Re: [rtcweb] No Plan

Paul Kyzivat <pkyzivat@alum.mit.edu> Mon, 03 June 2013 20:47 UTC

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Date: Mon, 03 Jun 2013 16:40:17 -0400
From: Paul Kyzivat <pkyzivat@alum.mit.edu>
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Subject: Re: [rtcweb] No Plan
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+1

The more we dig into this the more it looks like Plan B.

	Thanks,
	Paul

On 6/1/13 7:05 AM, Christer Holmberg wrote:
>
> Hi,
>
>>> The draft says:
>>>
>>>        "For the sake of interoperability this specification strongly advises
>>>        against the use of multiple m= lines for a single media type."
>>
>> This should probably be clarified. The above referred mostly to a
>> browser's expectations and default offers. Multiple m= lines can confuse
>> a number of existing legacy endpoints which is why they should be
>> avoided when initiating a session that could reach a similar device (and
>> by default this should be assumed for any session).
>>
>> If applications *know* that they need to have multiple m= lines of a
>> given type they can request this the same way they could do it with Plan B:
>>
>>     If the application wishes, it can request that a given
>>     media source be placed onto a separate m= line, by setting a new
>>     .content property on the desired MediaStreamTrack; the values for the
>>     .content property are those defined for the a=content attribute in
>>     [RFC4796].
>>
>> I'll make sure this is part of the next version.
>>
>> Does this make sense?
>
> I have nothing against a general recommendation to, for a given media type, have as few m- lines as possible.
>
> But, I do think the draft need to point out that it is not always possible, e.g. because:
>
> 1) m- lines have different characteristics (normally indicated using SDP attributes) that does not "fit" all content for the given media type;
> 2) different protocols are used for different m- lines, even if the media type is the same; or
> 3) the remote endpoint only supports a single (or, another given number) of sources per m- line.
>
> Etc.
>
> Regards,
>
> Christer
>
>
>
>
>
>> My understanding is that the usage of multiple m= lines for a single media type would not affect the mechanism as such, but I just want to verify that :)
>>
>> Also, there ARE "legacy" implementations that use multiple m= lines for a single media type (e.g. video enabled devices using two video m= lines: one for camera content, and one for slides).
>>
>> So, while I definitely think that legacy interoperability shall be taken into consideration, I would not like to make such strong statements. In my opinion (the draft also talks about it), the usage of multiple simultaneous SSRCs per m- line is a much bigger issue when it comes to legacy interoperability.
>>
>> Also, in CLUE we have been working on signaling scenarios with multiple m= lines per media type.
>   >
>>
>> Regards,
>>
>> Christer
>>
>>
>>
>> -----Original Message-----
>> From: rtcweb-bounces@ietf.org [mailto:rtcweb-bounces@ietf.org] On Behalf Of Emil Ivov
>> Sent: 29. toukokuuta 2013 22:00
>> To: rtcweb@ietf.org
>> Subject: [rtcweb] No Plan
>>
>> Hey all,
>>
>> Based on many of the discussions that we've had here, as well as many others that we've had offlist, it seemed like a good idea to investigate a negotiation alternative that relies on SDP and Offer/Answer just a little bit less.
>>
>> The following "no plan" draft attempts to present one such approach:
>>
>> http://tools.ietf.org/html/draft-ivov-rtcweb-noplan
>>
>> The draft relies on conventional use of SDP O/A but leaves the intricacies of multi-source scenarios to application-specific signalling, with potentially a little help from RTP.
>>
>> Hopefully, proponents of Plans A and B would find that the interoperability requirements that concerned them can still be met with "no plan". Of course they would have to be addressed by application-specific signalling and/or signalling gateways.
>>
>> Comments are welcome!
>>
>> Cheers,
>> Emil
>>
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>>
>
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