Re: [MMUSIC] Proposal for what bundle should say about demux

[Paul Kyzivat <[e-mail]>]

Colin,

(I guess you know I am naive about RTP, so take this with a grain of salt.)

In the below, you first split by SSRC. Then later you map to codec. That 
seems to suggest that each SSRC might have a unique mapping from PT to 
codec. Did you intend that?

I also don't understand below how you expect unknown SSRCs to be 
handled. ISTM that the flow you show could result in establishing a 
bunch of state for each received SSRC, whether that SSRC is of interest 
or not.

In some cases it may be that specific SSRCs are expected, and others can 
be ignored (or whatever). In other cases the recipient may not know what 
SSRCs to expect, but may have some other way of deciding which it wants 
to process.

That was the point of my thread about associating packets to m-lines. If 
you can at least associate a packet with one of the bundled m-lines, 
even if it is by a means other than SSRC, then it becomes easier to 
decide what to do with it. For instance, I think Cullen intends that in 
plan A you could associate packets with m-lines via the PT, and then for 
each m-line process only packets for the most recently seen SSRC. (So 
when a new SSRC is received, drop all state for the prior SSRC and 
establish a new state for the new one.)

The same approach can work using a header extension identifying a clue 
capture encoding to map to an m-line. When doing that, it won't be 
necessary for the PT to be unique across m-lines.

	Thanks,
	Paul

On 5/27/13 9:57 AM, Colin Perkins wrote:
> I don't agree with the phrasing about "packet processing pipelines", but can't tell if this is a terminology disagreement or a more fundamental disconnect. The way I see the demultiplexing logically working is:
>
>                      |
>                      | packets
>      +--             v
>      |           +----------+
>      |           |UDP socket|
>   A  |           +----------+
>      |        RTP ||  |  |
>      |   and RTCP ||  |  +------> SCTP
>      |            ||  +---------> STUN/ICE
>      +--          ||
>      +--          ||
>      |       split by SSRC
>      |       ||   ||   ||
>      |       ||   ||   ||
>   B  |      +--+ +--+ +--+
>      |      |PB| |PB| |PB| Playout buffer, process RTCP, FEC, etc.
>      |      +--+ +--+ +--+
>      +--      |   |     |
>      +--      |  /      |
>      |        +---+     |
>      |         /  |     |
>   C  |      +--+ +--+ +--+
>      |      |CR| |CR| |CR| Codecs and rendering
>      |      +--+ +--+ +--+
>      +--
>
> If your algorithm is for demultiplexing at layer C, i.e., to figure out what codec and rendering pipeline to use, then I think we're in agreement apart from terminology.
>
> For layer B, I believe the SSRC is the right thing to use to demultiplex, and fits with RTP and RTCP. This is where RTCP is processed, playout de-jitter buffering happens, FEC is processed, NACKs are sent, etc. It's logically independent of the decoding and rendering process since you can start filling your de-jitter buffer for an SSRC before you figure out if/how/where you're going to render that SSRC.
>
> For layer A, there was a clear-cut way of doing this with RTP, RTCP, and STUN. I haven't looked at SCTP enough to know how that affects things. I do think it's a logically separate issue, and should be documented separately to BUNDLE though, since it the same issues arise with non-bundled sessions.
>
> An implementation might merge these together, of course, but to avoid confusion the standards should be clear what level they're considering.
>
> Colin
>
>
>
> On 27 May 2013, at 14:15, Cullen Jennings (fluffy) wrote:
>> Great - sounds like we agree this algorithm will work.
>>
>> On May 27, 2013, at 6:41 AM, Colin Perkins <[e-mail]> wrote:
>>
>>> I'm not sure I agree.
>>>
>>> As I said in my previous message to the list, if we are agreed that the m= lines in a BUNDLE group form a single RTP session, then I believe we need unique payload types across all m= lines. In this case, BUNDLE can simply say that regular RTP source demultiplexing based on the SSRC has to be performed, then the payload type can be used to match sources to m= lines for those applications that care about doing so.
>>>
>>> If we're not agreed that the m= lines in a BUNDLE group form a single RTP session, then we have a lot more to discuss...
>>>
>>> Colin
>>>
>>>
>>>
>>> On 23 May 2013, at 19:02, Cullen Jennings (fluffy) wrote:
>>>> Here's is my proposal for roughly what the bundle draft should say about this demux topic
>>>>
>>>> Application will decide which packet processing pipeline to pass an given RTP/RTCP packet to based on what the application knows:
>>>>
>>>> 1) If future RFCs define new things (like RTP header extension), that explicitly specify the mapping, check if that future RFC is in use and if so then use that to form the mapping
>>>>
>>>> 2) If the PT type is uniquely identifies a mapping, use that to form the mapping
>>>>
>>>> 3) If application knows the SSRC the other side will use, use that to form the mapping
>>>>
>>>> 4) if there is no way to know which pipeline to pass it too, the packet MAY be discarded or the application MAY decide to buffer it until the mapping is known
>>>>
>>>> This is trivial to implement. It meets the requirements for Plan A, Plan B, UCIF, CLUE, and so on.
>>>>
>>>> We could swap the order of step 2 and 3, My thinking for this order was the only time it made any difference the order was if the PT were unique and indicated a different mapping than the SSRC. The only way this could happen is with a SSRC collision so the PT is the one that would be correct not the SSRC. If someone feels strongly the order of steps 2 and 3 should be the opposite way around, I can live with that.
>>>>
>>>>
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>>>
>>>
>>>
>>> --
>>> Colin Perkins
>>> http://csperkins.org/
>>>
>>>
>>>
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