Increase requirements for Plan-B detection

Before a SessionDescription was assumed to be Plan-B if it contained a
MediaName of `audio` or `video`. This PR Changes that behavior.

If SDPSemantics is UnifiedPlan the SessionDescription must contain a
MediaDescription with two SSRCes to be considered a Plan-B. Before if
the MediaDescription had name of `audio` or `video` it was assumed to be
Plan-B.

If SDPSemantics is PlanB or UnifiedPlanWithFallback it maintains the
existing behavior.

Resolves #2193

peerconnection.go

@@ -635,8 +635,8 @@ func (pc *PeerConnection) CreateOffer(options *OfferOptions) (SessionDescription
 		// in-parallel steps to create an offer
 		// https://w3c.github.io/webrtc-pc/#dfn-in-parallel-steps-to-create-an-offer
 		isPlanB := pc.configuration.SDPSemantics == SDPSemanticsPlanB
-		if pc.currentRemoteDescription != nil {
+		if pc.currentRemoteDescription != nil && isPlanB {
-			isPlanB = descriptionIsPlanB(pc.currentRemoteDescription)
+			isPlanB = descriptionPossiblyPlanB(pc.currentRemoteDescription)
 		}
 
 		// include unmatched local transceivers
@@ -1032,7 +1032,11 @@ func (pc *PeerConnection) SetRemoteDescription(desc SessionDescription) error {
 
 	var t *RTPTransceiver
 	localTransceivers := append([]*RTPTransceiver{}, pc.GetTransceivers()...)
-	detectedPlanB := descriptionIsPlanB(pc.RemoteDescription())
+	detectedPlanB := descriptionIsPlanB(pc.RemoteDescription(), pc.log)
+	if pc.configuration.SDPSemantics != SDPSemanticsUnifiedPlan {
+		detectedPlanB = descriptionPossiblyPlanB(pc.RemoteDescription())
+	}
+
 	weOffer := desc.Type == SDPTypeAnswer
 
 	if !weOffer && !detectedPlanB {
@@ -1355,7 +1359,7 @@ func (pc *PeerConnection) startRTPReceivers(remoteDesc *SessionDescription, curr
 	case SDPSemanticsPlanB:
 		remoteIsPlanB = true
 	case SDPSemanticsUnifiedPlanWithFallback:
-		remoteIsPlanB = descriptionIsPlanB(pc.RemoteDescription())
+		remoteIsPlanB = descriptionPossiblyPlanB(pc.RemoteDescription())
 	default:
 		// none
 	}
@@ -2281,7 +2285,12 @@ func (pc *PeerConnection) generateMatchedSDP(transceivers []*RTPTransceiver, use
 	}
 	isExtmapAllowMixed := isExtMapAllowMixedSet(remoteDescription.parsed)
 	localTransceivers := append([]*RTPTransceiver{}, transceivers...)
-	detectedPlanB := descriptionIsPlanB(remoteDescription)
+
+	detectedPlanB := descriptionIsPlanB(remoteDescription, pc.log)
+	if pc.configuration.SDPSemantics != SDPSemanticsUnifiedPlan {
+		detectedPlanB = descriptionPossiblyPlanB(remoteDescription)
+	}
+
 	mediaSections := []mediaSection{}
 	alreadyHaveApplicationMediaSection := false
 	for _, media := range remoteDescription.parsed.MediaDescriptions {

peerconnection_test.go

@@ -486,6 +486,7 @@ t=0 0
 a=group:BUNDLE audio
 a=msid-semantic: WMS 2867270241552712
 m=video 0 UDP/TLS/RTP/SAVPF 0
+a=mid:video
 c=IN IP4 192.168.84.254
 a=inactive
 m=audio 9 UDP/TLS/RTP/SAVPF 111

sdp.go

@@ -568,7 +568,29 @@ func getMidValue(media *sdp.MediaDescription) string {
 	return ""
 }
 
-func descriptionIsPlanB(desc *SessionDescription) bool {
+// SessionDescription contains a MediaSection with Multiple SSRCs, it is Plan-B
+func descriptionIsPlanB(desc *SessionDescription, log logging.LeveledLogger) bool {
+	if desc == nil || desc.parsed == nil {
+		return false
+	}
+
+	// Store all MIDs that already contain a track
+	midWithTrack := map[string]bool{}
+
+	for _, trackDetail := range trackDetailsFromSDP(log, desc.parsed) {
+		if _, ok := midWithTrack[trackDetail.mid]; ok {
+			return true
+		}
+		midWithTrack[trackDetail.mid] = true
+	}
+
+	return false
+}
+
+// SessionDescription contains a MediaSection with name `audio`, `video` or `data`
+// If only one SSRC is set we can't know if it is Plan-B or Unified. If users have
+// set fallback mode assume it is Plan-B
+func descriptionPossiblyPlanB(desc *SessionDescription) bool {
 	if desc == nil || desc.parsed == nil {
 		return false
 	}

sdpsemantics_test.go

@@ -325,7 +325,7 @@ func TestSDPSemantics_UnifiedPlanWithFallback(t *testing.T) {
 
