mid property specifies the negotiated media ID (
mid) which the local and remote peers have agreed upon to uniquely identify the stream's pairing of sender and receiver.
var mediaID = RTCRtpTransceiver.mid;
DOMString which uniquely identifies the pairing of source and destination of the transceiver's stream. Its value is taken from the media ID of the SDP m-line. This value is
null if negotiation has not completed.
|WebRTC 1.0: Real-time Communication Between Browsers
The definition of 'RTCRtpTransceiver.mid' in that specification.