[tcpPrague] ecn-l4s-id: Proposed Changed to Normative RTT Bias Text

Bob Briscoe <research@bobbriscoe.net> Fri, 30 October 2020 18:42 UTC

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From: Bob Briscoe <research@bobbriscoe.net>
To: tsvwg IETF list <tsvwg@ietf.org>
Cc: TCP Prague List <tcpPrague@ietf.org>, iccrg IRTF list <iccrg@irtf.org>, "De Schepper, Koen (Koen)" <koen.de_schepper@nokia.com>
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Date: Fri, 30 Oct 2020 18:42:50 +0000
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Subject: [tcpPrague] ecn-l4s-id: Proposed Changed to Normative RTT Bias Text
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The co-authors of ECN L4S ID have been reviewing the correctness of the 
normative 'Prague' requirements.
This is the first of 2 emails, about 2 of the requirements that we think 
ought to be reworded a little.

If you agree with the rationale, but think the new wording still doesn't 
capture the requirement well, pls suggest sthg better.
If you disagree with the rationale, pls discuss.

4.3.  Prerequisite Congestion Response
    o  A scalable congestion control MUST reduce or eliminate RTT bias
       over as wide a range of RTTs as possible, or at least over the
       typical range of RTTs that will interact in the intended
       deployment scenario (seeAppendix A.1.5  <https://tools.ietf.org/html/draft-ietf-tsvwg-ecn-l4s-id-10#appendix-A.1.5>  for rationale).

A scalable congestion control MUST eliminate RTT bias as much as 
possible in the range between the minimum likely RTT and typical RTTs 
in the intended deployment scenario  (see Appendix A.1.5 for rationale).

1/ "eliminate as much as possible" is stronger than "reduce or eliminate".
2/ This requirement was motivated by 'do no harm to others' relative to 
existing standard (RFC5681 Reno) congestion control. So there is no need 
to mandate that an L4S implementer does no harm to themselves, which 
window-based congestion controls tend to do at higher RTT. Of course, 
this doesn't preclude implementers reducing or eliminating RTT bias for 
larger than typical RTTs, but it removes any requirement to do so.



Bob Briscoe                               http://bobbriscoe.net/