code: @Override
    public Connection<T> newConnection(final Identifier destId) {
      final Connection<T> connection = connectionMap.get(destId);
  
      if (connection == null) {
        final Connection<T> newConnection = new NetworkConnection<>(this, destId);
        connectionMap.putIfAbsent(destId, newConnection);
        return connectionMap.get(destId);
      }
      return connection;
    }
docs: Creates a new connection.
  @param destId a destination identifier of NetworkConnectionService.