make migrated node has higher candidate priority (#546)

pkg/rtc/participant.go

@@ -36,6 +36,7 @@ const (
 	rttUpdateInterval = 5 * time.Second
 
 	stateActiveCond = 3 // reliableDCOpen,lossyDCOpen,PeerConnectionStateConnected
+	initNetWorkCost = 100
 )
 
 type pendingTrackInfo struct {
@@ -125,6 +126,7 @@ type ParticipantImpl struct {
 	onClaimsChanged     func(participant types.LocalParticipant)
 
 	activeCounter atomic.Int32
+	networkCost   int32
 }
 
 func NewParticipant(params ParticipantParams) (*ParticipantImpl, error) {
@@ -146,6 +148,7 @@ func NewParticipant(params ParticipantParams) (*ParticipantImpl, error) {
 		disallowedSubscriptions:  make(map[livekit.TrackID]livekit.ParticipantID),
 		connectedAt:              time.Now(),
 		rttUpdatedAt:             time.Now(),
+		networkCost:              initNetWorkCost,
 	}
 	p.version.Store(params.InitialVersion)
 	p.migrateState.Store(types.MigrateStateInit)
@@ -600,6 +603,9 @@ func (p *ParticipantImpl) SetMigrateState(s types.MigrateState) {
 		p.pendingOffer = nil
 		// in case of migration, subscriber data channel will not fire OnOpen callback
 		p.activeCounter.CAS(0, 2)
+
+		// use lower network cost to make sure migrated's node has higher priority candidate
+		p.networkCost--
 	}
 	p.lock.Unlock()
 	if s == types.MigrateStateComplete {

pkg/rtc/participant_signal.go

@@ -178,6 +178,7 @@ func (p *ParticipantImpl) sendIceCandidate(c *webrtc.ICECandidate, target liveki
 	// write candidate
 	p.params.Logger.Debugw("sending ice candidates",
 		"candidate", c.String(), "target", target)
+	ci.Candidate += fmt.Sprintf(" network-cost %d", p.networkCost)
 	trickle := ToProtoTrickle(ci)
 	trickle.Target = target
 	_ = p.writeMessage(&livekit.SignalResponse{