Re: [MMUSIC] Scope of RTP payload types in BUNDLE?

Cullen Jennings <fluffy@iii.ca> Wed, 05 June 2013 00:02 UTC

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From: Cullen Jennings <fluffy@iii.ca>
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Date: Mon, 3 Jun 2013 16:51:28 -0600
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To: Colin Perkins <csp@csperkins.org>
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Cc: "mmusic@ietf.org WG" <mmusic@ietf.org>, Christer Holmberg <christer.holmberg@ericsson.com>
Subject: Re: [MMUSIC] Scope of RTP payload types in BUNDLE?
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On May 27, 2013, at 12:52 PM, Colin Perkins <csp@csperkins.org> wrote:

> On 27 May 2013, at 19:43, Christer Holmberg wrote:
>>>>>>>> v=0
>>>>>>>> o=alice 2890844526 2890844526 IN IP4 host.anywhere.com s= c=IN IP4
>>>>>>>> host.anywhere.com
>>>>>>>> t=0 0
>>>>>>>> m=audio 49170 RTP/AVP 96
>>>>>>>> a=rtpmap:96 AMR-WB/16000
>>>>>>>> m=audio 49170 RTP/AVP ???
>>>>>>>> a=rtpmap:??? AMR-WB/16000
>>>>>>> 
>>>>>>> Not sure I get your point. You can have two different payload types that map to the same payload format in a single RTP session, since
>>>>>>> you can always distinguish what payload format is intended. You can't have the same payload type mapping to two different payload
>>>>>>> formats in a single RTP session, since you can't then infer what payload format was meant.
>>>>>> 
>>>>>> Please not that both PTs map to the SAME payload format :)
>>>>> 
>>>>> I thought I addressed that in my reply.
>>>> 
>>>> I am not sure you did - at least I didn't get it :)
>>>> 
>>>> Again, my understanding of what you said is that, for any payload format within an RTP session, the PT value has to be unique.
>>> 
>>> Correct.
>>> 
>>>> In the example above the same payload format, within the same RTP session, is used in two separate m- lines. But, I still can't use the same PT value for both m- lines, even if the payload format is the same, can I?
>> 
>>> Why not? I don't see any problem mapping the same payload type to the exact same payload format in two different m= lines.
>> 
>> But, when I receive media with that payload format, how do I know to which m- line it "belongs", as the same PT value is used for both m- lines?
> 
> 
> You don't. If you care about that, you need to either use distinct PT values for each m= line, or signal SSRCs. 
> 

+1 

I think this pretty much nails it so if we could come up some text that said this, and did not have the terminology confusing I had introduced about what happens at layer B, I think that would be excellent.