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

webrtc/p2p/base/p2ptransportchannel.h

 /*
  *  Copyright 2004 The WebRTC Project Authors. All rights reserved.
  *
  *  Use of this source code is governed by a BSD-style license
  *  that can be found in the LICENSE file in the root of the source
  *  tree. An additional intellectual property rights grant can be found
  *  in the file PATENTS.  All contributing project authors may
  *  be found in the AUTHORS file in the root of the source tree.
  */
 
@@ skipping 84 matching lines @@
   void MaybeStartGathering() override;
   IceGatheringState gathering_state() const override {
     return gathering_state_;
   }
   void AddRemoteCandidate(const Candidate& candidate) override;
   void RemoveRemoteCandidate(const Candidate& candidate) override;
   // Sets the parameters in IceConfig. We do not set them blindly. Instead, we
   // only update the parameter if it is considered set in |config|. For example,
   // a negative value of receiving_timeout will be considered "not set" and we
   // will not use it to update the respective parameter in |config_|.
+  // TODO(deadbeef): Use rtc::Optional instead of negative values.
   void SetIceConfig(const IceConfig& config) override;
   const IceConfig& config() const;
 
   // From TransportChannel:
   int SendPacket(const char* data,
                  size_t len,
                  const rtc::PacketOptions& options,
                  int flags) override;
   int SetOption(rtc::Socket::Option opt, int value) override;
   bool GetOption(rtc::Socket::Option opt, int* value) override;
@@ skipping 85 matching lines @@
 
  private:
   rtc::Thread* thread() { return worker_thread_; }
   bool IsGettingPorts() { return allocator_session()->IsGettingPorts(); }
 
   // A transport channel is weak if the current best connection is either
   // not receiving or not writable, or if there is no best connection at all.
   bool weak() const;
   void UpdateConnectionStates();
   void RequestSort();
+
+  // Returns an expanded write state based on Connection::write_state and other
+  // factors. Lower values are preferable.
+  int MappedWriteState(const cricket::Connection* a) const;
+
+  // The methods below return a positive value if a is preferable to b,
+  // a negative value if b is preferable, and 0 if they're equally preferable.
+  int CompareConnectionStates(const cricket::Connection* a,
+                              const cricket::Connection* b) const;
+  int CompareConnectionCandidates(const cricket::Connection* a,
+                                  const cricket::Connection* b) const;
+  // Compares first on states, then on candidates, then on RTT.
+  int CompareConnections(const cricket::Connection* a,
+                         const cricket::Connection* b) const;
+
+  bool ShouldSwitch(const cricket::Connection* a,
+                    const cricket::Connection* b) const;
   void SortConnections();
   void SwitchBestConnectionTo(Connection* conn);
   void UpdateState();
   void HandleAllTimedOut();
   void MaybeStopPortAllocatorSessions();
   TransportChannelState ComputeState() const;
 
   Connection* GetBestConnectionOnNetwork(rtc::Network* network) const;
   bool CreateConnections(const Candidate& remote_candidate,
                          PortInterface* origin_port);
@@ skipping 112 matching lines @@
   TransportChannelState state_ = TransportChannelState::STATE_INIT;
   IceConfig config_;
   int last_sent_packet_id_ = -1;  // -1 indicates no packet was sent before.
 
   RTC_DISALLOW_COPY_AND_ASSIGN(P2PTransportChannel);
 };
 
 }  // namespace cricket
 
 #endif  // WEBRTC_P2P_BASE_P2PTRANSPORTCHANNEL_H_