[Sip-implementors] SDP dynamic payload mapping problem.

[Christian Jansson]

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

[Paul Kyzivat]

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

Post by Christian Jansson
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.
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
"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.
"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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