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：
