[rtcweb] Proposal for a JS API for NoPlan (adding multiple sources without encoding them in SDP)

Peter Thatcher <pthatcher@google.com> Mon, 17 June 2013 12:57 UTC

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From: Peter Thatcher <pthatcher@google.com>
Date: Mon, 17 Jun 2013 05:57:05 -0700
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Subject: [rtcweb] Proposal for a JS API for NoPlan (adding multiple sources without encoding them in SDP)
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Google is in full support of "Plan B" for encoding multiple media sources
in SDP, and would like to see the Plan A vs. Plan B decision resolved soon.
 Recently, though, a third option, called "NoPlan" has been proposed, but
it lacked the details of what a JS API would look like for NoPlan.  Cullen
asked to see such an API proposal, and so I have worked with Emil to make
one.  He will be adding it to the NoPlan draft, but I will also include it
in this email.

Again, Google is in full support of "Plan B".  But if Plan A vs. Plan B
cannot be decided, then we support NoPlan with the following additions to
the WebRTC JS API as an option that allows implementing either Plan A or
Plan B  in Javascript.  And even if Plan A vs. Plan B is resolved, these
API additions would still be a big improvement for those WebRTC
applications that don't use SDP for signalling.

It is a bit long because I have added many comments and examples, but the
actually additions only include two new methods on PeerConnection and a few
new dictionaries.  So don't be overwhelmed :).

Intro: This follows the model of createDataChannel, which has a JS method
on PeerConnection that makes it possible to add data channels without going
through SDP.  Furthermore, just like createDataChannel allows 2 ways to
handle neogitation (the "I know what I'm doing;  Here's what I want to
send; Let me signal everything" mode and the "please take care of it for
me;  send an OPEN message" mode), this also has 2 ways to handle
negotiation (the "I know what I'm doing; Here's what I want to send; Let me
signal everything" mode and the "please take care of it for me;  send SDP
back and forth" mode).

Following the success of createDataChannel, this allows simple applications
to Just Work and more advanced applications to easily control what they
need to.  In particular, it's possible to use this API to implement either
Plan A or Plan B.

// The following two method are added to RTCPeerConnection
partial interface RTCPeerConnection {
 // Create a stream that is used to send a source stream.
 // The MediaSendStream.description can be used for signalling.
 // No media is sent until addStream(MediaSendStream) is called.
 LocalMediaStream createLocalStream(MediaStream sourceStream);

 // Create a stream that is used to receive media from the remote side,
 // given the parameters signalled from MedaiSendStream.description.
 MediaStream createRemoteStream(MediaStreamDescription description);

interface LocalMediaStream implements MediaStream {
  // This can be changed at any time, but especially before calling
  // PeerConnection.addStream
  attribute MediaStreamDescription description;

// Represents the parameters used to either send or receive a stream
// over a PeerConnection.
dictionary MediaStreamDescription {
  MediaStreamTrackDescription[] tracks;

// Represents the parameters used to either send or receive a track over //
a PeerConnection.  A track has many "flows", which can be grouped
// together.
dictionary MediaStreamTrackDescription {
  // Same as the MediaStreamTrack.id
  DOMString id;

  // Same as the MediaStreamTrack.kind
  DOMString kind;

  // A track can have many "flows", such as for Simulcast, FEC, etc.
  // And they can be grouped in arbitrary ways.
  MediaFlowDescription[] flows;
  MediaFlowGroup[] flowGroups;

// Represents the parameters used to either send or receive a "flow"
// over a PeerConnection.  A "flow" is a media that arrives with a
// single, unique SSRC.  One to many flows together make up the media
// for a track.  For example, there may be Simulcast, FEC, and RTX
// flows.
dictionay MediaFlowDescription {
  // The "flow id" must be unique to the track, but need not be unique
  // outside of the track (two tracks could both have a flow with the
  // same flow ID).
  DOMString id;

  // Each flow can go over its own transport.  If the JS sets this to a
  // transportId that doesn't have a transport setup already, the
  // browser will use SDP negotiation to setup a transport to back that
  // transportId.  If This is set to an MID in the SDP, then that MID's
  // transport is used.
  DOMString transportId;

