In the WebRTC draft, Section 4.3.2.2, there is a method addStream() on the RTCPeerConnection object. It is expected that this will add a stream to the local peer connection.  At the receiving end the onaddstream event is expected to fire when the SDP is passed into setRemoteDescription(). It is critical that it be possible to describe streams in such a way that the receiver can construct a MediaStream that consists of tracks in the same configuration as the stream added by the sender.  There is no specification within this document and its normative references defining how the labels used on MediaStream and MediaStreamTrack correspond to RTP constructs or how information about streams is conveyed between peers such that a receiver can successfully reconstruct a MediaStream in the same configuration that a sender used.