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Re: Mapping multiple media sources to few or many M-lines

From: Stefan Hakansson LK <stefan.lk.hakansson@ericsson.com>
Date: Thu, 25 Oct 2012 12:34:33 +0200
Message-ID: <508915B9.60208@ericsson.com>
To: public-webrtc@w3.org
Harald,

interesting. I will read and try to understand, and perhaps even provide 
a comment or two, but my immediate comment is: I think this info should 
be sent to the rtcweb and mmusic lists as well.

Much of this is about SDP and RTP sessions; and those protocols are 
developed/handled by IETF.

Stefan

PS Code seems to be missing between the SDP examples at the end.

On 10/24/2012 04:49 PM, Harald Alvestrand wrote:
> Cullen, Justin and I have been working on a proposal for this issue,
> which is one of the things we have to settle in order to know what we're
> designing our signalling for.
>
> One proposal is outlined below - it depends on defining a new attribute
> of a MediaStreamTrack called "content", and using that to direct tracks
> onto different M-lines when negotiating SDP.
>
> Details below.
>
> ==Proposal for controlling the allocation of multiple media sources to
> RTP sessions==
>
> ===Problem description===
>
> There are a number of applications that can be envisioned using WebRTC.
> The applications where one audio and one video stream is connected
> between two participants are trivial; there is no real controversy
> there. But other styles are more difficult.
> Two important cases are:
> A single PeerConnection is used to connect an end-user to a central
> non-mixing MCU (a "RTCP-terminating MCU" in RFC 5117 terminology) and
> the connection between the MCU and the user has a large number of audio
> and/or video tracks (for example, a “thumbnail strip” + one or more
> large video images).
> A single PeerConnection is used to connect an end-user to a non-RTCWEB
> SIP system, through a signalling gateway but not through a media
> gateway, using multiple video sessions that are distinguished by use of
> the “a=content” attribute (for example, a main video feed plus a
> presentation video feed).
>
> In the first case, we definitely want all the video sources in the same
> RTP session, which helps us be able to add or remove video sources with
> minimal overhead (no new ICE ports and NAT pinholes).
>
> In the second case, we want to have specific video sources on different
> RTP sessions, and we want to have exact control over which video streams
> get assigned to what RTP sessions.
> Solution Description
>
>
> The basic idea is to expose a new "content" property on
> MediaStreamTracks, as defined in RFC 4796, which would indicate the
> "usage" of the media in that particular track. When createOffer is
> called to create a session description, it will include a m= line for
> each [media, content] tuple that exists within the list of attached
> MediaStreamTracks.
>
> Since normally m= lines are omitted for tuples that have no associated
> MediaStreamTracks, the application can also include an empty m= line for
> a given tuple by specifying a constraint to createOffer, similar to how
> the existing OfferToReceiveAudio and OfferToReceiveVideo can be used to
> add empty m= lines for audio and video. The suggested form for this
> constraint is to use the existing OfferToReceiveAudio and
> OfferToReceiveVideo keys, but use the content property as the value,
> e.g. "OfferToReceiveVideo:slides".
>
> Individual MediaStreamTracks are represented via a=ssrc attributes on
> the appropriate m= lines; the MSID attribute on the a=ssrc line
> identifies the MediaStreamTrack. There can be an arbitrary number of
> MediaStreamTracks associated with a given m= line, including zero;
> demuxing of these MediaStreamTracks is performed done according to the
> SSRC specified with the a=ssrc attribute.
>
> By default, the content property for MediaStreamTracks is left empty.
> This means that MediaStreamTracks are by default associated with m=
> lines that have no a=content attribute.
>
> createAnswer works the same way as createOffer, using its attached
> MediaStreamTracks, and the constraints supplied; note that it will
> always include m= lines as needed to match the offer, even if no
> MediaStreamTracks are attached.
>
> Through this mechanism, applications that want to make use of multiple
> media streams can generate SDP that best matches what existing
> videoconferencing equipment expects, but this usage is not required;
> sophisticated applications can use the content property to assign their
> own grouping of MediaStreamTracks to m= lines, including the creation of
> individual m= lines for each MediaStreamTrack, or the combination of all
> video MediaStreamTracks into a single m= line. Of course, these
> applications could also do so by generating the SDP themselves and
> passing this SDP into setLocalDescription.
>
> ===Examples===
>
> MediaStream ms1 contains an audio track (denoted a0 in msid lines) and
> video track (v0), as obtained from getUserMedia(). The label of ms1 is
> <ms1.label>.
> MediaStream ms2 contains a single video track (also denoted v0, since
> it’s the first video track in its mediastream), taken from the desktop.
> The label of ms2 is <ms2.label>.
> PeerConnection pc exists, with no streams attached.
> pc.addStream(ms1, null);
> pc.createOffer(null);
>
> produces:
>
> <blah>
> m=audio
> a=ssrc:1234 msid:<ms1.label> a0
> m=video // nothing fancy
> a=ssrc:5678 msid:<ms1.label> v0
> pc.addStream(ms1, null);
> pc.addStream(ms2, null);
> pc.createOffer(null);
>
> produces:
>
> <blah>
> m=audio
> a=ssrc:1234 msid:<ms1.label> a0
> m=video // both tracks associated with one m= line
> a=ssrc:5678 msid:<ms1.label> v0
> a=ssrc:6789 msid:<ms2.label> v0
> pc.addStream(ms1, null);
> pc.createOffer({mandatory:{"OfferToReceiveVideo:slides"}});
>
> produces:
>
> <blah>
> m=audio
> a=ssrc:1234 msid:<ms1.label> a0
> m=video
> a=ssrc:5678 msid:<ms1.label> v0
> m=video // note this empty m= line
> a=content:slides
> pc.addStream(ms1, null);
> v2.content = "slides";
> pc.addStream(ms2, null);
> pc.createOffer(null);
>
> produces:
>
> <blah>
> m=audio
> a=ssrc:1234 msid:<ms1.label> a0
> m=video
> a=ssrc:5678 msid:<ms1.label> v0
> m=video // this m= line has an a=content attribute and a track
> a=content:slides
> a=ssrc:6789 msid:<ms2.label> v0
>
>
>
Received on Thursday, 25 October 2012 10:34:59 UTC

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