This was discussed a little while ago.
The answer is YES, you can map it differently in the answer - there must be a separate
mapping table for each endpoint. But once you have established a mapping between a
particular codec and a payload number for an endpoint, you must not associate a
different codec with that payload number *for that endpoint*.
Paul
Christian Jansson wrote:
>
> Is it allowed to map a different codec to particular dynamic
> payload number in the SDP answer than in the SDP offer?
> I think RFC 3264 is unclear on this.
>
> For example if an endpoint A sends an SDP offer like this:
>
> m=audio 10001 RTP/AVP 8 119
> a=rtpmap:8 PCMA/8000
> a=rtpmap:119 FooBar/8000
>
> Is B then allowed to answer with and SDP like this?
>
> m=audio 10001 RTP/AVP 8 119
> a=rtpmap:8 pcma/8000
> a=rtpmap:119 telephone-event/8000
>
> page 10 in RFC 3264:
> "In the case of RTP, if a particular codec was referenced with a
> specific payload type number in the offer, that same payload type
> number SHOULD be used for that codec in the answer."
> The "SHOULD" doesn't explicitly forbid anyone from using the same
> payload type number for a different codec in the SDP answer.
>
> page 15 RFC 3264:
> "However, in the
> case of RTP, the mapping from a particular dynamic payload type
> number to a particular codec within that media stream MUST NOT change
> for the duration of a session. For example, if A generates an offer
> with G.711 assigned to dynamic payload type number 46, payload type
> number 46 MUST refer to G.711 from that point forward in any offers
> or answers for that media stream within the session."
>
> This part seems to forbid the reuse of payload type numbers for different
> codecs, but is located in "8.3.2 Changing the Set of Media Formats".
>
> I really need to know if reusing dynamic payload numbers in SDP answers
> is ok or not?
>
> / Christian Jansson
>
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