tcpnal:

tcpnal:
  nal thread will call select() to block itself waiting incoming packets,
  but in 2 cases (1. new connection created 2. shutdown tcpnal) upper
  thread need wake up nal thread from sleep immediately.
  here we use a local socket which will be under select's monitoring to
  notify nal thread wakeup. again brings in unclean code into tcpnal.