Add backchannel support for rtsp server

internal/rtsp/rtsp.go

@@ -8,6 +8,7 @@ import (
 
 	"github.com/AlexxIT/go2rtc/internal/app"
 	"github.com/AlexxIT/go2rtc/internal/streams"
+	"github.com/AlexxIT/go2rtc/pkg/aac"
 	"github.com/AlexxIT/go2rtc/pkg/core"
 	"github.com/AlexxIT/go2rtc/pkg/rtsp"
 	"github.com/AlexxIT/go2rtc/pkg/tcp"
@@ -184,6 +185,23 @@ func tcpHandler(conn *rtsp.Conn) {
 				}
 			}
 
+            if query.Get("backchannel") == "1" {
+                conn.Medias = append(conn.Medias, &core.Media{
+                    Kind:      core.KindAudio,
+                    Direction: core.DirectionRecvonly,
+                    Codecs: []*core.Codec{
+						{Name: core.CodecOpus, ClockRate: 48000, Channels: 2},
+						{Name: core.CodecPCM, ClockRate: 16000},
+						{Name: core.CodecPCMA, ClockRate: 16000},
+						{Name: core.CodecPCMU, ClockRate: 16000},
+						{Name: core.CodecPCM, ClockRate: 8000},
+						{Name: core.CodecPCMA, ClockRate: 8000},
+						{Name: core.CodecPCMU, ClockRate: 8000},
+						{Name: core.CodecAAC, ClockRate: 16000, FmtpLine: aac.FMTP + "1408"},
+                    },
+                })
+            }
+
 			if s := query.Get("pkt_size"); s != "" {
 				conn.PacketSize = uint16(core.Atoi(s))
 			}

pkg/core/media.go

@@ -141,6 +141,10 @@ func MarshalSDP(name string, medias []*Media) ([]byte, error) {
 		}
 		md.WithCodec(codec.PayloadType, name, codec.ClockRate, codec.Channels, codec.FmtpLine)
 
+		if media.Direction != "" {
+			md.WithPropertyAttribute(media.Direction)
+		}
+
 		if media.ID != "" {
 			md.WithValueAttribute("control", media.ID)
 		}

pkg/rtsp/producer.go

@@ -16,6 +16,8 @@ func (c *Conn) GetTrack(media *core.Media, codec *core.Codec) (*core.Receiver, e
 		}
 	}
 
+    switch c.mode {
+    case core.ModeActiveProducer:
 		c.stateMu.Lock()
 		defer c.stateMu.Unlock()
 
@@ -37,6 +39,20 @@ func (c *Conn) GetTrack(media *core.Media, codec *core.Codec) (*core.Receiver, e
 		c.Receivers = append(c.Receivers, track)
 
 		return track, nil
+    case core.ModePassiveConsumer:
+        // Backchannel
+		c.stateMu.Lock()
+		defer c.stateMu.Unlock()
+
+		channel := byte(len(c.Senders)) * 2
+        track := core.NewReceiver(media, codec)
+        track.ID = channel
+        c.Receivers = append(c.Receivers, track)
+
+        return track, nil
+    default:
+        return nil, errors.New("rtsp: wrong mode for GetTrack")
+    }
 }
 
 func (c *Conn) Start() (err error) {

pkg/rtsp/server.go

@@ -129,6 +129,16 @@ func (c *Conn) Accept() error {
 				medias = append(medias, media)
 			}
 
+			for i, track := range c.Receivers {
+				media := &core.Media{
+					Kind:      core.GetKind(track.Codec.Name),
+					Direction: core.DirectionSendonly,
+					Codecs:    []*core.Codec{track.Codec},
+					ID:        "trackID=" + strconv.Itoa(i+len(c.Senders)),
+				}
+				medias = append(medias, media)
+			}
+
 			res.Body, err = core.MarshalSDP(c.SessionName, medias)
 			if err != nil {
 				return err
@@ -154,11 +164,20 @@ func (c *Conn) Accept() error {
 				c.state = StateSetup
 
 				if c.mode == core.ModePassiveConsumer {
-					if i := reqTrackID(req); i >= 0 && i < len(c.Senders) {
+					trackID := reqTrackID(req)
-						// mark sender as SETUP
+
-						c.Senders[i].Media.ID = MethodSetup
+					if trackID >= 0 {
-						tr = fmt.Sprintf("RTP/AVP/TCP;unicast;interleaved=%d-%d", i*2, i*2+1)
+						if trackID < len(c.Senders) {
+							c.Senders[trackID].Media.ID = MethodSetup
+							tr = fmt.Sprintf("RTP/AVP/TCP;unicast;interleaved=%d-%d", trackID*2, trackID*2+1)
 							res.Header.Set("Transport", tr)
+						} else if trackID >= len(c.Senders) && trackID < len(c.Senders)+len(c.Receivers) {
+							c.Receivers[trackID-len(c.Senders)].Media.ID = MethodSetup
+							tr = fmt.Sprintf("RTP/AVP/TCP;unicast;interleaved=%d-%d", trackID*2, trackID*2+1)
+							res.Header.Set("Transport", tr)
+						} else {
+							res.Status = "400 Bad Request"
+						}
 					} else {
 						res.Status = "400 Bad Request"
 					}