Restart ICE instead of restarting down connections.

static/sfu.js

@@ -643,7 +643,7 @@ function serverConnect() {
             let m = JSON.parse(e.data);
             switch(m.type) {
             case 'offer':
-                gotOffer(m.id, m.labels, m.offer);
+                gotOffer(m.id, m.labels, m.offer, !!m.renegotiate);
                 break;
             case 'answer':
                 gotAnswer(m.id, m.answer);
@@ -714,11 +714,12 @@ function sendRequest(value) {
     });
 }
 
-async function gotOffer(id, labels, offer) {
+async function gotOffer(id, labels, offer, renegotiate) {
     let c = down[id];
-    if(c) {
-        // a new offer with a known id does not indicate renegotiation,
-        // but a new connection that replaces the old one.
+    if(c && !renegotiate) {
+        // SDP is rather inflexible as to what can be renegotiated.
+        // Unless the server indicates that this is a renegotiation with
+        // all parameters unchanged, tear down the existing connection.
         delete(down[id])
         c.close(false);
         c = null;

webclient.go

@@ -179,6 +179,7 @@ type clientMessage struct {
 	Offer       *webrtc.SessionDescription `json:"offer,omitempty"`
 	Answer      *webrtc.SessionDescription `json:"answer,omitempty"`
 	Candidate   *webrtc.ICECandidateInit   `json:"candidate,omitempty"`
+	Renegotiate bool                       `json:"renegotiate,omitempty"`
 	Labels      map[string]string          `json:"labels,omitempty"`
 	Del         bool                       `json:"del,omitempty"`
 	Request     rateMap                    `json:"request,omitempty"`
@@ -393,8 +394,9 @@ func addDownTrack(c *webClient, conn *rtpDownConnection, remoteTrack upTrack, re
 	return s, nil
 }
 
-func negotiate(c *webClient, down *rtpDownConnection) error {
-	offer, err := down.pc.CreateOffer(nil)
+func negotiate(c *webClient, down *rtpDownConnection, renegotiate, restartIce bool) error {
+	options := webrtc.OfferOptions{ICERestart: restartIce}
+	offer, err := down.pc.CreateOffer(&options)
 	if err != nil {
 		return err
 	}
@@ -425,6 +427,7 @@ func negotiate(c *webClient, down *rtpDownConnection) error {
 		Type:        "offer",
 		Id:          down.id,
 		Offer:       &offer,
+		Renegotiate: renegotiate,
 		Labels:      labels,
 	})
 }
@@ -733,7 +736,7 @@ func clientLoop(c *webClient, conn *websocket.Conn) error {
 					return err
 				}
 				if down != nil {
-					err = negotiate(c, down)
+					err = negotiate(c, down, false, false)
 					if err != nil {
 						log.Printf("Negotiate: %v", err)
 						delDownConn(c, down.id)
@@ -769,7 +772,11 @@ func clientLoop(c *webClient, conn *websocket.Conn) error {
 						"unknown connection")
 					continue
 				}
-				tracks := make([]upTrack, len(down.tracks))
+				err := negotiate(c, down, true, true)
+				if err != nil {
+					tracks := make(
+						[]upTrack, len(down.tracks),
+					)
 					for i, t := range down.tracks {
 						tracks[i] = t.remote
 					}
@@ -777,6 +784,7 @@ func clientLoop(c *webClient, conn *websocket.Conn) error {
 						down.remote.Id(), down.remote,
 						tracks, down.remote.Label(),
 					)
+				}
 			case permissionsChangedAction:
 				c.write(clientMessage{
 					Type:        "permissions",