 // Assert that we can catch Remote SessionDescription that don't match our Semantics
 func TestSDPSemantics_SetRemoteDescription_Mismatch(t *testing.T) {
-	planBOffer := "v=0\r\no=- 4648475892259889561 3 IN IP4 127.0.0.1\r\ns=-\r\nt=0 0\r\na=group:BUNDLE video audio\r\na=ice-ufrag:1hhfzwf0ijpzm\r\na=ice-pwd:jm5puo2ab1op3vs59ca53bdk7s\r\na=fingerprint:sha-256 40:42:FB:47:87:52:BF:CB:EC:3A:DF:EB:06:DA:2D:B7:2F:59:42:10:23:7B:9D:4C:C9:58:DD:FF:A2:8F:17:67\r\nm=video 9 UDP/TLS/RTP/SAVPF 96\r\nc=IN IP4 0.0.0.0\r\na=rtcp:9 IN IP4 0.0.0.0\r\na=setup:passive\r\na=mid:video\r\na=sendonly\r\na=rtcp-mux\r\na=rtpmap:96 H264/90000\r\na=rtcp-fb:96 nack\r\na=rtcp-fb:96 goog-remb\r\na=fmtp:96 packetization-mode=1;profile-level-id=42e01f\r\na=ssrc:1505338584 cname:10000000b5810aac\r\nm=audio 9 UDP/TLS/RTP/SAVPF 111\r\nc=IN IP4 0.0.0.0\r\na=rtcp:9 IN IP4 0.0.0.0\r\na=setup:passive\r\na=mid:audio\r\na=sendonly\r\na=rtcp-mux\r\na=rtpmap:111 opus/48000/2\r\na=ssrc:697641945 cname:10000000b5810aac\r\n"
+	planBOffer := "v=0\r\no=- 4648475892259889561 3 IN IP4 127.0.0.1\r\ns=-\r\nt=0 0\r\na=group:BUNDLE video audio\r\na=ice-ufrag:1hhfzwf0ijpzm\r\na=ice-pwd:jm5puo2ab1op3vs59ca53bdk7s\r\na=fingerprint:sha-256 40:42:FB:47:87:52:BF:CB:EC:3A:DF:EB:06:DA:2D:B7:2F:59:42:10:23:7B:9D:4C:C9:58:DD:FF:A2:8F:17:67\r\nm=video 9 UDP/TLS/RTP/SAVPF 96\r\nc=IN IP4 0.0.0.0\r\na=rtcp:9 IN IP4 0.0.0.0\r\na=setup:passive\r\na=mid:video\r\na=sendonly\r\na=rtcp-mux\r\na=rtpmap:96 H264/90000\r\na=rtcp-fb:96 nack\r\na=rtcp-fb:96 goog-remb\r\na=fmtp:96 packetization-mode=1;profile-level-id=42e01f\r\na=ssrc:1505338584 cname:10000000b5810aac\r\na=ssrc:1 cname:trackB\r\nm=audio 9 UDP/TLS/RTP/SAVPF 111\r\nc=IN IP4 0.0.0.0\r\na=rtcp:9 IN IP4 0.0.0.0\r\na=setup:passive\r\na=mid:audio\r\na=sendonly\r\na=rtcp-mux\r\na=rtpmap:111 opus/48000/2\r\na=ssrc:697641945 cname:10000000b5810aac\r\n"
 	unifiedPlanOffer := "v=0\r\no=- 4648475892259889561 3 IN IP4 127.0.0.1\r\ns=-\r\nt=0 0\r\na=group:BUNDLE 0 1\r\na=ice-ufrag:1hhfzwf0ijpzm\r\na=ice-pwd:jm5puo2ab1op3vs59ca53bdk7s\r\na=fingerprint:sha-256 40:42:FB:47:87:52:BF:CB:EC:3A:DF:EB:06:DA:2D:B7:2F:59:42:10:23:7B:9D:4C:C9:58:DD:FF:A2:8F:17:67\r\nm=video 9 UDP/TLS/RTP/SAVPF 96\r\nc=IN IP4 0.0.0.0\r\na=rtcp:9 IN IP4 0.0.0.0\r\na=setup:passive\r\na=mid:0\r\na=sendonly\r\na=rtcp-mux\r\na=rtpmap:96 H264/90000\r\na=rtcp-fb:96 nack\r\na=rtcp-fb:96 goog-remb\r\na=fmtp:96 packetization-mode=1;profile-level-id=42e01f\r\na=ssrc:1505338584 cname:10000000b5810aac\r\nm=audio 9 UDP/TLS/RTP/SAVPF 111\r\nc=IN IP4 0.0.0.0\r\na=rtcp:9 IN IP4 0.0.0.0\r\na=setup:passive\r\na=mid:1\r\na=sendonly\r\na=rtcp-mux\r\na=rtpmap:111 opus/48000/2\r\na=ssrc:697641945 cname:10000000b5810aac\r\n"
 
 	report := test.CheckRoutines(t)