Read() handles distinct-SSRC RTX packets

constants.go

@@ -31,6 +31,11 @@ const (
 	generatedCertificateOrigin = "WebRTC"
 
 	sdesRepairRTPStreamIDURI = "urn:ietf:params:rtp-hdrext:sdes:repaired-rtp-stream-id"
+
+	// Attributes returned when Read() returns an RTX packet from a separate RTX stream (distinct SSRC)
+	attributeRtxPayloadType    = "rtx_payload_type"
+	attributeRtxSsrc           = "rtx_ssrc"
+	attributeRtxSequenceNumber = "rtx_sequence_number"
 )
 
 func defaultSrtpProtectionProfiles() []dtls.SRTPProtectionProfile {

rtpreceiver.go

@@ -7,13 +7,15 @@
 package webrtc
 
 import (
+	"encoding/binary"
 	"fmt"
 	"io"
 	"sync"
 	"time"
 
 	"github.com/pion/interceptor"
 	"github.com/pion/rtcp"
+	"github.com/pion/rtp"
 	"github.com/pion/srtp/v3"
 	"github.com/pion/webrtc/v4/internal/util"
 )
@@ -31,13 +33,19 @@ type trackStreams struct {
 	rtcpReadStream  *srtp.ReadStreamSRTCP
 	rtcpInterceptor interceptor.RTCPReader
 
-	repairReadStream  *srtp.ReadStreamSRTP
-	repairInterceptor interceptor.RTPReader
+	repairReadStream    *srtp.ReadStreamSRTP
+	repairInterceptor   interceptor.RTPReader
+	repairStreamChannel chan rtxPacketWithAttributes
 
 	repairRtcpReadStream  *srtp.ReadStreamSRTCP
 	repairRtcpInterceptor interceptor.RTCPReader
 }
 
+type rtxPacketWithAttributes struct {
+	rtxPacket  rtp.Packet
+	attributes interceptor.Attributes
+}
+
 // RTPReceiver allows an application to inspect the receipt of a TrackRemote
 type RTPReceiver struct {
 	kind      RTPCodecType
@@ -145,6 +153,7 @@ func (r *RTPReceiver) configureReceive(parameters RTPReceiveParameters) {
 			track: newTrackRemote(
 				r.kind,
 				parameters.Encodings[i].SSRC,
+				parameters.Encodings[i].RTX.SSRC,
 				parameters.Encodings[i].RID,
 				r,
 			),
@@ -388,8 +397,6 @@ func (r *RTPReceiver) receiveForRid(rid string, params RTPParameters, streamInfo
 }
 
 // receiveForRtx starts a routine that processes the repair stream
-// These packets aren't exposed to the user yet, but we need to process them for
-// TWCC
 func (r *RTPReceiver) receiveForRtx(ssrc SSRC, rsid string, streamInfo *interceptor.StreamInfo, rtpReadStream *srtp.ReadStreamSRTP, rtpInterceptor interceptor.RTPReader, rtcpReadStream *srtp.ReadStreamSRTCP, rtcpInterceptor interceptor.RTCPReader) error {
 	var track *trackStreams
 	if ssrc != 0 && len(r.tracks) == 1 {
@@ -411,13 +418,42 @@ func (r *RTPReceiver) receiveForRtx(ssrc SSRC, rsid string, streamInfo *intercep
 	track.repairInterceptor = rtpInterceptor
 	track.repairRtcpReadStream = rtcpReadStream
 	track.repairRtcpInterceptor = rtcpInterceptor
+	track.repairStreamChannel = make(chan rtxPacketWithAttributes)
 
