[rtcweb] JSEP: codecs in answer: MUST vs. MAY

[Jonathan Lennox <[e-mail]>]

JSEP (section 5.3.1) seems to be inconsistent about how whether locally-supported codecs that weren’t listed in a remote offer MUST, or MAY, be included in an answer:

   o  If codec preferences have been set for the associated transceiver,
      media formats MUST be generated in the corresponding order,
      regardless of what was offered, and MUST exclude any codecs not
      present in the codec preferences.

   o  Otherwise, the media formats on the m= line MUST be generated in
      the same order as those offered in the current remote description,
      excluding any currently unsupported formats.  Any currently
      available media formats that are not present in the current remote
      description MUST be added after all existing formats.

   o  In either case, the media formats in the answer MUST include at
      least one format that is present in the offer, but MAY include
      formats that are locally supported but not present in the offer,
      as mentioned in [RFC3264], Section 6.1 <https://tools.ietf.org/html/rfc3264#section-6.1>.  If no common format
      exists, the m= section is rejected as described above.

The first two paragraphs certainly seem to indicate that these codecs MUST be included in the answer; however, the third paragraph suddenly weakens this to a MAY.

Is the intent of the third paragraph simply to restate RFC 3264’s loose requirements, whereas the first two are JSEP’s more binding ones?  If so, I think this should be stated more clearly, otherwise the required behavior is unclear.