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

Sebastian Moeller <moeller0@gmx.de> Sat, 31 October 2020 15:50 UTC

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To: Bob Briscoe <research@bobbriscoe.net>
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Subject: Re: [tcpPrague] [tsvwg] ecn-l4s-id: Proposed Changed to Normative RTT Bias Text
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Dear list,
 
please note that both of these "requirements" hedge against the fact that RTT independence is almost impossible to achieve from the endpoints (as it is a consequence of buffering at the bottleneck), originally bzy stating "as wide a range of RTTs as possible" (which given the phisical constraints in reality is equal to the empty set). Replacing that non-requirement with a differently worded version ("as much as possible") does not change anything substantially, except for making TCP Prague appear less of a failure. 
IMNHO this requirement for an L4S compatible transport, should be replaced with a requirement for an L4S compliant AQM (as it is limited queueing at the bottleneck that causes most the issue) MUST at least perform as well as a dumb FIFO, a mark which DualQ at short RTTs misses badly. But I guess, a no regressions against the status quo requirement, will probably not apply to L4S.
 
Best Regards
        Sebastian
 
 
Gesendet: Freitag, 30. Oktober 2020 um 19:42 Uhr
Von: "Bob Briscoe" <research@bobbriscoe.net>
An: "tsvwg IETF list" <tsvwg@ietf.org>
Cc: "iccrg IRTF list" <iccrg@irtf.org>rg>, "TCP Prague List" <tcpPrague@ietf.org>rg>, "De Schepper, Koen (Koen)" <koen.de_schepper@nokia.com>
Betreff: [tsvwg] ecn-l4s-id: Proposed Changed to Normative RTT Bias Text
Folks,

The co-authors of ECN L4S ID have been reviewing the correctness of the normative 'Prague' requirements.
    See https://tools.ietf.org/html/draft-ietf-tsvwg-ecn-l4s-id-10#section-4.3" target="_blank" rel="nofollow">https://tools.ietf.org/html/draft-ietf-tsvwg-ecn-l4s-id-10#section-4.3
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
...
CURRENT:
   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 (see https://tools.ietf.org/html/draft-ietf-tsvwg-ecn-l4s-id-10#appendix-A.1.5" target="_blank" rel="nofollow">Appendix A.1.5 for rationale).

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

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.

Cheers


Bob
 
--
________________________________________________________________
Bob Briscoe                               http://bobbriscoe.net/" target="_blank" rel="nofollow">http://bobbriscoe.net/