Re: [MMUSIC] Offer/Answer PT Questions

[Christer Holmberg <[e-mail]>]

Hi Roman,

>>> 2. Is PT blocked from each direction independently or both directions at
>>> the same time. For instance if A sent an offer with PT=100, but B never
>>> sent an offer or an answer with PT=100, can B sent an offer with PT=100?
>>>
>>My understanding is that the answer to 2 has always been understood to
>>be that the PT number spaces in each direction are independent.  For
>>instance, RFC 3264 section 5.1 notes
>>
>>      Different payload type numbers may be needed in each direction
>>      because of interoperability concerns with H.323.
>
> Based on this, the answer to original question is that if
>
> 1.Alice sends an Offer, with the following PT values:
>
> PT 100: AMR-WB(mode-set=2)
> PT 111: AMR-WB(mode-set=2, OA)
> PT 0: PCMU
> 2.Bob sends the associated Answer:
>
> PT 100: AMR-WB (mode-set=2)
> PT 0: PCMU
>
> 3.Later Bob sends the following subsequent Offer:
> PT 111: AMR(mode-set=0,2,5,7)
>
> This offer should be accepted. PT is each direction is independent and Bob is allowed to reuse it.

Not necessarily. Even if PT is per direction is allowed as such, we still need to clarify whether the same PT/codec binding is per direction, or whether it applies to the whole session. 

> The complication here is that Alice will not be allowed to reuse PT 111 for AMR(mode-set=0,2,5,7) in the answer. Alice will need to pick a
> different PT for AMR(mode-set=0,2,5,7), if it is supported and end points will use asymmetric payload types.

So, you are saying that Alice is not allowed to reuse a PT value that Alice has previously used for another PT/codec binding - no matter if Bob supported the codec or not?

For example: assume Alice offered PT 111: VP8, but Bob did not include VP8 in the answer. Then Bob offers PT 111:AMR. Now, Alice is not allowed to use PT 111 for AMR, since Alice previously used it for VP8.

Regards,

Christer