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.

README.FRONTEND

@@ -65,7 +65,7 @@ You typically join a group and request media in the `onconnected` callback:
 ```javascript
 serverConnection.onconnected = function() {
     this.join(group, 'join', username, password); 
-    this.request('everything');
+    this.request({'':['audio','video']});
 }
 ```
 
@@ -110,10 +110,7 @@ serverConnection.ondownstream = function(stream) {
 }
 ```
 
-The `stream.labels` dictionary maps each track's id to one of `audio`,
+The `stream.label` field is one of `camera`, `screenshare` or `video`.
-`video` or `screenshare`.  Since `stream.labels` is already available at
-this point, you may set up an `audio` or `video` component straight away,
-or you may choose to wait until the `ondowntrack` callback is called.
 
 After a new stream is created, `ondowntrack` will be called whenever
 a track is added.
@@ -137,10 +134,10 @@ to push a stream to the server.  Given a `MediaStream` called `localStream`
 
 ```javascript
 let stream = serverConnection.newUpStream();
+stream.label = ...;
 stream.onerror = ...;
 stream.onstatus = ...;
 localStream.getTracks().forEach(t => {
-    c.labels[t.id] = t.kind;
     c.pc.addTrack(t, c.stream);
 });
 ```
@@ -150,8 +147,6 @@ the `localId` property of an existing stream, then the existing stream
 will be closed and the server will be informed that the new stream
 replaces the existing stream.
 
-See above for information about setting up the `labels` dictionary.
-
 ## Stream statistics
 
 Some statistics about streams are made available by calling the

README.PROTOCOL

@@ -135,7 +135,8 @@ A peer must explicitly request the streams that it wants to receive.
 ```
 
 The field `request` is a dictionary that maps the labels of requested
-tracks to the boolean `true`.
+streams to a list containing either 'audio', 'video' or both.  An entry
+with an empty key `''` serves as default.
 
 ## Pushing streams
 
@@ -145,11 +146,11 @@ A stream is created by the sender with the `offer` message:
 {
     type: 'offer',
     id: id,
+    label: label,
     replace: id,
     source: source-id,
     username: username,
     sdp: sdp,
-    labels: labels
 }
 ```
 
@@ -158,15 +159,14 @@ otherwise this message creates a new stream.  If the field `replace` is
 not empty, then this request additionally requests that an existing stream
 with the given id should be closed, and the new stream should replace it.
 
+The field `label` is one of `camera`, `screenshare` or `video`, as in the
+`request` message.
+
 The field `sdp` contains the raw SDP string (i.e. the `sdp` field of
 a JSEP session description).  Galène will interpret the `nack`,
 `nack pli`, `ccm fir` and `goog-remb` RTCP feedback types, and act
 accordingly.
 
-The field `labels` is a dictionary that maps track mids to one of `audio`,
-`video` or `screenshare`.  If a track does not appear in the `labels`
-dictionary, it should be ignored.
-
 The receiver may either abort the stream immediately (see below), or send
 an answer.
 

conn/conn.go

@@ -16,6 +16,7 @@ type Up interface {
 	AddLocal(Down) error
 	DelLocal(Down) bool
 	Id() string
+	Label() string
 	User() (string, string)
 }
 
@@ -23,7 +24,7 @@ type Up interface {
 type UpTrack interface {
 	AddLocal(DownTrack) error
 	DelLocal(DownTrack) bool
-	Label() string
+	Kind() webrtc.RTPCodecType
 	Codec() webrtc.RTPCodecCapability
 	// get a recent packet.  Returns 0 if the packet is not in cache.
