ 
> Coupled AQM will classify BBRv2 packets into the L4S queue. This should 
> have a very shallow ECN marking threshold (500us - 1ms), so even if the 
> flow (whether QUIC or TCP) is flying just under the available capacity, 
> bunching of packets means it is unlikely to completely avoid ECN marking 
> between probes. If it could avoid ECN marking, you are right that it 
> would get a bump of ECN marks during the probe. I haven't studied the 
> code, but when it experiences ECN marking I believe it switches into an 
> L4S ECN mode for a while, and uses ECN rather than delay probing to 
> track available capacity. I assume it switches back to BBR's delay 
> probing mode if it gets no ECN for a while (e.g. the bottleneck might 
> have moved). But I haven't looked at BBRv2's ECN behaviour in detail.

 [RWG] I have great concern about this 500uS to 1mS ECN marking threshold
given that I have recently learned the latest WiFi standards actually
run with a packet aggregation time of 4mS thus making it probably
impossible to have such traffic work in such a targeted low latency queue.

 [RWG] Has any consideration been given to what is already deployed on
the Internet as link layer technologies that would preclude the
operation of the L4S low latency queue?

Regards,
-- 
Rod Grimes                                                 rgrimes@freebsd.org