[AVTCORE] Francesca Palombini's No Objection on draft-ietf-payload-vp9-13: (with COMMENT)

Francesca Palombini via Datatracker <noreply@ietf.org> Tue, 01 June 2021 21:34 UTC

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Subject: [AVTCORE] Francesca Palombini's No Objection on draft-ietf-payload-vp9-13: (with COMMENT)
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Francesca Palombini has entered the following ballot position for
draft-ietf-payload-vp9-13: No Objection

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----------------------------------------------------------------------
COMMENT:
----------------------------------------------------------------------

Thank you for the work on this document.

I have some non blocking questions and comments, which I hope will help improve
the document.

Francesca

1. -----

   Timestamp:  The RTP timestamp indicates the time when the input frame
      was sampled, at a clock rate of 90 kHz.  If the input picture is
      encoded with multiple layer frames, all of the frames of the
      picture MUST have the same timestamp.

FP: I think it would be useful to add a reference to RFC 3550, regarding "RTP
timestamp". Also, I find it curious that RFC 3550 is not mentioned up to the
end of section 4.1 (I would think a reference to it would be present in the
introduction)

2. -----

      Otherwise, PID MUST NOT be present.  If the SS field was present
      in the stream's most recent start of a keyframe (i.e., non-
      flexible scalability mode is in use), then the PID MUST also be
      present in every packet.

FP: Is there any reason why this is not formulated in terms of V bit being set?
(I believe the rest of the text is consistently talking about bit being set)

3. -----

      described by "Reference indices" below.  This MUST only be set to
      1 if the I bit is also set to one; if the I bit is set to zero,
      then this MUST also be set to zero and ignored by receivers.  The

FP: Why is that the it MUST only be set to 1 if I is also set to 1? I was
looking for the motivation, but could not find it. Some more text would have
been helpful to me.

4. -----

   Z:  Not a reference frame for upper spatial layers.  If set to 1,
      indicates that frames with higher spatial layers SID+1 of the
      current and following pictures do not depend on the current

FP: I am not sure if the text it meant to say higher spatial layers than SID+1
(inclusive?)

5. -----

     The field MUST be present if the I bit is equal to one.  If set,
      the PID field MUST contain 15 bits; otherwise, it MUST contain 7

FP: "If set" - I understand by the context this should be "If M is set" (how
it's written now it could be interpreted by "if the PID field is set", which
does not make sense, but better be clear)

6. -----

      or 15-bit index.  The PID SHOULD start on a random number, and
      MUST wrap after reaching the maximum ID (0x7f or 0x7fff depending
      on the index size chosen).  The receiver MUST NOT assume that the

FP: So is the intention that the PID is increased by one for each picture? Does
the order matter? The way the text is written "reaching the maximum ID" would
suggest so, but I could not find any text about that, if I have missed it
please let me know.

7. -----

       SID-1 frame of the same picture, otherwise MUST set to zero.

FP: s/MUST set/MUST be set

8. -----

         depends on.  TL0PICIDX MUST be incremented when TID is equal to
         0.  The index SHOULD start on a random number, and MUST restart

FP: Does it matter by how much? If so, it should be stated.

9. -----

      temporal layer ID (TID), switch up point (U), and the R reference
      indices (P_DIFFs) are specified.

FP: I couldn't find the R bit defined anywhere. I assume its meaning is "if
set, P_DIFF is present" but this should be clearly stated in the text.

10. -----

FP: Please expand MCU, LRR on first use

11. -----

Section 7. IANA

FP: I checked the mailarchive for the subtype registration and could not find
it. I leave it to Murray to let me know if we are more lenient about subtype
requests, but I would have appreciated the registration being posted to the
media-types mailing list.