[rtcweb] On ptimes and maxptimes

[Justin Uberti <[e-mail]>]

>From the session this morning, I recall this consensus:

   - We will add recommendations on the frame sizes to use for PCMU/A to
   the rtcweb audio codec I-D (20, 30, 60 ms frames).
   - We will consider adding an a=maxptime attribute that represents the
   "minimum maxptime" of all the offered codecs. That is, if both Opus
   (maxptime of 120ms) and PCMU (maxptime of 60) are offered, a maxptime=60
   SHOULD be included.
      - Since maxptime spans all codecs, this means that some modes (e.g.
      Opus 120ms) will be unavailable, unless only Opus is offered.
   - We will consider adding an a=ptime attribute, set to the receiver's
   preferred frame size to receive. Again, this value spans all codecs, so the
   specified value may not be viable for all codecs (e.g. 30 ms works for PCMU
   but not for Opus)

After thinking about this some, I'm not sure the maxptime/ptime values will
lead to any different behavior than if they were not specified. Since there
is a complexity cost (especially if the values need to change based on
which codecs are offered), is there a clear upside here?