 	GetRTP(seqno uint16, result []byte) uint16

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),

rtpconn/webclient.go

@@ -59,7 +59,7 @@ type webClient struct {
 	password    string
 	permissions group.ClientPermissions
 	status      map[string]interface{}
-	requested   map[string]uint32
+	requested   map[string][]string
 	done        chan struct{}
 	writeCh     chan interface{}
 	writerDone  chan struct{}
@@ -122,53 +122,6 @@ func (c *webClient) PushClient(id, username string, permissions group.ClientPerm
 	})
 }
 
-type rateMap map[string]uint32
-
-func (v *rateMap) UnmarshalJSON(b []byte) error {
-	var m map[string]interface{}
-
-	err := json.Unmarshal(b, &m)
-	if err != nil {
-		return err
-	}
-
-	n := make(map[string]uint32, len(m))
-	for k, w := range m {
-		switch w := w.(type) {
-		case bool:
-			if w {
-				n[k] = ^uint32(0)
-			} else {
-				n[k] = 0
-			}
-		case float64:
-			if w < 0 || w >= float64(^uint32(0)) {
-				return errors.New("overflow")
-			}
-			n[k] = uint32(w)
-		default:
-			return errors.New("unexpected type in JSON map")
-		}
-	}
-	*v = n
-	return nil
-}
-
-func (v rateMap) MarshalJSON() ([]byte, error) {
-	m := make(map[string]interface{}, len(v))
-	for k, w := range v {
-		switch w {
-		case 0:
-			m[k] = false
-		case ^uint32(0):
-			m[k] = true
-		default:
-			m[k] = w
-		}
-	}
-	return json.Marshal(m)
-}
-
 type clientMessage struct {
 	Type             string                   `json:"type"`
 	Kind             string                   `json:"kind,omitempty"`
@@ -187,8 +140,8 @@ type clientMessage struct {
 	Time             int64                    `json:"time,omitempty"`
 	SDP              string                   `json:"sdp,omitempty"`
 	Candidate        *webrtc.ICECandidateInit `json:"candidate,omitempty"`
-	Labels           map[string]string        `json:"labels,omitempty"`
+	Label            string                   `json:"label,omitempty"`
-	Request          rateMap                  `json:"request,omitempty"`
+	Request          map[string][]string      `json:"request,omitempty"`
 	RTCConfiguration *webrtc.Configuration    `json:"rtcConfiguration,omitempty"`
 }
 
@@ -216,7 +169,7 @@ func getUpConns(c *webClient) []*rtpUpConnection {
 	return up
 }
 
-func addUpConn(c *webClient, id string, labels map[string]string, offer string) (*rtpUpConnection, bool, error) {
+func addUpConn(c *webClient, id, label string, offer string) (*rtpUpConnection, bool, error) {
 	c.mu.Lock()
 	defer c.mu.Unlock()
 
@@ -232,7 +185,7 @@ func addUpConn(c *webClient, id string, labels map[string]string, offer string)
 		return old, false, nil
 	}
 
-	conn, err := newUpConn(c, id, labels, offer)
+	conn, err := newUpConn(c, id, label, offer)
 	if err != nil {
 		return nil, false, err
 	}
@@ -540,33 +493,16 @@ func negotiate(c *webClient, down *rtpDownConnection, restartIce bool, replace s
 		return err
 	}
 
-	labels := make(map[string]string)
-	for _, t := range down.pc.GetTransceivers() {
-		var track webrtc.TrackLocal
-		if t.Sender() != nil {
-			track = t.Sender().Track()
-		}
-		if track == nil {
-			continue
-		}
-
-		for _, tr := range down.tracks {
-			if tr.track == track {
-				labels[t.Mid()] = tr.remote.Label()
-			}
-		}
-	}
-
 	source, username := down.remote.User()
 
 	return c.write(clientMessage{
 		Type:     "offer",
 		Id:       down.id,
+		Label:    down.remote.Label(),
 		Replace:  replace,
 		Source:   source,
 		Username: username,
 		SDP:      down.pc.LocalDescription().SDP,
-		Labels:   labels,
 	})
 }
 
@@ -582,8 +518,8 @@ func sendICE(c *webClient, id string, candidate *webrtc.ICECandidate) error {
