code: func (s *Stack) NetworkProtocolOption(network tcpip.NetworkProtocolNumber, option interface{}) *tcpip.Error {
  	netProto, ok := s.networkProtocols[network]
  	if !ok {
  		return tcpip.ErrUnknownProtocol
  	}
  	return netProto.Option(option)
  }
docs: // NetworkProtocolOption allows retrieving individual protocol level option
  // values. This method returns an error if the protocol is not supported or
  // option is not supported by the protocol implementation.
  // e.g.
  // var v ipv4.MyOption
  // err := s.NetworkProtocolOption(tcpip.IPv4ProtocolNumber, &v)
  // if err != nil {
  //   ...
  // }