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

Christer Holmberg <christer.holmberg@ericsson.com> Wed, 05 June 2013 17:01 UTC

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From: Christer Holmberg <christer.holmberg@ericsson.com>
To: Paul Kyzivat <pkyzivat@alum.mit.edu>
Thread-Topic: VS: [MMUSIC] Scope of RTP payload types in BUNDLE?
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Date: Wed, 05 Jun 2013 17:01:25 +0000
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Cc: "mmusic@ietf.org" <mmusic@ietf.org>, Cullen Jennings <fluffy@iii.ca>
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.
>
> Christer - I suspect we are on the same page.
>
> I agree there are many reasons one might want to bundle m-lines.
> But I assert that every one of those reasons will also require that you be able to associate incoming packets with a particular one of those m-lines.
>
> I don't understand why this is in question. I originally stated it as a truism because I thought it was self evident.

Me too. My assumption has from the start been that we need *A* mechanism to associate media with m- lines, as there might be m- line specific characteristics (I haven't gone through all SDP attributes etc with that in mind, though, so I can't give concrete examples at this point) that are needed in order to properly process the media. I thought that relying on PT and/or SSRC/CNAME would cover all cases, but obviously it doesn't. 

For example, I PERSONALLY think one can assume to receive an SDP Answer (including SSRC/CNAME information etc) before media is received. But, as Cullen has another opinion I don't want to waste time on arguing about that - and I am fully aware what 3264 says about receiving media before the SDP answer :)

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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