 	})
 }
 
-func gotOffer(c *webClient, id string, sdp string, labels map[string]string, replace string) error {
+func gotOffer(c *webClient, id, label string, sdp string, replace string) error {
-	up, _, err := addUpConn(c, id, labels, sdp)
+	up, _, err := addUpConn(c, id, label, sdp)
 	if err != nil {
 		return err
 	}
@@ -681,7 +617,7 @@ func gotICE(c *webClient, candidate *webrtc.ICECandidateInit, id string) error {
 	return conn.addICECandidate(candidate)
 }
 
-func (c *webClient) setRequested(requested map[string]uint32) error {
+func (c *webClient) setRequested(requested map[string][]string) error {
 	if c.group == nil {
 		return errors.New("attempted to request with no group joined")
 	}
@@ -701,8 +637,46 @@ func pushConns(c group.Client, g *group.Group) {
 	}
 }
 
-func (c *webClient) isRequested(label string) bool {
+func requestedTracks(c *webClient, up conn.Up, tracks []conn.UpTrack) []conn.UpTrack {
-	return c.requested[label] != 0
+	r, ok := c.requested[up.Label()]
+	if !ok {
+		r, ok = c.requested[""]
+	}
+	if !ok || len(r) == 0 {
+		return nil
+	}
+
+	var audio, video bool
+	for _, s := range r {
+		switch s {
+		case "audio":
+			audio = true
+		case "video":
+			video = true
+		default:
+			log.Printf("client requested unknown value %v", s)
+		}
+	}
+
+	var ts []conn.UpTrack
+	if audio {
+		for _, t := range tracks {
+			if t.Kind() == webrtc.RTPCodecTypeAudio {
+				ts = append(ts, t)
+				break
+			}
+		}
+	}
+	if video {
+		for _, t := range tracks {
+			if t.Kind() == webrtc.RTPCodecTypeVideo {
+				ts = append(ts, t)
+				break
+			}
+		}
+	}
+
+	return ts
 }
 
 func (c *webClient) PushConn(g *group.Group, id string, up conn.Up, tracks []conn.UpTrack, replace string) error {
@@ -871,16 +845,7 @@ func handleAction(c *webClient, a interface{}) error {
 		}
 		var tracks []conn.UpTrack
 		if a.conn != nil {
-			tracks = make([]conn.UpTrack,
+			tracks = requestedTracks(c, a.conn, a.tracks)
-				0, len(a.tracks),
-			)
-			for _, t := range a.tracks {
-				if c.isRequested(t.Label()) {
-					tracks = append(
-						tracks, t,
-					)
-				}
-			}
 		}
 
 		if len(tracks) == 0 {
@@ -923,9 +888,6 @@ func handleAction(c *webClient, a interface{}) error {
 			return nil
 		}
 		for _, u := range c.up {
-			if !u.complete() {
-				continue
-			}
 			tracks := u.getTracks()
 			replace := u.getReplace(false)
 
@@ -1051,7 +1013,7 @@ func leaveGroup(c *webClient) {
 	group.DelClient(c)
 	c.permissions = group.ClientPermissions{}
 	c.status = nil
-	c.requested = map[string]uint32{}
+	c.requested = make(map[string][]string)
 	c.group = nil
 }
 
@@ -1238,7 +1200,7 @@ func handleClientMessage(c *webClient, m clientMessage) error {
 			})
 			return c.error(group.UserError("not authorised"))
 		}
-		err := gotOffer(c, m.Id, m.SDP, m.Labels, m.Replace)
+		err := gotOffer(c, m.Id, m.Label, m.SDP, m.Replace)
 		if err != nil {
 			log.Printf("gotOffer: %v", err)
 			return failUpConnection(c, m.Id, "negotiation failed")

static/galene.js

@@ -342,13 +342,7 @@ function gotDownStream(c) {
         setMedia(c, false);
     };
     c.onnegotiationcompleted = function() {
-        let found = false;
+        resetMedia(c);
-        for(let key in c.labels) {
-            if(c.labels[key] === 'video')
-                found = true;
-        }
-        if(!found)
-            resetMedia(c);
     }
     c.onstatus = function(status) {
         setMediaStatus(c);
@@ -384,7 +378,7 @@ getButtonElement('presentbutton').onclick = async function(e) {
     // button a second time before the stream is set up and the button hidden.
