Adding IceConfig option to assume TURN/TURN candidate pairs will work.

webrtc/p2p/base/port.h

Index: webrtc/p2p/base/port.h
diff --git a/webrtc/p2p/base/port.h b/webrtc/p2p/base/port.h
index 06efd2d3c235c4e976fda6f610c187741ac7542f..8e8a2e09c31ede962b77581c8701e21953958232 100644
--- a/webrtc/p2p/base/port.h
+++ b/webrtc/p2p/base/port.h
@@ -178,7 +178,7 @@ class Port : public PortInterface, public rtc::MessageHandler,
   }
 
   // Identifies the generation that this port was created in.
-  uint32_t generation() { return generation_; }
+  uint32_t generation() const { return generation_; }
   void set_generation(uint32_t generation) { generation_ = generation; }
 
   const std::string username_fragment() const;
@@ -435,7 +435,7 @@ class Connection : public CandidatePairInterface,
   const Port* port() const { return port_; }
 
   // Implementation of virtual methods in CandidatePairInterface.
-  // Returns the description of the local port
+  // Returns the description of the local port.
   virtual const Candidate& local_candidate() const;
   // Returns the description of the remote port to which we communicate.
   virtual const Candidate& remote_candidate() const;
@@ -444,14 +444,22 @@ class Connection : public CandidatePairInterface,
   uint64_t priority() const;
 
   enum WriteState {
-    STATE_WRITABLE          = 0,  // we have received ping responses recently
-    STATE_WRITE_UNRELIABLE  = 1,  // we have had a few ping failures
-    STATE_WRITE_INIT        = 2,  // we have yet to receive a ping response
-    STATE_WRITE_TIMEOUT     = 3,  // we have had a large number of ping failures
+    STATE_WRITABLE = 0,           // We have received ping responses recently.
+    STATE_PRESUMED_WRITABLE = 1,  // We haven't received a ping response, but
+                                  // we presume we're writable, because we're a
+                                  // TURN-TURN connection and CreatePermission
+                                  // isn't required.
+    STATE_WRITE_UNRELIABLE = 2,   // We have had a few ping failures.
+    STATE_WRITE_INIT = 3,         // We have yet to receive a ping response.
+    STATE_WRITE_TIMEOUT = 4,  // We have had a large number of ping failures.
   };
 
   WriteState write_state() const { return write_state_; }
   bool writable() const { return write_state_ == STATE_WRITABLE; }
+  bool presumed_writable() const {

[honghaiz3, 2016/06/17 01:55:03]

When do we need to differentiate presumed_writable vs writable ?
Does it make sense just to let writable() return 
   write_state_ == STATE_WRITABLE ||
           write_state_ == STATE_PRESUMED_WRITABLE;

[Taylor Brandstetter, 2016/06/20 17:15:45]

There are a few notable places. For example, the logic for selecting a
connection to send a ping on. I assume if a connection is "presumed writable",
we should treat it as unwritable for the purpose of sending pings. Because if it
turns out that the "presumed writable" connection *isn't* writable, we want to
find out as soon as possible.

[honghaiz3, 2016/06/20 22:31:03]

If I understand correctly, that is the only place we may want a different
writable() method. I hope we could avoid two writable(writable and
presumed_writable) methods. From the p2ptransportchannel's perspective, when it
asks a connection whether it is writable, it should always return a single
result (basically PRESUMED_WRITABLE or WRITABLE). In the rare case (for stun
ping), can we just directly check the write_state() instead of having another
writable method if it is only called once or twice? 

[pthatcher1, 2016/06/21 06:20:50]

The problem is that if someone calls writable() and doesn't know the difference,
they might be surprised.  If we choose to make writable() == WRITABLE ||
PRESUMED_WRITABLE, then they might be surprised that it's writable when it's
only presumably writable.  If we choose to make writable() == WRITABLE, they
might be surprised that it's not writable when it's presumably writable.

There are 15 callers.  Perhaps we should remove writable() and make each one
check write_state() so that there is never ambiguity.

It would make it easier if we reversed the order of the enum so that a caller
could do this:

"if write_state() >= STATE_PRESUMED_WRITABLE" instead of "if write_state() ==
STATE_WRITABLE || write_state() == STATE_PRESUMED_WRITABLE"

Either that or we could remove writable() in favor of presumed_writable() and
certainly_writable().  But that seems a bit too much.

[Taylor Brandstetter, 2016/06/22 00:37:07]

My reasoning:

1. I don't want to change the meaning of writable(). I don't think anyone should
be surprised if a connection is not writable() before it's received a ping
response, because that's the way it's always worked.
2. I want to avoid checking write_state() directly from P2PTransportChannel
because that creates more fragile code (if we ever were to add a new state).

+    return write_state_ == STATE_WRITABLE ||
+           write_state_ == STATE_PRESUMED_WRITABLE;
+  }
   bool receiving() const { return receiving_; }
 
   // Determines whether the connection has finished connecting.  This can only
@@ -589,7 +597,10 @@ class Connection : public CandidatePairInterface,
   enum { MSG_DELETE = 0, MSG_FIRST_AVAILABLE };
 
   // Constructs a new connection to the given remote port.
-  Connection(Port* port, size_t index, const Candidate& candidate);
+  Connection(Port* port,
+             size_t index,
+             const Candidate& candidate,
+             const IceConfig& config);
 
   // Called back when StunRequestManager has a stun packet to send
   void OnSendStunPacket(const void* data, size_t size, StunRequest* req);
@@ -602,6 +613,10 @@ class Connection : public CandidatePairInterface,
   void OnConnectionRequestTimeout(ConnectionRequest* req);
   void OnConnectionRequestSent(ConnectionRequest* req);
 
+  // May be STATE_WRITE_INIT or STATE_PRESUMED_WRITABLE depending on ICE
+  // config and candidate types.
+  WriteState InitialWriteState() const;
+
   // Changes the state and signals if necessary.
   void set_write_state(WriteState value);
   void set_receiving(bool value);
@@ -650,6 +665,7 @@ class Connection : public CandidatePairInterface,
   int receiving_timeout_;
   int64_t time_created_ms_;
   int num_pings_sent_ = 0;
+  bool fully_relayed_createpermission_needed_;
 
   friend class Port;
   friend class ConnectionRequest;
@@ -658,7 +674,10 @@ class Connection : public CandidatePairInterface,
 // ProxyConnection defers all the interesting work to the port.
 class ProxyConnection : public Connection {
  public:
-  ProxyConnection(Port* port, size_t index, const Candidate& remote_candidate);
+  ProxyConnection(Port* port,
+                  size_t index,
+                  const Candidate& remote_candidate,
+                  const IceConfig& config);
 
   int Send(const void* data,
            size_t size,