Hi All,
I realize VAD negotiation was discussed here in the past but one thing
is still a bit unclear with me...
I understand RFC 3555 details an optional parameter "annex a" for G.723
and "annexb" for G.729 for specifying annex preferences. What is
unclear to me is whether there is a mechanism in place for the SDP
answerer to reject/alter the preferred VAD usage in the media stream,
and if so what is the appropriate method?
RFC 3264 states:
The interpretation of fmtp parameters in an offer depends on the
parameters. In many cases, those parameters describe specific
configurations of the media format, and should therefore be processed as
the media format value itself would be. This means that the same fmtp
parameters with the same values MUST be present in the answer if the
media format they describe is present in the answer. Other fmtp
parameters are more like parameters, for which it is perfectly
acceptable for each agent to use different values. In that case, the
answer MAY contain fmtp parameters, and those MAY have the same values
as those in the offer, or they MAY be different. SDP extensions that
define new parameters SHOULD specify the proper interpretation in
offer/answer.
The third and fourth sentences in this paragraph seem to contradict as
to whether it is permissible to change fmtp parameters. That said, is
there a mechanism for the answerer to if G.729 B was offered, to accept
G.729 but with only vanilla AnnexA format? For example, if an SDP offer
contained this:
m=audio 49170 RTP/AVP 18
fmtp:18 annexb=yes
If the answerer does not support G.729B (VAD) but supported vanilla
G.729 (or AnnexA), would it be required to:
1) add a new m-line, ie:
m=audio 0 RTP/AVP 18
fmtp:18 annexb=yes
m=audio 20001 RTP/AVP 18
fmtp:18 annexb=no
2) simply change the optional MIME media parameter to "no" in the
answer?
m=audio 2001 RTP/AVP 18
fmtp:18 annexb=no
3) reject the call?
Thanks for all your help,
Rich
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