Add support for RFC 4588

Respect in RTX with distinict SSRC + PayloadType

pkg/nack/responder_interceptor.go

@@ -4,6 +4,7 @@
 package nack
 
 import (
+	"encoding/binary"
 	"sync"
 
 	"github.com/pion/interceptor"
@@ -62,6 +63,11 @@ type ResponderInterceptor struct {
 type localStream struct {
 	sendBuffer *sendBuffer
 	rtpWriter  interceptor.RTPWriter
+
+	// Non-zero if Retransmissions should be sent on a distinct stream
+	rtxSsrc        uint32
+	rtxPayloadType uint8
+	rtxSequencer   rtp.Sequencer
 }
 
 // NewResponderInterceptor returns a new ResponderInterceptorFactor
@@ -108,7 +114,13 @@ func (n *ResponderInterceptor) BindLocalStream(info *interceptor.StreamInfo, wri
 	// error is already checked in NewGeneratorInterceptor
 	sendBuffer, _ := newSendBuffer(n.size)
 	n.streamsMu.Lock()
-	n.streams[info.SSRC] = &localStream{sendBuffer: sendBuffer, rtpWriter: writer}
+	n.streams[info.SSRC] = &localStream{
+		sendBuffer:     sendBuffer,
+		rtpWriter:      writer,
+		rtxSsrc:        info.SSRCRetransmission,
+		rtxPayloadType: info.PayloadTypeRetransmission,
+		rtxSequencer:   rtp.NewRandomSequencer(),
+	}
 	n.streamsMu.Unlock()
 
 	return interceptor.RTPWriterFunc(func(header *rtp.Header, payload []byte, attributes interceptor.Attributes) (int, error) {
@@ -139,8 +151,43 @@ func (n *ResponderInterceptor) resendPackets(nack *rtcp.TransportLayerNack) {
 	for i := range nack.Nacks {
 		nack.Nacks[i].Range(func(seq uint16) bool {
 			if p := stream.sendBuffer.get(seq); p != nil {
-				if _, err := stream.rtpWriter.Write(p.Header(), p.Payload(), interceptor.Attributes{}); err != nil {
-					n.log.Warnf("failed resending nacked packet: %+v", err)
+				if stream.rtxSsrc != 0 {
+					// Store the original sequence number and rewrite the sequence number.
+					originalSequenceNumber := p.Header().SequenceNumber
+					p.Header().SequenceNumber = stream.rtxSequencer.NextSequenceNumber()
+
+					// Rewrite the SSRC.
+					p.Header().SSRC = stream.rtxSsrc
+					// Rewrite the payload type.
+					p.Header().PayloadType = stream.rtxPayloadType
+
+					// Remove padding if present.
+					paddingLength := 0
+					originPayload := p.Payload()
+					if p.Header().Padding {
+						paddingLength = int(originPayload[len(originPayload)-1])
+						p.Header().Padding = false
+					}
+
+					// Write the original sequence number at the beginning of the payload.
+					payload := make([]byte, 2)
+					binary.BigEndian.PutUint16(payload, originalSequenceNumber)
+					payload = append(payload, originPayload[:len(originPayload)-paddingLength]...)
+
+					// Send RTX packet.
+					if _, err := stream.rtpWriter.Write(p.Header(), payload, interceptor.Attributes{}); err != nil {
+						n.log.Warnf("failed sending rtx packet: %+v", err)
+					}
+
+					// Resore the Padding and SSRC.
+					if paddingLength > 0 {
+						p.Header().Padding = true
+					}
+					p.Header().SequenceNumber = originalSequenceNumber
+				} else {
+					if _, err := stream.rtpWriter.Write(p.Header(), p.Payload(), interceptor.Attributes{}); err != nil {
+						n.log.Warnf("failed resending nacked packet: %+v", err)
+					}
 				}
 				p.Release()
 			}