Add Simulcast support

Resolves #1016

constants.go

@@ -10,5 +10,10 @@ const (
 	// Equal to UDP MTU
 	receiveMTU = 1460
 
+	// simulcastProbeCount is the amount of RTP Packets
+	// that handleUndeclaredSSRC will read and try to dispatch from
+	// mid and rid values
+	simulcastProbeCount = 10
+
 	mediaSectionApplication = "application"
 )

examples/simulcast/main.go

@@ -3,7 +3,6 @@ package main
 import (
 	"fmt"
 	"io"
-	"log"
 	"math/rand"
 	"net/url"
 	"time"
@@ -103,17 +102,13 @@ func main() {
 		panic(err)
 	}
 
-	// fmt.Printf("offer: %s\n", offer.SDP)
+	if err = peerConnection.SetRemoteDescription(offer); err != nil {
-	// Set the remote SessionDescription
-	err = peerConnection.SetRemoteDescription(offer)
-	if err != nil {
 		panic(err)
 	}
 
 	// Set a handler for when a new remote track starts
 	peerConnection.OnTrack(func(track *webrtc.Track, receiver *webrtc.RTPReceiver) {
-		fmt.Printf("Track has started\n")
+		fmt.Println("Track has started")
-		log.Println("Track has started", track)
 
 		// Start reading from all the streams and sending them to the related output track
 		rid := track.RID()
@@ -132,10 +127,9 @@ func main() {
 			}
 		}()
 		for {
-			var readErr error
 			// Read RTP packets being sent to Pion
 			packet, readErr := track.ReadRTP()
-			if err != nil {
+			if readErr != nil {
 				panic(readErr)
 			}
 
@@ -157,8 +151,6 @@ func main() {
 		panic(err)
 	}
 
-	fmt.Printf("answer: %s\n", answer.SDP)
-
 	// Sets the LocalDescription, and starts our UDP listeners
 	err = peerConnection.SetLocalDescription(answer)
 	if err != nil {

peerconnection.go

@@ -8,6 +8,7 @@ import (
 	"crypto/elliptic"
 	"crypto/rand"
 	"fmt"
+	"io"
 	"strconv"
 	"strings"
 	"sync"
@@ -877,24 +878,35 @@ func (pc *PeerConnection) SetRemoteDescription(desc SessionDescription) error {
 }
 
 func (pc *PeerConnection) startReceiver(incoming trackDetails, receiver *RTPReceiver) {
-	err := receiver.Receive(RTPReceiveParameters{
+	encodings := []RTPDecodingParameters{}
-		Encodings: RTPDecodingParameters{
+	if incoming.ssrc != 0 {
-			RTPCodingParameters{SSRC: incoming.ssrc},
+		encodings = append(encodings, RTPDecodingParameters{RTPCodingParameters{SSRC: incoming.ssrc}})
-		}})
+	}
-	if err != nil {
+	for _, rid := range incoming.rids {
+		encodings = append(encodings, RTPDecodingParameters{RTPCodingParameters{RID: rid}})
+	}
+
+	if err := receiver.Receive(RTPReceiveParameters{Encodings: encodings}); err != nil {
 		pc.log.Warnf("RTPReceiver Receive failed %s", err)
 		return
 	}
 
 	// set track id and label early so they can be set as new track information
 	// is received from the SDP.
-	receiver.Track().mu.Lock()
+	for i := range receiver.tracks {
-	receiver.Track().id = incoming.id
+		receiver.tracks[i].track.mu.Lock()
-	receiver.Track().label = incoming.label
+		receiver.tracks[i].track.id = incoming.id
-	receiver.Track().mu.Unlock()
+		receiver.tracks[i].track.label = incoming.label
+		receiver.tracks[i].track.mu.Unlock()
+	}
+
+	// We can't block and wait for a single SSRC
+	if incoming.ssrc == 0 {
+		return
+	}
 
 	go func() {
-		if err = receiver.Track().determinePayloadType(); err != nil {
+		if err := receiver.Track().determinePayloadType(); err != nil {
 			pc.log.Warnf("Could not determine PayloadType for SSRC %d", receiver.Track().SSRC())
 			return
 		}
@@ -922,7 +934,7 @@ func (pc *PeerConnection) startReceiver(incoming trackDetails, receiver *RTPRece
 }
 
