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

Paul Kyzivat <> Mon, 27 May 2013 18:38 UTC

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Subject: Re: [MMUSIC] Scope of RTP payload types in BUNDLE?
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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 s= c=IN IP4
>>>>>> 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.

>> 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.