Proposal for SctpTransport (and big picture diagram)

A missing piece of the current ORTC API is SCTP data channels.  I
think this part of WebRTC 1.0 is pretty good, we this exception of its
dependence on SDP and PeerConnection.  I propose we add a
createDataChannel method and a ondatachannel event similar to the one
on WebRTC 1.0's PeerConnection.  But instead of adding it to directly
to DtlsTransport, we create an SctpTransport which wraps a
DtlsTransport, like so:


[Constructor(RTCDtlsTransport)]
interface RTCSctpTransport {
 attribute RTCDtlsTransport transport;

 static RTCSctpCapabilities getCapabilities();

 void start(RTCSctpCapabilities remoteCaps);
 void stop();

 DataChannel createDataChannel(RTCDataChannelParameters);
 EventHandler<DataChannel> ondatachannel;
}

dictionary RTCSctpCapabilities {
 int maxMessageSize;
}

// Same as WebRTC 1.0 DataChannelInit
dictionary RTCDataChannelParameters {
 boolean outOfOrderAllowed;
 unsigned short maxRetransmitTime;
 unsigned short maxRetransmitNum;
 DOMString protocol;
 boolean preset;
 unsigned short stream;
}

This is similar to the good parts of WebRTC 1.0, but also signals
every thing without SDP, hooks up to DtlsTransport nicely, and gives
the programmer easy control of which transport to use (shared with
media or not).  Furthermore, it allows future expansion into other
types of data channels, or data channels built on other types of
transports.

Here is an example of how it would be used:

function initiate(signaller) {
  var dtls = ...;  // See ICE/DTLS example.
  var sctp = new RTCSctpTransport(dtls);

  signaller.sendInitiate({
    // ... include ICE/DTLS info from other example.
  sctpCapabilities: RTCSctpTransport.getCapabilities()
  }, function(remote) {
    sctp.start(remote.sctpCapabilities);
  });

  var channel = sctp.createDataChannel({...});
  channel.send("foo");
}

function accept(signaller, remote) {
  var dtls = ...;  // See ICE/DTLS example.
  signaller.sendAccept({
    // ... include ICE/DTLS info from other example.
    "sctpCapabilities": RTCSctpTransport.getCapabilities()
  });

  var sctp = new RTCSctpTransport(dtls);
  sctp.start(remote.sctpCapabilties);

  // Assume in-band signalling.  We could also easily add
  // RTCDataChannelParameters into the out-of-band signalling
  // And call .createDataChannel here with negotiated: true.

  sctp.ondatachannel = function(channel) {
    channel.onmessage = function(message) {
    if (message == "foo") {
      channel.send("bar");
    }
  }
}

Altogether, if our proposals for RtpSender, RtpReceiver, IceTransport,
DtlsTransport, and SctpTransport are accepted, the overall picture
would look like the one attached.