连通性检测过程

P2PTransportChannel::SortConnectionsAndUpdateState

P2PTransportChannel::MaybeStartPinging

P2PTransportChannel::CheckAndPing

BasicIceController::SelectConnectionToPing

H:\webrtc-20210315\webrtc-20210315\webrtc\webrtc-checkout\src\p2p\base\basic_ice_controller.cc

BasicIceController::FindNextPingableConnection

查找下一个可以ping的连接。

// Returns the next pingable connection to ping.
const Connection* BasicIceController::FindNextPingableConnection() {
  int64_t now = rtc::TimeMillis();
  // Rule 1: Selected connection takes priority over non-selected ones.
  if (selected_connection_ && selected_connection_->connected() &&
      selected_connection_->writable() &&
      WritableConnectionPastPingInterval(selected_connection_, now)) {
    return selected_connection_;
  }
  // Rule 2: If the channel is weak, we need to find a new writable and
  // receiving connection, probably on a different network. If there are lots of
  // connections, it may take several seconds between two pings for every
  // non-selected connection. This will cause the receiving state of those
  // connections to be false, and thus they won't be selected. This is
  // problematic for network fail-over. We want to make sure at least one
  // connection per network is pinged frequently enough in order for it to be
  // selectable. So we prioritize one connection per network.
  // Rule 2.1: Among such connections, pick the one with the earliest
  // last-ping-sent time.
  if (weak()) {
    std::vector<const Connection*> pingable_selectable_connections;
    absl::c_copy_if(GetBestWritableConnectionPerNetwork(),
                    std::back_inserter(pingable_selectable_connections),
                    [this, now](const Connection* conn) {
                      return WritableConnectionPastPingInterval(conn, now);
                    });
    auto iter = absl::c_min_element(
        pingable_selectable_connections,
        [](const Connection* conn1, const Connection* conn2) {
          return conn1->last_ping_sent() < conn2->last_ping_sent();
        });
    if (iter != pingable_selectable_connections.end()) {
      return *iter;
    }
  }
  // Rule 3: Triggered checks have priority over non-triggered connections.
  // Rule 3.1: Among triggered checks, oldest takes precedence.
  const Connection* oldest_triggered_check =
      FindOldestConnectionNeedingTriggeredCheck(now);
  if (oldest_triggered_check) {
    return oldest_triggered_check;
  }
  // Rule 4: Unpinged connections have priority over pinged ones.
  RTC_CHECK(connections_.size() ==
            pinged_connections_.size() + unpinged_connections_.size());
  // If there are unpinged and pingable connections, only ping those.
  // Otherwise, treat everything as unpinged.
  // TODO(honghaiz): Instead of adding two separate vectors, we can add a state
  // "pinged" to filter out unpinged connections.
  if (absl::c_none_of(unpinged_connections_,
                      [this, now](const Connection* conn) {
                        return this->IsPingable(conn, now);
                      })) {
    unpinged_connections_.insert(pinged_connections_.begin(),
                                 pinged_connections_.end());
    pinged_connections_.clear();
  }
  // Among un-pinged pingable connections, "more pingable" takes precedence.
  std::vector<const Connection*> pingable_connections;
  absl::c_copy_if(
      unpinged_connections_, std::back_inserter(pingable_connections),
      [this, now](const Connection* conn) { return IsPingable(conn, now); });
  auto iter = absl::c_max_element(
      pingable_connections,
      [this](const Connection* conn1, const Connection* conn2) {
        // Some implementations of max_element
        // compare an element with itself.
        if (conn1 == conn2) {
          return false;
        }
        return MorePingable(conn1, conn2) == conn2;
      });
  if (iter != pingable_connections.end()) {
    return *iter;
  }
  return nullptr;
}

P2PTransportChannel::PingConnection

当前角色是控制方，才会去判断是否提名，和使用candidate属性。

Connection::Ping

SentPing结构

nomination为0，不提名；大于0，提名