 // startRTPReceivers opens knows inbound SRTP streams from the RemoteDescription
-func (pc *PeerConnection) startRTPReceivers(incomingTracks map[uint32]trackDetails, currentTransceivers []*RTPTransceiver) {
+func (pc *PeerConnection) startRTPReceivers(incomingTracks []trackDetails, currentTransceivers []*RTPTransceiver) {
 	localTransceivers := append([]*RTPTransceiver{}, currentTransceivers...)
 
 	remoteIsPlanB := false
@@ -934,45 +946,54 @@ func (pc *PeerConnection) startRTPReceivers(incomingTracks map[uint32]trackDetai
 	}
 
 	// Ensure we haven't already started a transceiver for this ssrc
-	for ssrc := range incomingTracks {
+	for i := range incomingTracks {
-		for i := range localTransceivers {
+		if len(incomingTracks) <= i {
-			if t := localTransceivers[i]; (t.Receiver()) == nil || t.Receiver().Track() == nil || t.Receiver().Track().ssrc != ssrc {
+			break
+		}
+		incomingTrack := incomingTracks[i]
+
+		for _, t := range localTransceivers {
+			if (t.Receiver()) == nil || t.Receiver().Track() == nil || t.Receiver().Track().ssrc != incomingTrack.ssrc {
 				continue
 			}
 
-			delete(incomingTracks, ssrc)
+			incomingTracks = filterTrackWithSSRC(incomingTracks, incomingTrack.ssrc)
 		}
 	}
 
-	for ssrc, incoming := range incomingTracks {
+	for i := range incomingTracks {
-		for i := range localTransceivers {
+		for j := range localTransceivers {
-			t := localTransceivers[i]
+			if len(incomingTracks) <= i || len(localTransceivers) <= j {
+				break
+			}
+			t := localTransceivers[j]
+			incomingTrack := incomingTracks[i]
 
-			if t.Mid() != incoming.mid {
+			if t.Mid() != incomingTrack.mid {
 				continue
 			}
 
-			if (incomingTracks[ssrc].kind != t.kind) ||
+			if (incomingTrack.kind != t.kind) ||
 				(t.Direction() != RTPTransceiverDirectionRecvonly && t.Direction() != RTPTransceiverDirectionSendrecv) ||
 				(t.Receiver()) == nil ||
 				(t.Receiver().haveReceived()) {
 				continue
 			}
 
-			delete(incomingTracks, ssrc)
+			incomingTracks = append(incomingTracks[:i], incomingTracks[i+1:]...)
-			localTransceivers = append(localTransceivers[:i], localTransceivers[i+1:]...)
+			localTransceivers = append(localTransceivers[:j], localTransceivers[j+1:]...)
-			pc.startReceiver(incoming, t.Receiver())
+			pc.startReceiver(incomingTrack, t.Receiver())
 			break
 		}
 	}
 
 	if remoteIsPlanB {
-		for ssrc, incoming := range incomingTracks {
+		for _, incoming := range incomingTracks {
 			t, err := pc.AddTransceiverFromKind(incoming.kind, RtpTransceiverInit{
 				Direction: RTPTransceiverDirectionSendrecv,
 			})
 			if err != nil {
-				pc.log.Warnf("Could not add transceiver for remote SSRC %d: %s", ssrc, err)
+				pc.log.Warnf("Could not add transceiver for remote SSRC %d: %s", incoming.ssrc, err)
 				continue
 			}
 			pc.startReceiver(incoming, t.Receiver())
@@ -1036,16 +1057,18 @@ func (pc *PeerConnection) startSCTP() {
 	pc.sctpTransport.lock.Unlock()
 }
 