     button.disabled = true;
     try {
-        let id = findUpMedia('local');
+        let id = findUpMedia('camera');
         if(!id)
             await addLocalMedia();
     } finally {
@@ -394,12 +388,12 @@ getButtonElement('presentbutton').onclick = async function(e) {
 
 getButtonElement('unpresentbutton').onclick = function(e) {
     e.preventDefault();
-    closeUpMediaKind('local');
+    closeUpMedia('camera');
     resizePeers();
 };
 
 function changePresentation() {
-    let c = findUpMedia('local');
+    let c = findUpMedia('camera');
     if(c)
         addLocalMedia(c.localId);
 }
@@ -419,7 +413,7 @@ function setVisibility(id, visible) {
 function setButtonsVisibility() {
     let connected = serverConnection && serverConnection.socket;
     let permissions = serverConnection.permissions;
-    let local = !!findUpMedia('local');
+    let local = !!findUpMedia('camera');
     let video = !!findUpMedia('video');
     let canWebrtc = !(typeof RTCPeerConnection === 'undefined');
     let canFile =
@@ -516,7 +510,7 @@ document.getElementById('sharebutton').onclick = function(e) {
 
 document.getElementById('stopvideobutton').onclick = function(e) {
     e.preventDefault();
-    closeUpMediaKind('video');
+    closeUpMedia('video');
     resizePeers();
 };
 
@@ -556,12 +550,37 @@ getSelectElement('sendselect').onchange = async function(e) {
     }
 };
 
+/**
+ * @param {string} what
+ * @returns {Object<string,Array<string>>}
+ */
+
+function mapRequest(what) {
+    switch(what) {
+    case '':
+        return {};
+        break;
+    case 'audio':
+        return {'': ['audio']};
+        break;
+    case 'screenshare':
+        return {screenshare: ['audio','video'], '': ['audio']};
+        break;
+    case 'everything':
+        return {'': ['audio','video']}
+        break;
+    default:
+        displayError(`Unknown value ${what} in request`);
+        return {};
+    }
+}
+
 getSelectElement('requestselect').onchange = function(e) {
     e.preventDefault();
     if(!(this instanceof HTMLSelectElement))
         throw new Error('Unexpected type for this');
     updateSettings({request: this.value});
-    serverConnection.request(this.value);
+    serverConnection.request(mapRequest(this.value));
 };
 
 const activityDetectionInterval = 200;
@@ -1069,8 +1088,8 @@ async function addLocalMedia(localId) {
         return;
     }
 
-    c.kind = 'local';
     c.stream = stream;
+    c.label = 'camera';
 
     if(filter) {
         try {
@@ -1100,7 +1119,6 @@ async function addLocalMedia(localId) {
 
     let mute = getSettings().localMute;
     c.stream.getTracks().forEach(t => {
-        c.labels[t.id] = t.kind;
         if(t.kind == 'audio') {
             if(mute)
                 t.enabled = false;
@@ -1143,8 +1161,8 @@ async function addShareMedia() {
     }
 
     let c = newUpStream();
-    c.kind = 'screenshare';
     c.stream = stream;
+    c.label = 'screenshare';
     c.onclose = replace => {
         stopStream(stream);
         if(!replace)
@@ -1153,7 +1171,6 @@ async function addShareMedia() {
     stream.getTracks().forEach(t => {
         c.pc.addTrack(t, stream);
         t.onended = e => c.close();
-        c.labels[t.id] = 'screenshare';
     });
     c.onstats = gotUpStats;
     c.setStatsInterval(2000);
@@ -1184,8 +1201,8 @@ async function addFileMedia(file) {
     }
 
     let c = newUpStream();
-    c.kind = 'video';
     c.stream = stream;
+    c.label = 'video';
     c.onclose = function(replace) {
         stopStream(c.stream);
         let media = /** @type{HTMLVideoElement} */
@@ -1201,7 +1218,7 @@ async function addFileMedia(file) {
     stream.onaddtrack = function(e) {
         let t = e.track;
         if(t.kind === 'audio') {
-            let presenting = !!findUpMedia('local');
+            let presenting = !!findUpMedia('camera');
             let muted = getSettings().localMute;
             if(presenting && !muted) {
                 setLocalMute(true, true);
@@ -1209,13 +1226,11 @@ async function addFileMedia(file) {
             }
         }
         c.pc.addTrack(t, stream);
-        c.labels[t.id] = t.kind;
         c.onstats = gotUpStats;
         c.setStatsInterval(2000);
     };
     stream.onremovetrack = function(e) {
         let t = e.track;
-        delete(c.labels[t.id]);
 
         /** @type {RTCRtpSender} */
         let sender;
@@ -1229,7 +1244,12 @@ async function addFileMedia(file) {
             console.warn('Removing unknown track');
         }
 
-        if(Object.keys(c.labels).length === 0) {
+        let found = false;
+        c.pc.getSenders().forEach(s => {
+            if(s.track)
+                found = true;
+        });
+        if(!found) {
             stream.onaddtrack = null;
             stream.onremovetrack = null;
             c.close();
@@ -1254,28 +1274,28 @@ function stopStream(s) {
 }
 
 /**
- * closeUpMediaKind closes all up connections that correspond to a given
+ * closeUpMedia closes all up connections with the given label.  If label
- * kind of media.  If kind is null, it closes all up connections.
+ * is null, it closes all up connections.
