code: func (s *Stack) TransportProtocolOption(transport tcpip.TransportProtocolNumber, option interface{}) *tcpip.Error {
  	transProtoState, ok := s.transportProtocols[transport]
  	if !ok {
  		return tcpip.ErrUnknownProtocol
  	}
  	return transProtoState.proto.Option(option)
  }
docs: // TransportProtocolOption 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.
  // var v tcp.SACKEnabled
  // if err := s.TransportProtocolOption(tcpip.TCPProtocolNumber, &v); err != nil {
  //   ...
  // }