H:\webrtc-20210315\webrtc-20210315\webrtc\webrtc-checkout\src\p2p\base\p2p_transport_channel.cc
你只能裁你同一个network上的connection,多主机上每个network最终都决胜一个最优的connection,最终的网路调整就在每个网卡的最优connection切换。

1. 对于Controlling端，每次更新UpdateState后(Connection状态发生变动必然会触发UpdateState)，就会面临裁剪.
2. 对于Controlled端，只有当选出了Selected_Connection后，并nominated后才会进行裁剪.

https://zhuanlan.zhihu.com/p/443154955

SortConnectionsAndUpdateState

// Sort the available connections to find the best one.  We also monitor
// the number of available connections and the current state.
void P2PTransportChannel::SortConnectionsAndUpdateState(
    IceControllerEvent reason_to_sort) {
  RTC_DCHECK_RUN_ON(network_thread_);
  // Make sure the connection states are up-to-date since this affects how they
  // will be sorted.
  UpdateConnectionStates();
  // Any changes after this point will require a re-sort.
  sort_dirty_ = false;
  // If necessary, switch to the new choice. Note that |top_connection| doesn't
  // have to be writable to become the selected connection although it will
  // have higher priority if it is writable.
  MaybeSwitchSelectedConnection(
      reason_to_sort, ice_controller_->SortAndSwitchConnection(reason_to_sort));
  // The controlled side can prune only if the selected connection has been
  // nominated because otherwise it may prune the connection that will be
  // selected by the controlling side.
  // TODO(honghaiz): This is not enough to prevent a connection from being
  // pruned too early because with aggressive nomination, the controlling side
  // will nominate every connection until it becomes writable.
  if (ice_role_ == ICEROLE_CONTROLLING ||
      (selected_connection_ && selected_connection_->nominated())) {
    PruneConnections();
  }
  // Check if all connections are timedout.
  bool all_connections_timedout = true;
  for (const Connection* conn : connections()) {
    if (conn->write_state() != Connection::STATE_WRITE_TIMEOUT) {
      all_connections_timedout = false;
      break;
    }
  }
  // Now update the writable state of the channel with the information we have
  // so far.
  if (all_connections_timedout) {
    HandleAllTimedOut();
  }
  // Update the state of this channel.
  UpdateState();
  // Also possibly start pinging.
  // We could start pinging if:
  // * The first connection was created.
  // * ICE credentials were provided.
  // * A TCP connection became connected.
  MaybeStartPinging();
}

P2PTransportChannel::PruneConnections

void P2PTransportChannel::PruneConnections() {
  RTC_DCHECK_RUN_ON(network_thread_);
  std::vector<const Connection*> connections_to_prune =
      ice_controller_->PruneConnections();
  for (const Connection* conn : connections_to_prune) {
    FromIceController(conn)->Prune();
  }
}

PruneConnections

H:\webrtc-20210315\webrtc-20210315\webrtc\webrtc-checkout\src\p2p\base\basic_ice_controller.cc
获取可以裁剪的connections

std::vector<const Connection*> BasicIceController::PruneConnections() {
  // We can prune any connection for which there is a connected, writable
  // connection on the same network with better or equal priority.  We leave
  // those with better priority just in case they become writable later (at
  // which point, we would prune out the current selected connection).  We leave
  // connections on other networks because they may not be using the same
  // resources and they may represent very distinct paths over which we can
  // switch. If |best_conn_on_network| is not connected, we may be reconnecting
  // a TCP connection and should not prune connections in this network.
  // See the big comment in CompareConnectionStates.
  //
  // An exception is made for connections on an "any address" network, meaning
  // not bound to any specific network interface. We don't want to keep one of
  // these alive as a backup, since it could be using the same network
  // interface as the higher-priority, selected candidate pair.
  std::vector<const Connection*> connections_to_prune;
  auto best_connection_by_network = GetBestConnectionByNetwork();
  for (const Connection* conn : connections_) {
    const Connection* best_conn = selected_connection_;
    if (!rtc::IPIsAny(conn->network()->ip())) {
      // If the connection is bound to a specific network interface (not an
      // "any address" network), compare it against the best connection for
      // that network interface rather than the best connection overall. This
      // ensures that at least one connection per network will be left
      // unpruned.
      best_conn = best_connection_by_network[conn->network()];
    }
    // Do not prune connections if the connection being compared against is
    // weak. Otherwise, it may delete connections prematurely.
    if (best_conn && conn != best_conn && !best_conn->weak() &&
        CompareConnectionCandidates(best_conn, conn) >= 0) {
      connections_to_prune.push_back(conn);
    }
  }
  return connections_to_prune;
}

BasicIceController::GetBestConnectionByNetwork

遍历所有连接的connections_，获取每个网络的最优connection。

std::map<const rtc::Network*, const Connection*>
BasicIceController::GetBestConnectionByNetwork() const {
  // |connections_| has been sorted, so the first one in the list on a given
  // network is the best connection on the network, except that the selected
  // connection is always the best connection on the network.
  std::map<const rtc::Network*, const Connection*> best_connection_by_network;
  if (selected_connection_) {
    best_connection_by_network[selected_connection_->network()] =
        selected_connection_;
  }
  // TODO(honghaiz): Need to update this if |connections_| are not sorted.
  for (const Connection* conn : connections_) {
    const rtc::Network* network = conn->network();
    // This only inserts when the network does not exist in the map.
    best_connection_by_network.insert(std::make_pair(network, conn));
  }
  return best_connection_by_network;
}

Connection::Prune

此时，裁剪的connection已经被标记了，然后等待这些资源被释放掉。