Match libwebrtc's TURN protocol priority

Today, all relay candidates from Pion have the same priority.
This PR attempts to reproduce libwebrtc's behavior, where the
TURN servers candidates priority is based on the underlying
relay protocol. UDP are preferred over TCP, which are preferred
over the TLS options.

candidate_relay.go

@@ -70,6 +70,23 @@ func NewCandidateRelay(config *CandidateRelayConfig) (*CandidateRelay, error) {
 	}, nil
 }
 
+// LocalPreference returns the local preference for this candidate
+func (c *CandidateRelay) LocalPreference() uint16 {
+	// These preference values come from libwebrtc
+	// https://github.com/mozilla/libwebrtc/blob/1389c76d9c79839a2ca069df1db48aa3f2e6a1ac/p2p/base/turn_port.cc#L61
+	var relayPreference uint16
+	switch c.relayProtocol {
+	case relayProtocolTLS, relayProtocolDTLS:
+		relayPreference = 2
+	case tcp:
+		relayPreference = 1
+	default:
+		relayPreference = 0
+	}
+
+	return c.candidateBase.LocalPreference() + relayPreference
+}
+
 // RelayProtocol returns the protocol used between the endpoint and the relay server.
 func (c *CandidateRelay) RelayProtocol() string {
 	return c.relayProtocol

gather.go

@@ -632,7 +632,7 @@ func (a *Agent) gatherCandidatesRelay(ctx context.Context, urls []*stun.URI) { /
 
 				relAddr = conn.LocalAddr().(*net.UDPAddr).IP.String() //nolint:forcetypeassert
 				relPort = conn.LocalAddr().(*net.UDPAddr).Port        //nolint:forcetypeassert
-				relayProtocol = "dtls"
+				relayProtocol = relayProtocolDTLS
 				locConn = &fakenet.PacketConn{Conn: conn}
 			case url.Proto == stun.ProtoTypeTCP && url.Scheme == stun.SchemeTypeTURNS:
 				tcpAddr, resolvErr := a.net.ResolveTCPAddr(NetworkTypeTCP4.String(), turnServerAddr)
@@ -662,7 +662,7 @@ func (a *Agent) gatherCandidatesRelay(ctx context.Context, urls []*stun.URI) { /
 
 				relAddr = conn.LocalAddr().(*net.TCPAddr).IP.String() //nolint:forcetypeassert
 				relPort = conn.LocalAddr().(*net.TCPAddr).Port        //nolint:forcetypeassert
-				relayProtocol = "tls"
+				relayProtocol = relayProtocolTLS
 				locConn = turn.NewSTUNConn(conn)
 			default:
 				a.log.Warnf("Unable to handle URL in gatherCandidatesRelay %v", url)

ice.go

@@ -83,3 +83,8 @@ func (t GatheringState) String() string {
 		return ErrUnknownType.Error()
 	}
 }
+
+const (
+	relayProtocolDTLS = "dtls"
+	relayProtocolTLS  = "tls"
+)