Re: [MMUSIC] Offer/Answer PT Questions - text proposal

[Christer Holmberg <[e-mail]>]

Hi,

>>>> I put together some text:
>>>> 
>>>> "The scope of a mapping from a particular dynamic payload type 
>>>> number to a codec (or codec configuration) is for a given RTP media flow direction within a session.
>>> 
>>> I think this conflicts with various RTP-related RFCs and drafts, although whether this conflict causes practical 
>>> problems is unclear. RFC 3551 indicates that the binding between RTP payload type and RTP payload format is per 
>>> RTP session, not per sender within an RTP session:
>>> 
>>>    mechanisms for defining dynamic payload type bindings have been
>>>    specified in the Session Description Protocol (SDP) and in other
>>>    protocols such as ITU-T Recommendation H.323/H.245.  These mechanisms
>>>    associate the registered name of the encoding/payload format, along
>>>    with any additional required parameters, such as the RTP timestamp
>>>    clock rate and number of channels, with a payload type number.  This
>>>    association is effective only for the duration of the RTP session in
>>>    which the dynamic payload type binding is made.  This association
>>>    applies only to the RTP session for which it is made, thus the
>>>    numbers can be re-used for different encodings in different sessions
>>>    so the number space limitation is avoided.
>>> 
>>> (note: reuse in different sessions, not different participants within a session) and:
> 
> Does a sendrecv m-line established via O/A create one or two RTP sessions?

One, since RTP sessions are defined by shared SSRC space, not directionality of media.

>> If one, then it seems this conflicts with 3264 allowing the two ends to use different PTs for the same codec configuration.
>
> That depends exactly what 3264 allows: if the two ends use different payload types to refer to the same codec, that’s okay; but if 
> the two ends use the same payload type to refer to different codecs, that problematic.

From a 3264 perspective, "use" doesn't automatically mean that the PT is ever sent on the wire - it only means that a PT/codec mapping has been indicated in an offer or answer.

Regards,

Christer