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

Colin Perkins <csp@csperkins.org> Mon, 27 May 2013 18:41 UTC

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From: Colin Perkins <csp@csperkins.org>
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Subject: Re: [MMUSIC] Scope of RTP payload types in BUNDLE?
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On 27 May 2013, at 19:38, Paul Kyzivat wrote:
> On 5/27/13 2:10 PM, Colin Perkins wrote:
>> On 27 May 2013, at 18:55, 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.
> 
> Now you're both confused. What you are trying to say is that for any PT value withing an RTP session the payload format must be unique.
> 
> E.g., two different PTs may map to the same format, but one PT may not map to two different formats.
> 
> But I continue to argue that we could relax that, as long as there is something else that selects which PT mapping to use.

Yes, but why?

>>> 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. Mapping the same payload type to two different payload formats is the problem.
> 
> It depends. *If* you are trying to use the PT to associate packets to an m-line, then indeed you might need to use different PT values for the same payload format in order to accomplish that.


Sure, but no-one is arguing against that. The problem is using the same PT for different payload formats. 

-- 
Colin Perkins
http://csperkins.org/