Add an RTX-associated TrackLocal

rtpsender.go

@@ -30,6 +30,8 @@ type trackEncoding struct {
 	context *baseTrackLocalContext
 
 	ssrc SSRC
+
+	rtx *trackEncoding
 }
 
 // RTPSender allows an application to control how a given Track is encoded and transmitted to a remote peer
@@ -199,12 +201,19 @@ func (r *RTPSender) AddEncoding(track TrackLocal) error {
 }
 
 func (r *RTPSender) addEncoding(track TrackLocal) {
-	trackEncoding := &trackEncoding{
+	baseTrackEncoding := &trackEncoding{
 		track: track,
 		ssrc:  SSRC(randutil.NewMathRandomGenerator().Uint32()),
 	}
 
-	r.trackEncodings = append(r.trackEncodings, trackEncoding)
+	if r.api.settingEngine.distinctSSRCRTX {
+		baseTrackEncoding.rtx = &trackEncoding{
+			track: track,
+			ssrc:  SSRC(randutil.NewMathRandomGenerator().Uint32()),
+		}
+	}
+
+	r.trackEncodings = append(r.trackEncodings, baseTrackEncoding)
 }
 
 // Track returns the RTCRtpTransceiver track, or nil
@@ -339,6 +348,63 @@ func (r *RTPSender) Send(parameters RTPSendParameters) error {
 		)
 
 		writeStream.interceptor.Store(rtpInterceptor)
+
+		if trackEncoding.rtx != nil {
+			// also handle the rtx track. check the sdp for the associated
+			// a=fmtp:<pt> apt=<rtx-pt> line (see https://datatracker.ietf.org/doc/html/rfc4588#section-8.1)
+
+			codecs := r.api.mediaEngine.getCodecsByKind(trackEncoding.track.Kind())
+			var rtxCodec *RTPCodecParameters
+			for _, c := range codecs {
+				if fmt.Sprintf("apt=%d", c.PayloadType) == codec.SDPFmtpLine {
+					rtxCodec = &c
+					break
+				}
+			}
+
+			if rtxCodec == nil {
+				return fmt.Errorf("no RTX codec found for %s", codec.SDPFmtpLine)
+			}
+
+			rtxSRTPStream := &srtpWriterFuture{ssrc: parameters.Encodings[idx].RTX.SSRC, rtpSender: r}
+			rtxWriteStream := &interceptorToTrackLocalWriter{}
+
+			trackEncoding.rtx.srtpStream = rtxSRTPStream
+			trackEncoding.rtx.ssrc = parameters.Encodings[idx].RTX.SSRC
+			trackEncoding.rtx.context = &baseTrackLocalContext{
+				id:              r.id,
+				params:          r.api.mediaEngine.getRTPParametersByKind(trackEncoding.track.Kind(), []RTPTransceiverDirection{RTPTransceiverDirectionSendonly}),
+				ssrc:            parameters.Encodings[idx].RTX.SSRC,
+				writeStream:     rtxWriteStream,
+				rtcpInterceptor: trackEncoding.rtcpInterceptor,
+			}
+
+			trackEncoding.rtx.context.params.Codecs = []RTPCodecParameters{*rtxCodec}
+
+			trackEncoding.rtx.streamInfo = *createStreamInfo(
+				fmt.Sprintf("%s-rtx", r.id),
+				parameters.Encodings[idx].RTX.SSRC,
+				rtxCodec.PayloadType,
+				rtxCodec.RTPCodecCapability,
+				parameters.HeaderExtensions,
+			)
+
+			trackEncoding.rtx.rtcpInterceptor = r.api.interceptor.BindRTCPReader(
+				interceptor.RTCPReaderFunc(func(in []byte, a interceptor.Attributes) (n int, attributes interceptor.Attributes, err error) {
+					n, err = trackEncoding.rtx.srtpStream.Read(in)
+					return n, a, err
+				}),
+			)
+
+			rtxRTPInterceptor := r.api.interceptor.BindLocalStream(
+				&trackEncoding.rtx.streamInfo,
+				interceptor.RTPWriterFunc(func(header *rtp.Header, payload []byte, _ interceptor.Attributes) (int, error) {
+					return rtxSRTPStream.WriteRTP(header, payload)
+				}),
+			)
+
+			rtxWriteStream.interceptor.Store(rtxRTPInterceptor)
+		}
 	}
 