-// drainSRTP pulls and discards RTP/RTCP packets that don't match any a:ssrc lines
+func (pc *PeerConnection) handleUndeclaredSSRC(rtpStream io.Reader, ssrc uint32) error {
+	remoteDescription := pc.RemoteDescription()
+	if remoteDescription == nil {
+		return fmt.Errorf("remote Description has not been set yet")
+	}
+
 	// If the remote SDP was only one media section the ssrc doesn't have to be explicitly declared
-func (pc *PeerConnection) drainSRTP() {
-	handleUndeclaredSSRC := func(ssrc uint32) bool {
-		if remoteDescription := pc.RemoteDescription(); remoteDescription != nil {
 	if len(remoteDescription.parsed.MediaDescriptions) == 1 {
 		onlyMediaSection := remoteDescription.parsed.MediaDescriptions[0]
 		for _, a := range onlyMediaSection.Attributes {
 			if a.Key == ssrcStr {
-						return false
+				return fmt.Errorf("single media section has an explicit SSRC")
 			}
 		}
 
@@ -1061,33 +1084,88 @@ func (pc *PeerConnection) drainSRTP() {
 			Direction: RTPTransceiverDirectionSendrecv,
 		})
 		if err != nil {
-					pc.log.Warnf("Could not add transceiver for remote SSRC %d: %s", ssrc, err)
+			return fmt.Errorf("could not add transceiver for remote SSRC %d: %s", ssrc, err)
-					return false
 		}
 		pc.startReceiver(incoming, t.Receiver())
-				return true
+		return nil
+	}
+
+	//  Simulcast no longer uses SSRCes, but RID instead. We then use that value to populate rest of Track Data
+	matchedSDPMap, err := matchedAnswerExt(pc.RemoteDescription().parsed, pc.api.settingEngine.getSDPExtensions())
+	if err != nil {
+		return err
+	}
+
+	sdesMidExtMap := getExtMapByURI(matchedSDPMap, sdp.SDESMidURI)
+	sdesStreamIDExtMap := getExtMapByURI(matchedSDPMap, sdp.SDESRTPStreamIDURI)
+	if sdesMidExtMap == nil || sdesStreamIDExtMap == nil {
+		return fmt.Errorf("mid and rid RTP Extensions required for Simulcast")
+	}
+
+	b := make([]byte, receiveMTU)
+	var mid, rid string
+	for readCount := 0; readCount <= simulcastProbeCount; readCount++ {
+		i, err := rtpStream.Read(b)
+		if err != nil {
+			return err
+		}
+
+		maybeMid, maybeRid, payloadType, err := handleUnknownRTPPacket(b[:i], sdesMidExtMap, sdesStreamIDExtMap)
+		if err != nil {
+			return err
+		}
+
+		if maybeMid != "" {
+			mid = maybeMid
+		}
+		if maybeRid != "" {
+			rid = maybeRid
+		}
+
+		if mid == "" || rid == "" {
+			continue
+		}
+
+		codec, err := pc.api.mediaEngine.getCodec(payloadType)
+		if err != nil {
+			return err
+		}
+
+		for _, t := range pc.GetTransceivers() {
+			if t.Mid() != mid || t.Receiver() == nil {
+				continue
+			}
+
+			track, err := t.Receiver().receiveForRid(rid, codec, ssrc)
+			if err != nil {
+				return err
+			}
+			pc.onTrack(track, t.Receiver())
+			return nil
 		}
 	}
 
-		return false
+	return fmt.Errorf("incoming SSRC failed Simulcast probing")
 }
 
+// undeclaredMediaProcessor handles RTP/RTCP packets that don't match any a:ssrc lines
+func (pc *PeerConnection) undeclaredMediaProcessor() {
 	go func() {
 		for {
 			srtpSession, err := pc.dtlsTransport.getSRTPSession()
 			if err != nil {
-				pc.log.Warnf("drainSRTP failed to open SrtpSession: %v", err)
+				pc.log.Warnf("undeclaredMediaProcessor failed to open SrtpSession: %v", err)
 				return
 			}
 
-			_, ssrc, err := srtpSession.AcceptStream()
+			stream, ssrc, err := srtpSession.AcceptStream()
 			if err != nil {
 				pc.log.Warnf("Failed to accept RTP %v", err)
 				return
 			}
 
