Re: [rtcweb] No Plan

Emil Ivov <emcho@jitsi.org> Tue, 04 June 2013 16:12 UTC

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Date: Tue, 04 Jun 2013 15:08:49 +0300
From: Emil Ivov <emcho@jitsi.org>
Organization: Jitsi
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To: Jim Barnett <Jim.Barnett@genesyslab.com>
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Cc: Jonathan Lennox <jonathan@vidyo.com>, "rtcweb@ietf.org" <rtcweb@ietf.org>
Subject: Re: [rtcweb] No Plan
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Hey Jim,

On 04.06.13, 01:16, Jim Barnett wrote:
> Just checking if I understand No Plan:  since the bundling is added via JS APIs, and not SDP,

I am not sure where you got this. No Plan does depend on the browser to 
provide support for BUNDLE and to generate the SDP accordingly.

Emil

> then if the app looks at
> The localDescription and remoteDescription attributes of the PeerConnection, it won't be able to tell if sources are bundled or not (since localDescription and remoteDescription are defined to contain SDP)
>
> - Jim
> -----Original Message-----
> From: rtcweb-bounces@ietf.org [mailto:rtcweb-bounces@ietf.org] On Behalf Of Jonathan Lennox
> Sent: Monday, June 03, 2013 6:03 PM
> To: Paul Kyzivat
> Cc: rtcweb@ietf.org
> Subject: Re: [rtcweb] No Plan
>
>
> On Jun 3, 2013, at 4:40 PM, Paul Kyzivat <pkyzivat@alum.mit.edu> wrote:
>
>> +1
>>
>> The more we dig into this the more it looks like Plan B.
>>
>> 	Thanks,
>> 	Paul
>
> I think No Plan and Plan B are largely isomorphic in a WebRTC context.  The primary difference is that rather than using a=ssrc and a=receive-ssrc (and whatever other Plan B attributes would be needed) to control sources within an m-line, instead direct Javascript APIs are used for the same semantics. It's up to the application to communicate the relevant information end-to-end, in parallel to however it communicates the SDP blob.
>
> As I see it, the primary advantages of No Plan, then, are that:
>
> a) It allows "implicit" signaling of sources -- where that's sufficient -- by just sending RTP traffic.  This prevents problems related to the timing and synchronization between signaling and media.  (Note that Plan B probably actually needs this as well, for legacy interop.)
>
> b) It carves a Comment 22 corner out of the SDP-based APIs, thus decoupling source signaling from SDP offer/answer.  This is for a scenario where there aren't any issues of legacy compatibility, and there isn't any (or at least much) existing IETF standardization -- as best I can recall, legacy compatibility and existing standardization were the primary motivations for using SDP as the WebRTC API.
>
> Other than that, I think the two proposals are pretty similar for WebRTC, and it would probably be a Simple Matter of Programming to translate between the two (though you have to worry about the O/A state on the Plan B side).
>
> In particular, if you have disaggregated media, you need multiple m-lines of the same type.  The No Plan document's suggestion that you offer only one m-line of a given media type is guidance on how to avoid interop failures on initial offers to legacy devices, I think, not an essential aspect of the proposal.
>
>
> In a SIP context, No Plan requires defining a separate signaling channel for this source information, in scenarios where something beyond implicit signaling is needed.  As Emil has stated, I think that something like the SIP/XCon conference package, or something like the CLUE channel, would be the obvious places to start.
>
> I think SIP, even more than RTCWeb, is where you'd want something lighter-weight and better-designed than SDP offer/answer to signal source changes.
>
>> 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
>>>>
>>>> --
>>>> https://jitsi.org
>>>> _______________________________________________
>>>> rtcweb mailing list
>>>> rtcweb@ietf.org
>>>> https://www.ietf.org/mailman/listinfo/rtcweb
>>>> .
>>>>
>>>
>>> --
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>>>
>>
>> _______________________________________________
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>> rtcweb@ietf.org
>> https://www.ietf.org/mailman/listinfo/rtcweb
>>
>
> --
> Jonathan Lennox
> jonathan@vidyo.com
>
>
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>
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