Fix active tcp port to 9

webrtc/p2p/base/tcpport.cc

Index: webrtc/p2p/base/tcpport.cc
diff --git a/webrtc/p2p/base/tcpport.cc b/webrtc/p2p/base/tcpport.cc
index 2590d0aca893610551ab9af824701361afb27371..721acc2007035eb688d9a7aa072fb43475ea8477 100644
--- a/webrtc/p2p/base/tcpport.cc
+++ b/webrtc/p2p/base/tcpport.cc
@@ -184,10 +184,13 @@ void TCPPort::PrepareAddress() {
   } else {
     LOG_J(LS_INFO, this) << "Not listening due to firewall restrictions.";
     // Note: We still add the address, since otherwise the remote side won't
-    // recognize our incoming TCP connections.
-    AddAddress(rtc::SocketAddress(ip(), 0), rtc::SocketAddress(ip(), 0),
-               rtc::SocketAddress(), TCP_PROTOCOL_NAME, "", TCPTYPE_ACTIVE_STR,
-               LOCAL_PORT_TYPE, ICE_TYPE_PREFERENCE_HOST_TCP, 0, true);
+    // recognize our incoming TCP connections. According to
+    // https://tools.ietf.org/html/rfc6544#section-4.5, for active candidate,
+    // the port must be set to the discard port, i.e. 9.
+    AddAddress(rtc::SocketAddress(ip(), DISCARD_PORT),
+               rtc::SocketAddress(ip(), 0), rtc::SocketAddress(),
+               TCP_PROTOCOL_NAME, "", TCPTYPE_ACTIVE_STR, LOCAL_PORT_TYPE,
+               ICE_TYPE_PREFERENCE_HOST_TCP, 0, true);
   }
 }