1、调用逻辑

首先看穿透之前的ping（stun bindingrequest）的调用流程

穿透时的4次握手.jpg

2、具体方法分析

看图中OnCheckAndPing的代码

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;
  if (rtc::Time64() >= 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);
}

UpdateConnectionStates就是对connection的状态做出更新，这里先暂时不管。

2.1,ping的频率的设定

来看

//p2ptransportchannel.cc 212L
// for pinging.  When the socket is writable, we will use only 1 Kbps because
// we don't want to degrade the quality on a modem.  These numbers should work
// well on a 28.8K modem, which is the slowest connection on which the voice
// quality is reasonable at all.
static const int PING_PACKET_SIZE = 60 * 8;
// STRONG_PING_INTERVAL (480ms) is applied when the best connection is both
// writable and receiving.
static const int STRONG_PING_INTERVAL = 1000 * PING_PACKET_SIZE / 1000;
// WEAK_PING_INTERVAL (48ms) is applied when the best connection is either not
// writable or not receiving.
const int WEAK_PING_INTERVAL = 1000 * PING_PACKET_SIZE / 10000;

 // 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;

上述方法的目的是确定ping的频率，在best_connection不可读，不可写的情况下，使用较快频率的ping，在best_connection可读，可写时使用较慢频率的ping。

2.2、FindNextPingableConnection，寻找下一个需要ping的connection

// Returns the next pingable connection to ping.  This will be the oldest
// pingable connection unless we have a connected, writable connection that is
// past the maximum acceptable ping interval. When reconnecting a TCP
// connection, the best connection is disconnected, although still WRITABLE
// while reconnecting. The newly created connection should be selected as the
// ping target to become writable instead. See the big comment in
// CompareConnections.
Connection* P2PTransportChannel::FindNextPingableConnection() {
  int64_t now = rtc::Time64();
  Connection* conn_to_ping = nullptr;
  if (best_connection_ && best_connection_->connected() &&
      best_connection_->writable() &&
      (best_connection_->last_ping_sent() + config_.max_strong_interval <=
       now)) {
    conn_to_ping = best_connection_;
  } else {
    conn_to_ping = FindConnectionToPing(now);
  }
  return conn_to_ping;
}

对该方法的源码进行详细分析，可以得出webrtc选择一个connection来进行ping操作的流程是：