Do not switch best connection on the controlled side too frequently

webrtc/p2p/base/p2ptransportchannel.cc

Index: webrtc/p2p/base/p2ptransportchannel.cc
diff --git a/webrtc/p2p/base/p2ptransportchannel.cc b/webrtc/p2p/base/p2ptransportchannel.cc
index fc1f0a4e6769ee96ce28f851d0f7cbc368fbf69c..7b5e1e70e9b6af1c51c35f7d02b79ec5354e13d9 100644
--- a/webrtc/p2p/base/p2ptransportchannel.cc
+++ b/webrtc/p2p/base/p2ptransportchannel.cc
@@ -200,6 +200,39 @@ bool ShouldSwitch(cricket::Connection* a_conn,
   return b_conn->rtt() <= a_conn->rtt() + kMinImprovement;
 }
 
+// Determines whether we should switch the best connection to |new_connection|
+// based the writable state, the last nominated connection, and the last
+// connection receiving data. This is to prevent that the controlled side may
+// switch the best connection too frequently when the controlling side is doing
+// aggressive nominations. With this implementation, the controlled side will
+// switch the best connection if the new nominated connection is writable, and
+// i) the best connection is nullptr, or
+// ii) the best connection was not nominated, or
+// iii) the new connection has been nominated twice consecutively, or
+// iv) the new connection is nominated and is receiving data, or
+// v) the new connection has lower cost or high priority.
+// TODO(honghaiz): Stop the aggressive nomination on the controlling side and
+// implement the ice-renomination option.
+bool ShouldSwitchOnNominatedOrDataReceived(
+    cricket::Connection* best_connection,
+    cricket::Connection* new_connection,
+    cricket::Connection* last_nominated,
+    cricket::Connection* last_data_received) {
+  RTC_CHECK(new_connection != nullptr);
+  if (!new_connection->writable()) {
+    return false;
+  }
+  if (best_connection == nullptr || !best_connection->nominated()) {
+    return true;
+  }
+  if (new_connection == last_nominated ||
+      new_connection == last_data_received) {
+    return true;
+  }
+  // Compare the network cost and priority.
+  return CompareConnectionCandidates(best_connection, new_connection) < 0;
+}
+
 }  // unnamed namespace
 
 namespace cricket {
@@ -238,8 +271,6 @@ P2PTransportChannel::P2PTransportChannel(const std::string& transport_name,
       worker_thread_(rtc::Thread::Current()),
       incoming_only_(false),
       error_(0),
-      best_connection_(NULL),
-      pending_best_connection_(NULL),
       sort_dirty_(false),
       remote_ice_mode_(ICEMODE_FULL),
       ice_role_(ICEROLE_UNKNOWN),
@@ -729,20 +760,25 @@ void P2PTransportChannel::OnNominated(Connection* conn) {
   ASSERT(worker_thread_ == rtc::Thread::Current());
   ASSERT(ice_role_ == ICEROLE_CONTROLLED);
 
-  if (conn->write_state() == Connection::STATE_WRITABLE) {
-    if (best_connection_ != conn) {
-      pending_best_connection_ = NULL;
-      LOG(LS_INFO) << "Switching best connection on controlled side: "
-                   << conn->ToString();
-      SwitchBestConnectionTo(conn);
-      // Now we have selected the best connection, time to prune other existing
-      // connections and update the read/write state of the channel.
-      RequestSort();
-    }
+  if (best_connection_ == conn) {
+    return;
+  }
+
+  if (ShouldSwitchOnNominatedOrDataReceived(best_connection_, conn,
+                                            last_nominated_conn_,
+                                            last_data_received_conn_)) {
+    last_nominated_conn_ = nullptr;
+    last_data_received_conn_ = nullptr;
+    LOG(LS_INFO) << "Switching best connection on controlled side: "
+                 << conn->ToString();
+    SwitchBestConnectionTo(conn);
+    // Now we have selected the best connection, time to prune other existing
+    // connections and update the read/write state of the channel.
+    RequestSort();
   } else {
-    LOG(LS_INFO) << "Not switching the best connection on controlled side yet,"
-                 << " because it's not writable: " << conn->ToString();
-    pending_best_connection_ = conn;
+    LOG(LS_INFO) << "Not switching the best connection on controlled side yet: "
+                 << conn->ToString();
+    last_nominated_conn_ = conn;
   }
 }
 
@@ -1444,9 +1480,18 @@ void P2PTransportChannel::OnConnectionStateChange(Connection* connection) {
 
   // Update the best connection if the state change is from pending best
   // connection and role is controlled.
-  if (ice_role_ == ICEROLE_CONTROLLED) {
-    if (connection == pending_best_connection_ && connection->writable()) {
-      pending_best_connection_ = NULL;
+  if (ice_role_ == ICEROLE_CONTROLLED && connection == last_nominated_conn_) {
+    // This essentially requires that the connection is both last nominated
+    // and receiving data, or the best connection is nullptr, in order
+    // to switch to |connection|.
+    if (connection == best_connection_) {
+      // If it is already the best connection, reset the last_nominated_conn_
+      last_nominated_conn_ = nullptr;
+    } else if (ShouldSwitchOnNominatedOrDataReceived(
+                   best_connection_, connection, nullptr,
+                   last_data_received_conn_)) {
+      last_nominated_conn_ = nullptr;
+      last_data_received_conn_ = nullptr;
       LOG(LS_INFO) << "Switching best connection on controlled side"
                    << " because it's now writable: " << connection->ToString();
       SwitchBestConnectionTo(connection);
@@ -1489,8 +1534,8 @@ void P2PTransportChannel::OnConnectionDestroyed(Connection* connection) {
   LOG_J(LS_INFO, this) << "Removed connection ("
     << static_cast<int>(connections_.size()) << " remaining)";
 
-  if (pending_best_connection_ == connection) {
-    pending_best_connection_ = NULL;
+  if (last_nominated_conn_ == connection) {
+    last_nominated_conn_ = nullptr;
   }
 
   // If this is currently the best connection, then we need to pick a new one.
@@ -1559,9 +1604,17 @@ void P2PTransportChannel::OnReadPacket(Connection* connection,
 
   // May need to switch the sending connection based on the receiving media path
   // if this is the controlled side.
-  if (ice_role_ == ICEROLE_CONTROLLED && !best_nominated_connection() &&
-      connection->writable() && best_connection_ != connection) {
+  if (ice_role_ != ICEROLE_CONTROLLED || best_connection_ == connection) {
+    return;
+  }
+
+  if (ShouldSwitchOnNominatedOrDataReceived(best_connection_, connection,
+                                            last_nominated_conn_, nullptr)) {
     SwitchBestConnectionTo(connection);
+    last_data_received_conn_ = nullptr;
+    last_nominated_conn_ = nullptr;
+  } else {
+    last_data_received_conn_ = connection;
   }
 }