-			if !handleUndeclaredSSRC(ssrc) {
+			if err := pc.handleUndeclaredSSRC(stream, ssrc); err != nil {
-				pc.log.Warnf("Incoming unhandled RTP ssrc(%d), OnTrack will not be fired", ssrc)
+				pc.log.Errorf("Incoming unhandled RTP ssrc(%d), OnTrack will not be fired. %v", ssrc, err)
 			}
 		}
 	}()
@@ -1096,7 +1174,7 @@ func (pc *PeerConnection) drainSRTP() {
 		for {
 			srtcpSession, err := pc.dtlsTransport.getSRTCPSession()
 			if err != nil {
-				pc.log.Warnf("drainSRTP failed to open SrtcpSession: %v", err)
+				pc.log.Warnf("undeclaredMediaProcessor failed to open SrtcpSession: %v", err)
 				return
 			}
 
@@ -1694,13 +1772,14 @@ func (pc *PeerConnection) startRTP(isRenegotiation bool, remoteDesc *SessionDesc
 
 			t.Receiver().Track().mu.Lock()
 			ssrc := t.Receiver().Track().ssrc
-			if _, ok := trackDetails[ssrc]; ok {
+
-				incoming := trackDetails[ssrc]
+			if details := trackDetailsForSSRC(trackDetails, ssrc); details != nil {
-				t.Receiver().Track().id = incoming.id
+				t.Receiver().Track().id = details.id
-				t.Receiver().Track().label = incoming.label
+				t.Receiver().Track().label = details.label
 				t.Receiver().Track().mu.Unlock()
 				continue
 			}
+
 			t.Receiver().Track().mu.Unlock()
 
 			if err := t.Receiver().Stop(); err != nil {
@@ -1721,7 +1800,7 @@ func (pc *PeerConnection) startRTP(isRenegotiation bool, remoteDesc *SessionDesc
 	pc.startRTPSenders(currentTransceivers)
 
 	if !isRenegotiation {
-		pc.drainSRTP()
+		pc.undeclaredMediaProcessor()
 		if haveApplicationMediaSection(remoteDesc.parsed) {
 			pc.startSCTP()
 		}

rtpcodingparameters.go

@@ -4,6 +4,7 @@ package webrtc
 // This is a subset of the RFC since Pion WebRTC doesn't implement encoding/decoding itself
 // http://draft.ortc.org/#dom-rtcrtpcodingparameters
 type RTPCodingParameters struct {
+	RID         string `json:"rid"`
 	SSRC        uint32 `json:"ssrc"`
 	PayloadType uint8  `json:"payloadType"`
 }

rtpreceiveparameters.go

@@ -2,5 +2,5 @@ package webrtc
 
 // RTPReceiveParameters contains the RTP stack settings used by receivers
 type RTPReceiveParameters struct {
-	Encodings RTPDecodingParameters
+	Encodings []RTPDecodingParameters
 }

rtpreceiver.go

@@ -11,19 +11,24 @@ import (
 	"github.com/pion/srtp"
 )
 
+// trackStreams maintains a mapping of RTP/RTCP streams to a specific track
+// a RTPReceiver may contain multiple streams if we are dealing with Multicast
+type trackStreams struct {
+	track          *Track
+	rtpReadStream  *srtp.ReadStreamSRTP
+	rtcpReadStream *srtp.ReadStreamSRTCP
+}
+
 // RTPReceiver allows an application to inspect the receipt of a Track
 type RTPReceiver struct {
 	kind      RTPCodecType
 	transport *DTLSTransport
 
-	track *Track
+	tracks []trackStreams
 
 	closed, received chan interface{}
 	mu               sync.RWMutex
 
-	rtpReadStream  *srtp.ReadStreamSRTP
-	rtcpReadStream *srtp.ReadStreamSRTCP
-
 	// A reference to the associated api object
 	api *API
 }
@@ -40,6 +45,7 @@ func (api *API) NewRTPReceiver(kind RTPCodecType, transport *DTLSTransport) (*RT
 		api:       api,
 		closed:    make(chan interface{}),
 		received:  make(chan interface{}),
+		tracks:    []trackStreams{},
 	}, nil
 }
 
@@ -51,11 +57,28 @@ func (r *RTPReceiver) Transport() *DTLSTransport {
 	return r.transport
 }
 
