Label streams, not tracks.

We used to label tracks individually, in a view to using the labelling
for simulcast.  Since then, the WebRTC community has converged on a
different strategy, where multiple tracks share a single mid and
are labelled with the rid extension.

We now label whole streams, which is simpler, and use the track's
kind (and, in the future, the rid) to disambiguate.  This changes the
protocol in two ways:

  * in offers, the "labels" dictionary is replaced by a single "label"
    field; and
  * the syntax of the "request" message has changed.

rtpconn/rtpconn.go

@@ -313,6 +313,10 @@ func (up *rtpUpTrack) Label() string {
 	return up.label
 }
 
+func (up *rtpUpTrack) Kind() webrtc.RTPCodecType {
+	return up.track.Kind()
+}
+
 func (up *rtpUpTrack) Codec() webrtc.RTPCodecCapability {
 	return up.track.Codec().RTPCodecCapability
 }
@@ -328,10 +332,10 @@ func (up *rtpUpTrack) hasRtcpFb(tpe, parameter string) bool {
 
 type rtpUpConnection struct {
 	id            string
+	label         string
 	userId        string
 	username      string
 	pc            *webrtc.PeerConnection
-	labels        map[string]string
 	iceCandidates []*webrtc.ICECandidateInit
 
 	mu      sync.Mutex
@@ -363,6 +367,10 @@ func (up *rtpUpConnection) Id() string {
 	return up.id
 }
 
+func (up *rtpUpConnection) Label() string {
+	return up.label
+}
+
 func (up *rtpUpConnection) User() (string, string) {
 	return up.userId, up.username
 }
@@ -413,33 +421,6 @@ func (up *rtpUpConnection) flushICECandidates() error {
 	return err
 }
 
-func getTrackMid(pc *webrtc.PeerConnection, track *webrtc.TrackRemote) string {
-	for _, t := range pc.GetTransceivers() {
-		if t.Receiver() != nil && t.Receiver().Track() == track {
-			return t.Mid()
-		}
-	}
-	return ""
-}
-
-// called locked
-func (up *rtpUpConnection) complete() bool {
-	for mid := range up.labels {
-		found := false
-		for _, t := range up.tracks {
-			m := getTrackMid(up.pc, t.track)
-			if m == mid {
-				found = true
-				break
-			}
-		}
-		if !found {
-			return false
-		}
-	}
-	return true
-}
-
 // pushConnNow pushes a connection to all of the clients in a group
 func pushConnNow(up *rtpUpConnection, g *group.Group, cs []group.Client) {
 	up.mu.Lock()
@@ -460,17 +441,11 @@ func pushConnNow(up *rtpUpConnection, g *group.Group, cs []group.Client) {
 // pushConn schedules a call to pushConnNow
 func pushConn(up *rtpUpConnection, g *group.Group, cs []group.Client) {
 	up.mu.Lock()
-	if up.complete() {
-		up.mu.Unlock()
-		pushConnNow(up, g, cs)
-		return
-	}
-
 	up.pushed = false
 	up.mu.Unlock()
 
 	go func(g *group.Group, cs []group.Client) {
-		time.Sleep(4 * time.Second)
+		time.Sleep(200 * time.Millisecond)
 		up.mu.Lock()
 		pushed := up.pushed
 		up.pushed = true
@@ -481,7 +456,7 @@ func pushConn(up *rtpUpConnection, g *group.Group, cs []group.Client) {
 	}(g, cs)
 }
 
-func newUpConn(c group.Client, id string, labels map[string]string, offer string) (*rtpUpConnection, error) {
+func newUpConn(c group.Client, id string, label string, offer string) (*rtpUpConnection, error) {
 	var o sdp.SessionDescription
 	err := o.Unmarshal([]byte(offer))
 	if err != nil {
@@ -506,31 +481,13 @@ func newUpConn(c group.Client, id string, labels map[string]string, offer string
 		}
 	}
 
-	up := &rtpUpConnection{id: id, pc: pc, labels: labels}
+	up := &rtpUpConnection{id: id, label: label, pc: pc}
 
 	pc.OnTrack(func(remote *webrtc.TrackRemote, receiver *webrtc.RTPReceiver) {
 		up.mu.Lock()
 
-		mid := getTrackMid(pc, remote)
-		if mid == "" {
-			log.Printf("Couldn't get track's mid")
-			return
-		}
-
-		label, ok := up.labels[mid]
-		if !ok {
-			log.Printf("Couldn't get track's label")
-			isvideo := remote.Kind() == webrtc.RTPCodecTypeVideo
-			if isvideo {
-				label = "video"
-			} else {
-				label = "audio"
-			}
-		}
-
 		track := &rtpUpTrack{
 			track:      remote,
-			label:      label,
 			cache:      packetcache.New(minPacketCache(remote)),
 			rate:       estimator.New(time.Second),
 			jitter:     jitter.New(remote.Codec().ClockRate),