 	close(r.sendCalled)

sdp.go

@@ -392,7 +392,13 @@ func addSenderSDP(
 
 		sendParameters := sender.GetParameters()
 		for _, encoding := range sendParameters.Encodings {
-			media = media.WithMediaSource(uint32(encoding.SSRC), track.StreamID() /* cname */, track.StreamID() /* streamLabel */, track.ID())
+			if encoding.RTX.SSRC == 0 {
+				media = media.WithMediaSource(uint32(encoding.SSRC), track.StreamID() /* cname */, track.StreamID() /* streamLabel */, track.ID())
+			} else {
+				media = media.WithValueAttribute(sdp.AttrKeySSRCGroup, fmt.Sprintf("FID %d %d", encoding.SSRC, encoding.RTX.SSRC)).
+					WithMediaSource(uint32(encoding.SSRC), track.StreamID() /* cname */, track.StreamID() /* streamLabel */, track.ID()).
+					WithMediaSource(uint32(encoding.RTX.SSRC), track.StreamID() /* cname */, track.StreamID() /* streamLabel */, track.ID())
+			}
 			if !isPlanB {
 				media = media.WithPropertyAttribute("msid:" + track.StreamID() + " " + track.ID())
 			}

settingengine.go

@@ -101,6 +101,7 @@ type SettingEngine struct {
 	srtpProtectionProfiles                    []dtls.SRTPProtectionProfile
 	receiveMTU                                uint
 	iceMaxBindingRequests                     *uint16
+	distinctSSRCRTX                           bool
 }
 
 // getReceiveMTU returns the configured MTU. If SettingEngine's MTU is configured to 0 it returns the default
@@ -491,3 +492,8 @@ func (e *SettingEngine) SetSCTPRTOMax(rtoMax time.Duration) {
 func (e *SettingEngine) SetICEBindingRequestHandler(bindingRequestHandler func(m *stun.Message, local, remote ice.Candidate, pair *ice.CandidatePair) bool) {
 	e.iceBindingRequestHandler = bindingRequestHandler
 }
+
+// SetDistinctSSRCRTX configures if RTX should be sent with distinct SSRC instead of in-band.
+func (e *SettingEngine) SetDistinctSSRCRTX(distinctSSRCRTX bool) {
+	e.distinctSSRCRTX = distinctSSRCRTX
+}

track_local.go

@@ -28,8 +28,7 @@ type TrackLocalContext interface {
 	// both PeerConnections and the SSRC/PayloadTypes
 	HeaderExtensions() []RTPHeaderExtensionParameter
 
-	// SSRC requires the negotiated SSRC of this track
-	// This track may have multiple if RTX is enabled
+	// SSRC requires the negotiated SSRC of this track.
 	SSRC() SSRC
 
 	// WriteStream returns the WriteStream for this TrackLocal. The implementer writes the outbound
@@ -63,8 +62,7 @@ func (t *baseTrackLocalContext) HeaderExtensions() []RTPHeaderExtensionParameter
 	return t.params.HeaderExtensions
 }
 
-// SSRC requires the negotiated SSRC of this track
-// This track may have multiple if RTX is enabled
+// SSRC requires the negotiated SSRC of this track.
 func (t *baseTrackLocalContext) SSRC() SSRC {
 	return t.ssrc
 }
@@ -111,4 +109,9 @@ type TrackLocal interface {
 
 	// Kind controls if this TrackLocal is audio or video
 	Kind() RTPCodecType
+
+	// RTXTrackLocal returns the retransmission TrackLocal for this TrackLocal.
+	// If this TrackLocal does not have an RTX TrackLocal or is an RTX TrackLocal itself,
+	// this will return nil.
+	RTXTrackLocal() TrackLocal
 }