-// Track returns the RTCRtpTransceiver track
+// Track returns the RtpTransceiver track
 func (r *RTPReceiver) Track() *Track {
 	r.mu.RLock()
 	defer r.mu.RUnlock()
-	return r.track
+
+	if len(r.tracks) != 1 {
+		return nil
+	}
+	return r.tracks[0].track
+}
+
+// Tracks returns the RtpTransceiver tracks
+// A RTPReceiver to support Simulcast may now have multiple tracks
+func (r *RTPReceiver) Tracks() []*Track {
+	r.mu.RLock()
+	defer r.mu.RUnlock()
+
+	tracks := []*Track{}
+	for i := range r.tracks {
+		tracks = append(tracks, r.tracks[i].track)
+	}
+	return tracks
 }
 
 // Receive initialize the track and starts all the transports
@@ -69,30 +92,32 @@ func (r *RTPReceiver) Receive(parameters RTPReceiveParameters) error {
 	}
 	defer close(r.received)
 
-	r.track = &Track{
+	if len(parameters.Encodings) == 1 && parameters.Encodings[0].SSRC != 0 {
+		t := trackStreams{
+			track: &Track{
 				kind:     r.kind,
-		ssrc:     parameters.Encodings.SSRC,
+				ssrc:     parameters.Encodings[0].SSRC,
 				receiver: r,
+			},
 		}
 
-	srtpSession, err := r.transport.getSRTPSession()
+		var err error
+		t.rtpReadStream, t.rtcpReadStream, err = r.streamsForSSRC(parameters.Encodings[0].SSRC)
 		if err != nil {
 			return err
 		}
 
-	r.rtpReadStream, err = srtpSession.OpenReadStream(parameters.Encodings.SSRC)
+		r.tracks = append(r.tracks, t)
-	if err != nil {
+	} else {
-		return err
+		for _, encoding := range parameters.Encodings {
+			r.tracks = append(r.tracks, trackStreams{
+				track: &Track{
+					kind:     r.kind,
+					rid:      encoding.RID,
+					receiver: r,
+				},
+			})
 		}
-
-	srtcpSession, err := r.transport.getSRTCPSession()
-	if err != nil {
-		return err
-	}
-
-	r.rtcpReadStream, err = srtcpSession.OpenReadStream(parameters.Encodings.SSRC)
-	if err != nil {
-		return err
 	}
 
 	return nil
@@ -102,7 +127,7 @@ func (r *RTPReceiver) Receive(parameters RTPReceiveParameters) error {
 func (r *RTPReceiver) Read(b []byte) (n int, err error) {
 	select {
 	case <-r.received:
-		return r.rtcpReadStream.Read(b)
+		return r.tracks[0].rtcpReadStream.Read(b)
 	case <-r.closed:
 		return 0, io.ErrClosedPipe
 	}
@@ -141,13 +166,11 @@ func (r *RTPReceiver) Stop() error {
 
 	select {
 	case <-r.received:
-		if r.rtcpReadStream != nil {
+		for i := range r.tracks {
-			if err := r.rtcpReadStream.Close(); err != nil {
+			if err := r.tracks[i].rtcpReadStream.Close(); err != nil {
 				return err
 			}
-		}
+			if err := r.tracks[i].rtpReadStream.Close(); err != nil {
-		if r.rtpReadStream != nil {
-			if err := r.rtpReadStream.Close(); err != nil {
 				return err
 			}
 		}
@@ -158,8 +181,72 @@ func (r *RTPReceiver) Stop() error {
 	return nil
 }
 
