Ping connections with the fast rate until each connection is pinged at least three times.

webrtc/p2p/base/p2ptransportchannel.cc

 /*
  *  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 1284 matching lines @@
 void P2PTransportChannel::OnSort() {
   // Resort the connections based on the new statistics.
   SortConnections();
 }
 
 // Handle queued up check-and-ping request
 void P2PTransportChannel::OnCheckAndPing() {
   // Make sure the states of the connections are up-to-date (since this affects
   // which ones are pingable).
   UpdateConnectionStates();
-  // When the best connection is either not receiving or not writable,
-  // switch to weak ping interval.
-  int ping_interval = weak() ? weak_ping_interval_ : STRONG_PING_INTERVAL;
+  // When the best connection is receiving and writable, and all connections
+  // have been pinged sufficiently, switch to strong (longer) ping interval.
+  bool all_connections_pinged_sufficiently = std::all_of(
+      connections_.begin(), connections_.end(), [](Connection* conn) {
+        return !conn->active() || conn->num_pings_sent() >= kMinNumPingsSent;
+      });
+  int ping_interval = (all_connections_pinged_sufficiently && !weak())
+                          ? STRONG_PING_INTERVAL

[pthatcher1, 2016/05/25 17:29:13]

This might be more clear as :

bool need_more_pings_at_weak_interval = std::any_of(...,
  return conn->active() && conn->num_pings_sent() <
MIN_PINGS_AT_WEAK_INTERVAL));

int ping_interval = (weak() || need_more_pings_at_weak_interval) ?
weak_ping_interval_ : STRONG_PING_INTERVAL;

[honghaiz3, 2016/05/25 18:40:02]

Done.

+                          : weak_ping_interval_;
   if (rtc::TimeMillis() >= last_ping_sent_ms_ + ping_interval) {
     Connection* conn = FindNextPingableConnection();
     if (conn) {
       PingConnection(conn);
       MarkConnectionPinged(conn);
     }
   }
   int delay = std::min(ping_interval, check_receiving_interval_);
   thread()->PostDelayed(delay, this, MSG_CHECK_AND_PING);
 }
@@ skipping 342 matching lines @@
 
   // During the initial state when nothing has been pinged yet, return the first
   // one in the ordered |connections_|.
   return *(std::find_if(connections_.begin(), connections_.end(),
                         [conn1, conn2](Connection* conn) {
                           return conn == conn1 || conn == conn2;
                         }));
 }
 
 }  // namespace cricket