[MMUSIC] Query: payload type collision with offer/answer

[Harald Alvestrand <[e-mail]>]

Hello,
I have encountered an issue, and I'm not sure it's an issue or not. SDP 
wisdom is sought.

Suppose two entities exchange the following offer/answer:

offer from X:

m=video 97
a=rtpmap:97 foo

answer from Y:

m=video 98
a=rtpmap:98 foo

It's clear from RFC 3264 that now X must send codec foo with payload 
type 98, and Y must send codec foo with payload type 97. That's not the 
question.

But suppose these are the offer and answer:

offer from X:

m=video 97 98
a=rtpmap:97 foo
a=rtpmap:98 bar

answer from Y:

m=video 97 98
a=rtpmap:97 bar
a=rtpmap:98 foo

That is, the payload types collide.

It would be possible to write code so that X sends foo with 98 and bar 
with 97, while Y sends foo with 97 and bar with 98. But is it conformant 
with the specs to do so?

And if this has a clear answer - which paragraph of which RFC makes this 
clear?
(Yes, this discussion is triggered from a real world problem.)

                    Harald