 	go func() {
 		b := make([]byte, r.api.settingEngine.getReceiveMTU())
 		for {
-			if _, _, readErr := track.repairInterceptor.Read(b, nil); readErr != nil {
+			i, attributes, err := track.repairInterceptor.Read(b, nil)
+			if err != nil {
+				return
+			}
+
+			pkt := &rtp.Packet{}
+			if err := pkt.Unmarshal(b[:i]); err != nil {
 				return
 			}
+
+			if len(pkt.Payload) < 2 {
+				// BWE probe packet, ignore
+				continue
+			}
+
+			// RTX packets have a different payload format. Move the OSN in the payload to the RTP header and rewrite the
+			// payload type and SSRC, so that we can return RTX packets to the caller 'transparently' i.e. in the same format
+			// as non-RTX RTP packets
+			attributes.Set(attributeRtxPayloadType, pkt.Header.PayloadType)
+			attributes.Set(attributeRtxSsrc, pkt.Header.SSRC)
+			attributes.Set(attributeRtxSequenceNumber, pkt.Header.SequenceNumber)
+			pkt.Header.PayloadType = uint8(track.track.PayloadType())
+			pkt.Header.SSRC = uint32(track.track.SSRC())
+			pkt.Header.SequenceNumber = binary.BigEndian.Uint16(pkt.Payload[:2])
+			pkt.Payload = pkt.Payload[2:]
+
+			select {
+			case <-r.closed:
+				return
+			case track.repairStreamChannel <- rtxPacketWithAttributes{rtxPacket: *pkt, attributes: attributes}:
+			}
 		}
 	}()
 	return nil
@@ -455,3 +491,25 @@ func (r *RTPReceiver) setRTPReadDeadline(deadline time.Time, reader *TrackRemote
 	}
 	return fmt.Errorf("%w: %d", errRTPReceiverWithSSRCTrackStreamNotFound, reader.SSRC())
 }
+
+// readRTX returns an RTX packet if one is available on the RTX track, otherwise returns nil
+func (r *RTPReceiver) readRTX(reader *TrackRemote) (*rtp.Packet, interceptor.Attributes) {
+	if !reader.HasRTX() {
+		return nil, interceptor.Attributes{}
+	}
+
+	select {
+	case <-r.received:
+	default:
+		return nil, interceptor.Attributes{}
+	}
+
+	if t := r.streamsForTrack(reader); t != nil {
+		select {
+		case rtxPacketReceived := <-t.repairStreamChannel:
+			return &rtxPacketReceived.rtxPacket, rtxPacketReceived.attributes
+		default:
+		}
+	}
+	return nil, interceptor.Attributes{}
+}

track_remote.go

@@ -24,6 +24,7 @@ type TrackRemote struct {
 	payloadType PayloadType
 	kind        RTPCodecType
 	ssrc        SSRC
+	rtxSsrc     SSRC
 	codec       RTPCodecParameters
 	params      RTPParameters
 	rid         string
@@ -33,10 +34,11 @@ type TrackRemote struct {
 	peekedAttributes interceptor.Attributes
 }
 
-func newTrackRemote(kind RTPCodecType, ssrc SSRC, rid string, receiver *RTPReceiver) *TrackRemote {
+func newTrackRemote(kind RTPCodecType, ssrc, rtxSsrc SSRC, rid string, receiver *RTPReceiver) *TrackRemote {
 	return &TrackRemote{
 		kind:     kind,
 		ssrc:     ssrc,
+		rtxSsrc:  rtxSsrc,
 		rid:      rid,
 		receiver: receiver,
 	}
@@ -125,13 +127,24 @@ func (t *TrackRemote) Read(b []byte) (n int, attributes interceptor.Attributes,
 		}
 	}
 
-	n, attributes, err = r.readRTP(b, t)
-	if err != nil {
-		return
+	// If there's a separate RTX track and an RTX packet is available, return that
+	if rtxPacket, rtxAttributes := r.readRTX(t); rtxPacket != nil {
+		n, err = rtxPacket.MarshalTo(b)
+		attributes = rtxAttributes
+		if err != nil {
+			return 0, nil, err
+		}
+	} else {
+		// If there's no separate RTX track (or there's a separate RTX track but no RTX packet waiting), wait for and return
+		// a packet from the main track
+		n, attributes, err = r.readRTP(b, t)
+		if err != nil {
+			return
+		}
+		err = t.checkAndUpdateTrack(b)
 	}
 
-	err = t.checkAndUpdateTrack(b)
-	return
+	return n, attributes, err
 }
 
 // checkAndUpdateTrack checks payloadType for every incoming packet
@@ -197,3 +210,17 @@ func (t *TrackRemote) peek(b []byte) (n int, a interceptor.Attributes, err error
 func (t *TrackRemote) SetReadDeadline(deadline time.Time) error {
 	return t.receiver.setRTPReadDeadline(deadline, t)
 }
+
+// RtxSSRC returns the RTX SSRC for a track, or 0 if track does not have a separate RTX stream
+func (t *TrackRemote) RtxSSRC() SSRC {
+	t.mu.RLock()
+	defer t.mu.RUnlock()
+	return t.rtxSsrc
+}
+
+// HasRTX returns true if the track has a separate RTX stream
+func (t *TrackRemote) HasRTX() bool {
+	t.mu.RLock()
+	defer t.mu.RUnlock()
+	return t.rtxSsrc != 0
+}