-// readRTP should only be called by a track, this only exists so we can keep state in one place
+func (r *RTPReceiver) streamsForTrack(t *Track) *trackStreams {
-func (r *RTPReceiver) readRTP(b []byte) (n int, err error) {
+	for i := range r.tracks {
-	<-r.received
+		if r.tracks[i].track == t {
-	return r.rtpReadStream.Read(b)
+			return &r.tracks[i]
+		}
+	}
+	return nil
+}
+
+// readRTP should only be called by a track, this only exists so we can keep state in one place
+func (r *RTPReceiver) readRTP(b []byte, reader *Track) (n int, err error) {
+	<-r.received
+	if t := r.streamsForTrack(reader); t != nil {
+		return t.rtpReadStream.Read(b)
+	}
+
+	return 0, fmt.Errorf("unable to find stream for Track with SSRC(%d)", reader.SSRC())
+}
+
+// receiveForRid is the sibling of Receive expect for RIDs instead of SSRCs
+// It populates all the internal state for the given RID
+func (r *RTPReceiver) receiveForRid(rid string, codec *RTPCodec, ssrc uint32) (*Track, error) {
+	r.mu.Lock()
+	defer r.mu.Unlock()
+
+	for i := range r.tracks {
+		if r.tracks[i].track.RID() == rid {
+			r.tracks[i].track.mu.Lock()
+			r.tracks[i].track.kind = codec.Type
+			r.tracks[i].track.codec = codec
+			r.tracks[i].track.ssrc = ssrc
+			r.tracks[i].track.mu.Unlock()
+
+			var err error
+			r.tracks[i].rtpReadStream, r.tracks[i].rtcpReadStream, err = r.streamsForSSRC(ssrc)
+			if err != nil {
+				return nil, err
+			}
+
+			return r.tracks[i].track, nil
+		}
+	}
+
+	return nil, fmt.Errorf("no trackStreams found for SSRC(%d)", ssrc)
+}
+
+func (r *RTPReceiver) streamsForSSRC(ssrc uint32) (*srtp.ReadStreamSRTP, *srtp.ReadStreamSRTCP, error) {
+	srtpSession, err := r.transport.getSRTPSession()
+	if err != nil {
+		return nil, nil, err
+	}
+
+	rtpReadStream, err := srtpSession.OpenReadStream(ssrc)
+	if err != nil {
+		return nil, nil, err
+	}
+
+	srtcpSession, err := r.transport.getSRTCPSession()
+	if err != nil {
+		return nil, nil, err
+	}
+
+	rtcpReadStream, err := srtcpSession.OpenReadStream(ssrc)
+	if err != nil {
+		return nil, nil, err
+	}
+
+	return rtpReadStream, rtcpReadStream, nil
 }

rtptransceiver.go

@@ -5,6 +5,9 @@ package webrtc
 import (
 	"fmt"
 	"sync/atomic"
+
+	"github.com/pion/rtp"
+	"github.com/pion/sdp/v2"
 )
 
 // RTPTransceiver represents a combination of an RTPSender and an RTPReceiver that share a common mid.
@@ -150,3 +153,27 @@ func satisfyTypeAndDirection(remoteKind RTPCodecType, remoteDirection RTPTransce
 
 	return nil, localTransceivers
 }
+
+// handleUnknownRTPPacket consumes a single RTP Packet and returns information that is helpful
+// for demuxing and handling an unknown SSRC (usually for Simulcast)
+func handleUnknownRTPPacket(buf []byte, sdesMidExtMap, sdesStreamIDExtMap *sdp.ExtMap) (mid, rid string, payloadType uint8, err error) {
+	rp := &rtp.Packet{}
+	if err = rp.Unmarshal(buf); err != nil {
+		return
+	}
+
+	if !rp.Header.Extension {
+		return
+	}
+
+	payloadType = rp.PayloadType
+	if payload := rp.GetExtension(uint8(sdesMidExtMap.Value)); payload != nil {
+		mid = string(payload)
+	}
+
+	if payload := rp.GetExtension(uint8(sdesStreamIDExtMap.Value)); payload != nil {
+		rid = string(payload)
+	}
+
+	return
+}

sdp.go

@@ -12,12 +12,34 @@ import (
 	"github.com/pion/sdp/v2"
 )
 
+// trackDetails represents any media source that can be represented in a SDP
+// This isn't keyed by SSRC because it also needs to support rid based sources
 type trackDetails struct {
 	mid   string
 	kind  RTPCodecType
 	label string
 	id    string
 	ssrc  uint32
+	rids  []string
+}
+
+func trackDetailsForSSRC(trackDetails []trackDetails, ssrc uint32) *trackDetails {
+	for i := range trackDetails {
+		if trackDetails[i].ssrc == ssrc {
+			return &trackDetails[i]
+		}
+	}
+	return nil
+}
+
+func filterTrackWithSSRC(incomingTracks []trackDetails, ssrc uint32) []trackDetails {
+	filtered := []trackDetails{}
+	for i := range incomingTracks {
+		if incomingTracks[i].ssrc != ssrc {
+			filtered = append(filtered, incomingTracks[i])
+		}
+	}
+	return filtered
 }
 
