The read-only sdpMLineIndex property on the RTCIceCandidate interface is a zero-based index of the m-line describing the media associated with the candidate.

This property can be configured by specifying the value of the sdpMLineIndex property in the candidateInfo options object that is passed to the RTCIceCandidate() constructor. If you call the constructor with an m-line string instead of the options object, the value of sdpMLineIndex is extracted from the specified candidate m-line string.

A number containing a 0-based index into the set of m-lines providing media descriptions, indicating which media source is associated with the candidate, or null if no such association is available.