  *
- * @param {string} kind
+ * @param {string} label
 */
-function closeUpMediaKind(kind) {
+function closeUpMedia(label) {
     for(let id in serverConnection.up) {
         let c = serverConnection.up[id];
-        if(kind && c.kind != kind)
+        if(label && c.label !== label)
             continue
         c.close();
     }
 }
 
 /**
- * @param {string} kind
+ * @param {string} label
  * @returns {Stream}
  */
-function findUpMedia(kind) {
+function findUpMedia(label) {
     for(let id in serverConnection.up) {
         let c = serverConnection.up[id]
-        if(c.kind === kind)
+        if(c.label === label)
             return c;
     }
     return null;
@@ -1289,7 +1309,7 @@ function muteLocalTracks(mute) {
         return;
     for(let id in serverConnection.up) {
         let c = serverConnection.up[id];
-        if(c.kind === 'local') {
+        if(c.label === 'camera') {
             let stream = c.stream;
             stream.getTracks().forEach(t => {
                 if(t.kind === 'audio') {
@@ -1370,7 +1390,7 @@ async function setMedia(c, isUp, mirror, video) {
     showVideo();
     resizePeers();
 
-    if(!isUp && isSafari() && !findUpMedia('local')) {
+    if(!isUp && isSafari() && !findUpMedia('camera')) {
         // Safari doesn't allow autoplay unless the user has granted media access
         try {
             let stream = await navigator.mediaDevices.getUserMedia({audio: true});
@@ -1429,10 +1449,10 @@ function addCustomControls(media, container, c) {
 
     let volume = getVideoButton(controls, 'volume');
     let stopsharing = getVideoButton(topcontrols, 'video-stop');
-    if (c.kind !== "screenshare") {
+    if (c.label !== "screenshare") {
         stopsharing.remove();
     }
-    if(c.kind === 'local') {
+    if(c.label === 'camera') {
         volume.remove();
     } else {
         setVolumeButton(media.muted,
@@ -1820,9 +1840,9 @@ async function gotJoined(kind, group, perms, message) {
     if(typeof RTCPeerConnection === 'undefined')
         displayWarning("This browser doesn't support WebRTC");
     else
-        this.request(getSettings().request);
+        this.request(mapRequest(getSettings().request));
 
-    if(serverConnection.permissions.present && !findUpMedia('local')) {
+    if(serverConnection.permissions.present && !findUpMedia('camera')) {
         if(present) {
             if(present === 'mike')
                 updateSettings({video: ''});

static/protocol.js

@@ -212,8 +212,8 @@ function ServerConnection() {
   * @property {boolean} [noecho]
   * @property {string} [sdp]
   * @property {RTCIceCandidate} [candidate]
-  * @property {Object<string,string>} [labels]
+  * @property {string} [label]
-  * @property {Object<string,(boolean|number)>} [request]
+  * @property {Object<string,Array<string>>} [request]
   * @property {Object<string,any>} [rtcConfiguration]
   */
 
@@ -296,7 +296,7 @@ ServerConnection.prototype.connect = async function(url) {
             case 'handshake':
                 break;
             case 'offer':
-                sc.gotOffer(m.id, m.labels, m.source, m.username,
+                sc.gotOffer(m.id, m.label, m.source, m.username,
                             m.sdp, m.replace);
                 break;
             case 'answer':
@@ -439,32 +439,14 @@ ServerConnection.prototype.leave = function(group) {
 /**
  * request sets the list of requested media types.
  *
- * @param {string} what - One of '', 'audio', 'screenshare' or 'everything'.
+ * @param {Object<string,Array<string>>} what
+ *     - A dictionary that maps labels to a sequence of 'audio' and 'video'.
+ *       An entry with an empty label '' provides the default.
  */
 ServerConnection.prototype.request = function(what) {
-    /** @type {Object<string,boolean>} */
-    let request = {};
-    switch(what) {
-    case '':
-        request = {};
-        break;
-    case 'audio':
-        request = {audio: true};
-        break;
-    case 'screenshare':
-        request = {audio: true, screenshare: true};
-        break;
-    case 'everything':
-        request = {audio: true, screenshare: true, video: true};
-        break;
-    default:
-        console.error(`Unknown value ${what} in request`);
-        break;
-    }
-
     this.send({
         type: 'request',
-        request: request,
+        request: what,
     });
 };
 
@@ -622,14 +604,14 @@ ServerConnection.prototype.groupAction = function(kind, message) {
  * Called when we receive an offer from the server.  Don't call this.