  // The SSRC used to send the flow.
  unsigned int ssrc;

  // When used as receive parameters, this indicates the possible list
  // of codecs that might come in for this flow.  For exmample, a given
  // receive flow could be setup to receive any of OPUS, ISAC, or PCMU.
  // When used as send parameters, this indicates that the first codec
  // should be used, but the browser can use send other codecs if it
  // needs to because of either bandwidth or CPU constraints.
  MediaCodecDescription[] codecs;

dictionary MediaFlowGroup {
  DOMString type;  // "SIM" for Simulcast, "FEC" for FEC, etc
  DOMString[] flowids;

dictionary MediaCodecDescription {
  unsigned byte payloadType;
  DOMString name;
  unsigned int? clockRate;
  unsigned int? bitRate;
  // A grab bag of other fmtp that will need to be further defined.
  MediaCodecParam[] params;

dictionary MediaCodecParam {
  DOMString key;
  DOMString value;


- When LocalMediaStreams are added using addStream, onnegotiatedneeded is
not called, and those streams are never reflected in future SDP exchanges.
 Indeed, it would be impossible to put them in the SDP without first
resolving if that would be Plan A SDP or Plan B SDP.

- Just like piles of attributes would need to be defined for Plan A and for
Plan B, similar attributes would need to be defined here (Luckily,  much
work has already been done figuring out what those parameters are :).


- Either Plan A or Plan B or could be implemented in Javascript using this
- It exposes all the same functionality to the Javascript as SDP, but in a
much nicer format that is much easier to work with.
- Any other signalling mechanism, such as Jingle or CLUE could be
implemented using this API.
- There is almost no risk of signalling glare.
- Debugging errors with misconfigured descriptions should be much easier
with this than with large SDP blobs.


- Now there are two slightly different ways to add streams: by creating a
LocalMediaStream first, and not.  This is, however, analogous to setting
"negotiated: true" in createDataChannel.  On way is "Just Work", and the
other is more advanced control.

- All the options in MediaCodecDescription are a bit complicated.  Really,
this is only necessary because Plan A requires being able to specify codec
parameters per SSRC, and set each flow on different transports.  If we did
not have this requirement, we could simplify.

Example Usage:

// Imagine I have MyApp, handles creating a PeerConnection,
// signalling, and rendering streams.  This is how the new API could be
// used.
var peerConnection = MyApp.createPeerConnection();

// On sender side:
var stream = MyApp.getMediaStream();
var localStream = peerConnection.createSendStream(stream);
sendStream.description = MyApp.modifyStream(localStream.description)
MyApp.signalAddStream(localStream.description, function(response)) {
  if (!response.rejected) {
    // Media will not be sent.

// On receiver side:
MyApp.onAddStreamSignalled = function(streamDescription) {
  var stream = peerConnection.createReceiveStream(streamDescription);

// In this exchange, the MediaStreamDescription signalled from the
// sender to the receiver may have looked something like this:

  tracks: [
    id: "audio1",
    kind: "audio",
    flows: [
      id: "main",
      transportId: "transport1",
      ssrc: 1111,
      codecs: [
        payloadType: 111,
        name: "opus",
        // ... more codec details
        payloadType: 112,
        name: "pcmu",
        // ... more codec details
    id: "video1",
    kind: "video",
    flows: [
      id: "sim0",
      transportId: "transport2",
      ssrc: 2222,
      codecs: [
        payloadType: 122,
        name: "vp8"
        // ... more codec details
     id: "sim1",
     transportId: "transport2",
     ssrc: 2223,
     codecs: [
       payloadType: 122,
       name: "vp8",
       // ... more codec details
     id: "sim2",
     transportId: "transport2",
     ssrc: 2224,
     codecs: [
       payloadType: 122,
       name: "vp8",
       // ... more codec details

     id: "sim0fec",
     transportId: "transport2",
     ssrc: 2225,
     codecs: [
       payloadType: 122,
       name: "vp8",
       // ...
   flowGroups: [
     semantics: "SIM",
     ssrcs: [2222, 2223, 2224]
     semantics: "FEC",
     ssrcs: [2222, 2225]

Constructive feedback is welcome :).