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

Christer Holmberg <christer.holmberg@ericsson.com> Wed, 05 June 2013 16:24 UTC

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From: Christer Holmberg <christer.holmberg@ericsson.com>
To: Paul Kyzivat <pkyzivat@alum.mit.edu>, Cullen Jennings <fluffy@iii.ca>
Thread-Topic: [MMUSIC] Scope of RTP payload types in BUNDLE?
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Date: Wed, 05 Jun 2013 16:23:57 +0000
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Subject: Re: [MMUSIC] Scope of RTP payload types in BUNDLE?
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Hi,

I think there are many reasons why one would want to use different m- lines. Eventhough the transport characteristics are the same (if you use BUNDLE, that is), there other things that may be different. Just to give a simple example: the direction attribute value, content attribute value, and anything related to the "scope"/"context" of the m- line.

Regards,

Christer




-----Alkuperäinen viesti-----
Lähettäjä: mmusic-bounces@ietf.org [mailto:mmusic-bounces@ietf.org] Puolesta Paul Kyzivat
Lähetetty: 5. kesäkuuta 2013 18:53
Vastaanottaja: Cullen Jennings
Kopio: mmusic@ietf.org
Aihe: Re: [MMUSIC] Scope of RTP payload types in BUNDLE?

On 6/3/13 6:49 PM, Cullen Jennings wrote:
>
> On May 27, 2013, at 1:13 PM, Paul Kyzivat <pkyzivat@alum.mit.edu> wrote:
>
>> On 5/27/13 2:52 PM, Colin Perkins wrote:
>>
>>>>>> 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.
>>
>> I assert that if you don't care about associating a packet to a particular m-line in a bundle that you don't need to use bundle.
>
> I'm not sure this makes a huge difference to your argument but I don't think I agree with the above.
>
> It seems that even if you don't care which m-line the media get associated with, if you want to use a single port for all the m-line, you still need bundle.

I don't understand. Why do you want to use multiple m-lines on one port? 
If you don't need to associate the packets with a particular one of those m-lines, then why didn't you use a single m-line?

E.g., if your reason is because one is audio and another is video then you certainly want to associate the packets with one or the other.

ISTM that the whole point of Plan A is that each m-line is associated with a specific use/rendering, and so it is essential to associate incoming packets with a particular one of the m-lines.

	Thanks,
	Paul

>> So if you used bundle, you must care, and so there must be a way to achieve that association.
>>
>> The reason you care is because doing so allows you to discover all the other things in that media section of the SDP that may be pertinent to this packet.
>>
>> 	Thanks,
>> 	Paul
>
>

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