Re: [MMUSIC] BUNDLE TEXT: De-mux procedures (June 19th)

[Christer Holmberg <[e-mail]>]

Hi,

>> *IF* you have a way to find the right m= line, couldn't PT X have 
>> different codec configurations in different m= lines?
>
> Yes, I think so.

RTP seems to forbid it, though.

>> Let's assume that it could ... but then how would you know that your 
>> peer also supports the same demuxing technique as yourself?
>
> Muxing/demuxing (in the sense of associating to a particular m-line) MUST be consistent on both ends. (The sender knows which m-line he associates a packet with, and expects that the receiver will make the same association.)
>
> There must be enough signaling to get the two sides in sync about this.

When it comes to PT values, each endpoint decides which PT values it wants to other endpoint to send.

So, even if Alice uses the same PT value in multiple "m=" lines, and have some way to find the "m=" line when media is received, Bob can still assign unique PT values in each "m=" line.

I don't think there is a requirement to have symmetric PT values in both directions, eventhough I know implementations often do that.


>> In SIP for example, when constructing a first offer, you may want to 
>> map both Opus and VP8 to 96, then add SSRCs for demuxing. This means 
>> that you expect your peer to support SSRC demuxing and you can't 
>> expect that unless we mandate it.
>
> In this case there will be two m-lines, one audio and one video, each using PT 96. And the offer specifies a specific ssrc for each m-line.
>
> If the answerer does't understand or support muxing by ssrc it will then note that if it accepted both m-lines it would not be able to associate packets with m-lines. So it then has two choices in constructing its answer:
>
> - reject the bundle, but accept both m-lines
> - accept the bundle, but reject one of the m-lines.

Again, my understanding is that the Offerer indicates which PT values it wants to RECEIVE. So, it it assigns the same PT value to both audio and video, it will receive the same PT value in the media packets.

But, the Answerer can indicate different PT values for audio and video in the SDP Answer, and it will receive different PT values in the media packets.

Regards,

Christer