 // SDPSectionType specifies media type sections
@@ -31,8 +53,8 @@ const (
 )
 
 // extract all trackDetails from an SDP.
-func trackDetailsFromSDP(log logging.LeveledLogger, s *sdp.SessionDescription) map[uint32]trackDetails {
+func trackDetailsFromSDP(log logging.LeveledLogger, s *sdp.SessionDescription) []trackDetails {
-	incomingTracks := map[uint32]trackDetails{}
+	incomingTracks := []trackDetails{}
 	rtxRepairFlows := map[uint32]bool{}
 
 	for _, media := range s.MediaDescriptions {
@@ -78,7 +100,7 @@ func trackDetailsFromSDP(log logging.LeveledLogger, s *sdp.SessionDescription) m
 							continue
 						}
 						rtxRepairFlows[uint32(rtxRepairFlow)] = true
-						delete(incomingTracks, uint32(rtxRepairFlow)) // Remove if rtx was added as track before
+						incomingTracks = filterTrackWithSSRC(incomingTracks, uint32(rtxRepairFlow)) // Remove if rtx was added as track before
 					}
 				}
 
@@ -99,31 +121,52 @@ func trackDetailsFromSDP(log logging.LeveledLogger, s *sdp.SessionDescription) m
 					log.Warnf("Failed to parse SSRC: %v", err)
 					continue
 				}
+
 				if rtxRepairFlow := rtxRepairFlows[uint32(ssrc)]; rtxRepairFlow {
 					continue // This ssrc is a RTX repair flow, ignore
 				}
-				if existingValues, ok := incomingTracks[uint32(ssrc)]; ok && existingValues.label != "" && existingValues.id != "" {
-					continue // This ssrc is already fully defined
-				}
 
 				if len(split) == 3 && strings.HasPrefix(split[1], "msid:") {
 					trackLabel = split[1][len("msid:"):]
 					trackID = split[2]
 				}
 
-				// Plan B might send multiple a=ssrc lines under a single m= section. This is also why a single trackDetails{}
+				isNewTrack := true
-				// is not defined at the top of the loop over s.MediaDescriptions.
+				trackDetails := &trackDetails{}
-				incomingTracks[uint32(ssrc)] = trackDetails{
+				for i := range incomingTracks {
-					mid:   midValue,
+					if incomingTracks[i].ssrc == uint32(ssrc) {
-					kind:  codecType,
+						trackDetails = &incomingTracks[i]
-					label: trackLabel,
+						isNewTrack = false
-					id:    trackID,
-					ssrc:  uint32(ssrc),
 					}
 				}
+
+				trackDetails.mid = midValue
+				trackDetails.kind = codecType
+				trackDetails.label = trackLabel
+				trackDetails.id = trackID
+				trackDetails.ssrc = uint32(ssrc)
+
+				if isNewTrack {
+					incomingTracks = append(incomingTracks, *trackDetails)
+				}
 			}
 		}
 
+		if rids := getRids(media); len(rids) != 0 && trackID != "" && trackLabel != "" {
+			newTrack := trackDetails{
+				mid:   midValue,
+				kind:  codecType,
+				label: trackLabel,
+				id:    trackID,
+				rids:  []string{},
+			}
+			for rid := range rids {
+				newTrack.rids = append(newTrack.rids, rid)
+			}
+
+			incomingTracks = append(incomingTracks, newTrack)
+		}
+	}
 	return incomingTracks
 }
 
@@ -269,8 +312,15 @@ func addTransceiverSDP(d *sdp.SessionDescription, isPlanB bool, dtlsFingerprints
 		}
 	}
 
