Subject: [MMUSIC] comments on draft-ietf-mmusic-sdp-g723-g729-03

From: <[email-masked]>
Date: Thu, 27 Jun 2013 08:22:23 +0000

Hi

I reviewed draft-ietf-mmusic-sdp-g723-g729-03 and I have some comments. They are quite symmetrical between G723 and G729.

** Section 3

1)
-----
   Based on the above, it is best not to use comfort noise frames if the
   SDP offer or answer indicates that comfort noise is not supported.
-----
The words "it is best not to use" are not strong enough, because it can not work if one of the party do not support comfort noise frames (SID frames in the ITU-T Annexes specs). It is rather a "MUST NOT use".

The word "restrict the use" in the RFC3551 quote is to be interpreted as "forbid the use".

** Section 3.1

2)
After the first paragraph, I suggest to insert the new paragraph below:

---NEW
 The Annex A of the codec concerns both sending and receiving, so both
 sides of a bi-directional session MUST have the same setting.  If
 one party indicates that it does not support Annex A, Annex A  must be
 deactivated both ways.
-----
That defines the general rationale of the following paragraphs.

3)
-----
   When the offer has G723 with annexa=no then the answer MUST NOT have
   annexa=yes for G723.  Thus the annexa parameter can be turned off by
   the answerer, but cannot be turned on.
-----
I don't think this new requirement is needed.
Anyway implementation that will not implement this RFC-to-be will not comply with it.

Plus, with the following paragraph, it breaks the equivalence between annexa=yes and annexa omitted.

Note that RFC3264 allows the SDP answer to include media format that are not present in the SDP offer, so it could be the same for the Annex A support.

4)
-----
   When the offer has G723 with annexa=no, the reason for not mandating
   that the answer MUST have annexa=no for G723 is that there are there
   implementations that omit the annexa parameter in answer and expect
   the least common denominator to be used.
-----

Add the following sentence at the end of the paragraph, to be explicit about the "least common denominator":
   In such cases the offerer and answerer MUST proceed as if G723 is 
   negotiated with annexa=no.

** Section 3.2

5)
After the first two paragraphs, I suggest to insert the new paragraph below:

---NEW
 The Annex B of the codec concerns both sending and receiving, so both
 sides of a bi-directional session MUST have the same setting.  If
 one party indicates that it does not support Annex B, Annex B must be
 deactivated both ways.
-----
That defines the general rationale of the following paragraphs.

6)
-----
   When the offer or answer has G729 and the annexb parameter is absent,
   it MUST be considered as if the offer or answer has G729 with
   annexb=yes.
-----
This paragraph is redundant with what is said before.
Remove it.

7)
-----
   When the offer has G729 with annexb=no then the answer MUST NOT have
   annexb=yes for G729.  Thus the annexb parameter can be turned off by
   the answerer, but cannot be turned on.
-----
I don't think this new requirement is needed.
For the same reason as for G723

8)
-----
   When the offer has G729 with annexa=no, the reason for not mandating
   that the answer MUST have annexa=no for G729 is that there are there
   implementations that omit the annexa parameter in answer and expect
   the least common denominator to be used.
-----

Replace annexa by annexb  (copy/paste error I guess)

And add the following sentence at the end of the paragraph, to be explicit about the "least common denominator":
   In such cases the offerer and answerer MUST proceed as if G729 is 
   negotiated with annexb=no.

Best regards,
Aurelien

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