  *
  * @param {string} id
- * @param {Object<string, string>} labels
+ * @param {string} label
  * @param {string} source
  * @param {string} username
  * @param {string} sdp
  * @param {string} replace
  * @function
  */
-ServerConnection.prototype.gotOffer = async function(id, labels, source, username, sdp, replace) {
+ServerConnection.prototype.gotOffer = async function(id, label, source, username, sdp, replace) {
     let sc = this;
 
     if(sc.up[id]) {
@@ -669,6 +651,7 @@ ServerConnection.prototype.gotOffer = async function(id, labels, source, usernam
             return;
         }
         c = new Stream(this, id, oldLocalId || newLocalId(), pc, false);
+        c.label = label;
         sc.down[id] = c;
 
         c.pc.onicecandidate = function(e) {
@@ -689,32 +672,15 @@ ServerConnection.prototype.gotOffer = async function(id, labels, source, usernam
         };
 
         c.pc.ontrack = function(e) {
-            let label = e.transceiver && c.labelsByMid[e.transceiver.mid];
-            if(label) {
-                c.labels[e.track.id] = label;
-            } else {
-                console.warn("Couldn't find label for track");
-            }
             c.stream = e.streams[0];
             if(c.ondowntrack) {
                 c.ondowntrack.call(
-                    c, e.track, e.transceiver, label, e.streams[0],
+                    c, e.track, e.transceiver, e.streams[0],
                 );
             }
         };
     }
 
-    c.labelsByMid = labels;
-    c.labels = {};
-    c.pc.getTransceivers().forEach(transceiver => {
-        let label = c.labelsByMid[transceiver.mid];
-        let track = transceiver.receiver && transceiver.receiver.track;
-        if(label && track) {
-            c.labels[track.id] = label;
-        } else if(!track) {
-            console.warn("Couldn't find track for label");
-        }
-    });
     c.source = source;
     c.username = username;
 
@@ -883,12 +849,6 @@ function Stream(sc, id, localId, pc, up) {
      * @const
      */
     this.up = up;
-    /**
-     * For up streams, one of "local" or "screenshare".
-     *
-     * @type {string}
-     */
-    this.kind = null;
     /**
      * For down streams, the id of the client that created the stream.
      *
@@ -916,17 +876,11 @@ function Stream(sc, id, localId, pc, up) {
      */
     this.stream = null;
     /**
-     * Track labels, indexed by track id.
+     * The label assigned by the originator to this stream.
      *
-     * @type {Object<string,string>}
+     * @type {string}
      */
-    this.labels = {};
+    this.label = null;
-    /**
-     * Track labels, indexed by mid.
-     *
-     * @type {Object<string,string>}
-     */
-    this.labelsByMid = {};
     /**
      * The id of the stream that we are currently replacing.
      *
@@ -1004,7 +958,7 @@ function Stream(sc, id, localId, pc, up) {
      * If the stream parameter differs from its previous value, then it
      * indicates that the old stream has been discarded.
      *
-     * @type{(this: Stream, track: MediaStreamTrack, transceiver: RTCRtpTransceiver, label: string, stream: MediaStream) => void}
+     * @type{(this: Stream, track: MediaStreamTrack, transceiver: RTCRtpTransceiver, stream: MediaStream) => void}
      */
     this.ondowntrack = null;
     /**
@@ -1164,17 +1118,6 @@ Stream.prototype.negotiate = async function (restartIce) {
         throw(new Error("Didn't create offer"));
     await c.pc.setLocalDescription(offer);
 
-    // mids are not known until this point
-    c.pc.getTransceivers().forEach(t => {
-        if(t.sender && t.sender.track) {
-            let label = c.labels[t.sender.track.id];
-            if(label)
-                c.labelsByMid[t.mid] = label;
-            else
-                console.warn("Couldn't find label for track");
-        }
-    });
-
     c.sc.send({
         type: 'offer',
         source: c.sc.id,
@@ -1182,7 +1125,7 @@ Stream.prototype.negotiate = async function (restartIce) {
         kind: this.localDescriptionSent ? 'renegotiate' : '',
         id: c.id,
         replace: this.replace,
-        labels: c.labelsByMid,
+        label: c.label,
         sdp: c.pc.localDescription.sdp,
     });
     this.localDescriptionSent = true;