From 8680cd591fd152081d91cd554fe0890fbf91641c Mon Sep 17 00:00:00 2001
From: =?UTF-8?q?Jos=C3=A9=20M=C3=A9lan=C3=A7on?= <jmelancon@genetec.com>
Date: Wed, 22 Nov 2023 07:47:32 -0500
Subject: [PATCH] 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 | 17 +++++++++++++++++
 gather.go          |  4 ++--
 ice.go             |  5 +++++
 3 files changed, 24 insertions(+), 2 deletions(-)

diff --git a/candidate_relay.go b/candidate_relay.go
index 449d077..fa5297b 100644
--- a/candidate_relay.go
+++ b/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
diff --git a/gather.go b/gather.go
index 931d368..507b7fc 100644
--- a/gather.go
+++ b/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)
diff --git a/ice.go b/ice.go
index bd55120..73262dd 100644
--- a/ice.go
+++ b/ice.go
@@ -83,3 +83,8 @@ func (t GatheringState) String() string {
 		return ErrUnknownType.Error()
 	}
 }
+
+const (
+	relayProtocolDTLS = "dtls"
+	relayProtocolTLS  = "tls"
+)