[AVT] Codec parameter mismatch in SDP offer and answer

["Somogyi, Gabor (NSN - HU/Budapest)" <gabor.somogyi at nsn.com>]

Sorry if that topic was discussed earlier. I was not able to find the
proper answer, though.

What happens if codec parameters in the SDP answer differ from the ones
in relevant offer? Maybe each codec specific SDP format description
shell address that issue, as it may not be possible to give a generic
answer.

RFC 3952 (iLBC) clearly defines that "mode" parameter is bi-directional,
meaning that in both direction the same "mode" must be used. And if the
parameter differs in the offer and the answer, then value "30" must be
used. That sounds fine.

In rfc3047-bis-09 proposal I saw no such definition. Is it possible for
the answerer to reduce the offered bitrate? Must the same bitrate be
used in both directions?

I saw such interworking issues between live products of different
vendors. G.729 (RFC 3555) defines optional parameter "annexb". If the
offerer indicated the willingness of using Annex B, then could the
answerer indicate that it is not willing to use discontinuous
transmission (as it cannot generate comfort noise)? When the offerer
sent its offer, it may had set up its DSPs, so might not be happy about
even "downgrading" codec settings. Same applies to many other codecs, as
well.

BR,
Gabor Somogyi