The proposal for RTCRtpSender /Receiver sent to the mailing list on April 28 is reproduced below. The April 28 meeting seemed to agree that this is a sensible direction, but details may need adjusting before integrating.

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.