Update of RTCRtpSender "doohickey" proposal

Update of the proposal from earlier this year, addressing the feedback from
the list. Mainly, the issues with AddStream and AddTrack have been resolved
by adding an explicit |stream| parameter to AddTrack.


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I suggest we call the SendDoohickeys RTCRtpSenders and the corresponding
receive-side objects RTCRtpReceivers. These objects allow control of how a
MediaStreamTrack is encoded and sent on the wire, including "hold" state,
prioritization, and multiple encodings (e.g. simulcast).

You get a RTCRtpSender as a return value from AddTrack (which replaces
AddStream). You get a RTCRtpReceiver as an argument to the new onaddtrack
callback (which replaces onaddstream). The actual track object can be
obtained as a property from the RTCRtpReceiver (see below).

For getting access to ICE/DTLS info, both RTCRtpSenders and RTCRtpReceivers
can also have a reference to a RTCDtlsTransport object, which will have its
own state variables, including the RTCIceConnectionState of that particular
transport, and the .remoteCertificates for the DTLS connection. This allows
applications to monitor the state of individual transports, as well as
inspect the certificates for individual transports.

The actual interface is as follows:

// used with RTCRtpSender

interface RTCDtlsTransport {

 attribute RTCIceConnectionState state;

 sequence<ArrayBuffer> getRemoteCertificates();

 //... more stuff as needed

};

// used with RTCRtpSender

interface RTCRtpEncodingParams {

    double               priority = 1.0;  // relative priority of this
encoding

    unsigned int       maxBitrate = null;  // maximum bits to use for this
encoding

    boolean             active;  // sending or "paused/onhold"

};

// the "send" doohickey

interface RTCRtpSender {

 readonly attribute MediaStreamTrack track;

 readonly attribute RTCDtlsTransport transport;

 // 1-N encodings; in the future, N can be > 1, for simulcast or layered
coding

 // Each EncodingParams specifies the details of what to send (e.g. bitrate)

 sequence<RTCRtpEncodingParams> getEncodings();

};

// the "receive" doohickey

interface RTCRtpReceiver {

  readonly attribute RTCDtlsTransport transport;

  readonly attribute MediaStreamTrack track;

  // more stuff as needed

};

// parameter to the onaddtrack event

interface RemoteTrackEvent : Event {

 readonly attribute RtpReceiver receiver;

 readonly attribute MediaStreamTrack track;

 readonly attribute MediaStream stream;

};

partial interface RTCPeerConnection {

 // because a track can be part of multiple streams, the id parameter

  // indicates which particular stream should be referenced in signaling

 RtpSender addTrack(track, streamId);  // replaces addStream



 void removeTrack(RtpSender);  // replaces removeStream

 sequence<RtpSender> getSenders();

 sequence<RtpReceiver> getReceivers();

 EventHandler onaddtrack;  // replaces onaddstream; event object is
RemoteTrackEvent.

                                        // note that onremovestream is not
needed, since tracks are 'removed'

                                        // simply by progressing to the
ENDED state

};

For backcompat, addStream, removeStream, getLocalStreams, getRemoteStreams,
and onaddstream can be trivially polyfilled in JS, so there is minimal
impact on current applications.

All together, the pipeline looks like this:

Source ---> MST ---> RtpSender ---> DtlsTransport ---> (The Internet) --->
DtlsTransport ---> RtpReceiver ---> MST ---> <video/>

Each RtpSender/Receiver references a single MST, although a single
RtpSender/Receiver can send/receive multiple encodings (e.g. simulcast).

There are N RtpSenders/Receivers per DtlsTransport; N is controlled by the
policy specified for BUNDLE.