WebRTC RTCConfiguration中的sdp_sem

// Configure the SDP semantics used by this PeerConnection. Note that the
// WebRTC 1.0 specification requires kUnifiedPlan semantics. The
// RtpTransceiver API is only available with kUnifiedPlan semantics.
//
// kPlanB will cause PeerConnection to create offers and answers with at
// most one audio and one video m= section with multiple RtpSenders and
// RtpReceivers specified as multiple a=ssrc lines within the section. This
// will also cause PeerConnection to ignore all but the first m= section of
// the same media type.
//
// kUnifiedPlan will cause PeerConnection to create offers and answers with
// multiple m= sections where each m= section maps to one RtpSender and one
// RtpReceiver (an RtpTransceiver), either both audio or both video. This
// will also cause PeerConnection to ignore all but the first a=ssrc lines
// that form a Plan B stream.
//
// For users who only send at most one audio and one video track, this
// choice does not matter and should be left as kDefault.
//
// For users who wish to send multiple audio/video streams and need to stay
// interoperable with legacy WebRTC implementations, specify kPlanB.
//
// For users who wish to send multiple audio/video streams and/or wish to
// use the new RtpTransceiver API, specify kUnifiedPlan.
SdpSemantics sdp_semantics = SdpSemantics::kDefault;