+	if len(mediaSection.ridMap) > 0 {
+		recvRids := make([]string, 0, len(mediaSection.ridMap))
+
 		for rid := range mediaSection.ridMap {
 			media.WithValueAttribute("rid", rid+" recv")
+			recvRids = append(recvRids, rid)
+		}
+		// Simulcast
+		media.WithValueAttribute("simulcast", "recv "+strings.Join(recvRids, ";"))
 	}
 
 	for _, mt := range transceivers {
@@ -561,3 +611,21 @@ func remoteExts(session *sdp.SessionDescription) (map[SDPSectionType]map[int]sdp
 	}
 	return remoteExtMaps, nil
 }
+
+// GetExtMapByURI return a copy of the extmap matching the provided
+// URI. Note that the extmap value will change if not yet negotiated
+func getExtMapByURI(exts map[SDPSectionType][]sdp.ExtMap, uri string) *sdp.ExtMap {
+	for _, extList := range exts {
+		for _, extMap := range extList {
+			if extMap.URI.String() == uri {
+				return &sdp.ExtMap{
+					Value:     extMap.Value,
+					Direction: extMap.Direction,
+					URI:       extMap.URI,
+					ExtAttr:   extMap.ExtAttr,
+				}
+			}
+		}
+	}
+	return nil
+}

sdp_test.go

@@ -213,27 +213,27 @@ func TestTrackDetailsFromSDP(t *testing.T) {
 
 		tracks := trackDetailsFromSDP(nil, s)
 		assert.Equal(t, 3, len(tracks))
-		if _, ok := tracks[1000]; ok {
+		if trackDetail := trackDetailsForSSRC(tracks, 1000); trackDetail != nil {
 			assert.Fail(t, "got the unknown track ssrc:1000 which should have been skipped")
 		}
-		if track, ok := tracks[2000]; !ok {
+		if track := trackDetailsForSSRC(tracks, 2000); track == nil {
 			assert.Fail(t, "missing audio track with ssrc:2000")
 		} else {
 			assert.Equal(t, RTPCodecTypeAudio, track.kind)
 			assert.Equal(t, uint32(2000), track.ssrc)
 			assert.Equal(t, "audio_trk_label", track.label)
 		}
-		if track, ok := tracks[3000]; !ok {
+		if track := trackDetailsForSSRC(tracks, 3000); track == nil {
 			assert.Fail(t, "missing video track with ssrc:3000")
 		} else {
 			assert.Equal(t, RTPCodecTypeVideo, track.kind)
 			assert.Equal(t, uint32(3000), track.ssrc)
 			assert.Equal(t, "video_trk_label", track.label)
 		}
-		if _, ok := tracks[4000]; ok {
+		if track := trackDetailsForSSRC(tracks, 4000); track != nil {
 			assert.Fail(t, "got the rtx track ssrc:3000 which should have been skipped")
 		}
-		if track, ok := tracks[5000]; !ok {
+		if track := trackDetailsForSSRC(tracks, 5000); track == nil {
 			assert.Fail(t, "missing video track with ssrc:5000")
 		} else {
 			assert.Equal(t, RTPCodecTypeVideo, track.kind)

track.go

@@ -27,6 +27,7 @@ type Track struct {
 	label       string
 	ssrc        uint32
 	codec       *RTPCodec
+	rid         string
 
 	packetizer rtp.Packetizer
 
@@ -42,6 +43,16 @@ func (t *Track) ID() string {
 	return t.id
 }
 
+// RID gets the RTP Stream ID of this Track
+// With Simulcast you will have multiple tracks with the same ID, but different RID values.
+// In many cases a Track will not have an RID, so it is important to assert it is non-zero
+func (t *Track) RID() string {
+	t.mu.RLock()
+	defer t.mu.RUnlock()
+
+	return t.rid
+}
+
 // PayloadType gets the PayloadType of the track
 func (t *Track) PayloadType() uint8 {
 	t.mu.RLock()
@@ -95,7 +106,7 @@ func (t *Track) Read(b []byte) (n int, err error) {
 	}
 	t.mu.RUnlock()
 
-	return r.readRTP(b)
+	return r.readRTP(b, t)
 }
 
 // ReadRTP is a convenience method that wraps Read and unmarshals for you