[tsvwg] L4S ECN marking: MUST be immediate vs SHOULD NOT be delayed
Bob Briscoe <research@bobbriscoe.net> Tue, 21 December 2021 16:26 UTC
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To: tsvwg IETF list <tsvwg@ietf.org>, "tsvwg-chairs@ietf.org" <tsvwg-chairs@ietf.org>
From: Bob Briscoe <research@bobbriscoe.net>
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Date: Tue, 21 Dec 2021 16:26:15 +0000
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Subject: [tsvwg] L4S ECN marking: MUST be immediate vs SHOULD NOT be delayed
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List, chairs, It has been pointed out (offlist) that the normative text on smoothing delay at the end of this section: https://datatracker.ietf.org/doc/html/draft-ietf-tsvwg-ecn-l4s-id#section-5.2 could be expressed as a MUST rather than SHOULD, if stated the other way round, which would make it less open to interpretation. Current: Also, L4S CE marking needs to be interpreted as an unsmoothed signal, in contrast to the Classic approach in which AQMs filter variations before signalling congestion. An L4S AQM SHOULD NOT smooth or filter out variations in the queue before signalling congestion. In the L4S architecture [I-D.ietf-tsvwg-l4s-arch <https://datatracker.ietf.org/doc/html/draft-ietf-tsvwg-ecn-l4s-id#ref-I-D.ietf-tsvwg-l4s-arch>], the sender, not the network, is responsible for smoothing out variations. This requirement is worded as 'SHOULD NOT' rather than 'MUST NOT' to allow for the case where the signals from a Classic smoothed AQM are coupled with those from an unsmoothed L4S AQM. Nonetheless, the spirit of the requirement is for all systems to expect that L4S ECN signalling is unsmoothed and unfiltered, which is important for interoperability. Proposed: Also, in the L4S architecture [I-D.ietf-tsvwg-l4s-arch <https://datatracker.ietf.org/doc/html/draft-ietf-tsvwg-ecn-l4s-id#ref-I-D.ietf-tsvwg-l4s-arch>], the sender, not the network, is responsible for smoothing out variations in the queue. So, an L4S AQM MUST signal congestion as soon as possible. Then, an L4S sender generally interprets CE marking as an unsmoothed signal. This requirement does not prevent an L4S AQM from mixing in additional congestion signals that are smoothed, such as the signals from a Classic smoothed AQM that are coupled with unsmoothed L4S signals in the coupled DualQ [I-D.ietf-tsvwg-aqm-dualq-coupled <https://datatracker.ietf.org/doc/html/draft-ietf-tsvwg-ecn-l4s-id#ref-I-D.ietf-tsvwg-aqm-dualq-coupled>]. But only as long as the onset of congestion can be signalled immediately, and can be interpreted by the sender as if it has been signalled immediately, which is important for interoperability This also needs a tweak to the earlier text in §4.4. "Filtering or Smoothing of ECN Feedback" that refers forward to here: Current: Section 5.2 <https://datatracker.ietf.org/doc/html/draft-ietf-tsvwg-ecn-l4s-id#section-5.2> below specifies that an L4S AQM is expected to signal L4S ECN without filtering or smoothing. Proposed: Section 5.2 <https://datatracker.ietf.org/doc/html/draft-ietf-tsvwg-ecn-l4s-id#section-5.2> below specifies that an L4S AQM is expected to signal L4S ECN immediately, to avoid introducing delay due to filtering or smoothing. Imminently, I shall prepare a new revision of the draft to wrap up the few changes subsequent to draft-22, which were mainly push-back against changes intended to address WGLC comments. I'll include the above hopefully uncontroversial change, unless someone can give a good reason not to. Bob -- ________________________________________________________________ Bob Briscoe http://bobbriscoe.net/