code: def pre_dissect(self, s):
          
          length = len(s)
          if length < _NTP_PACKET_MIN_SIZE:
              err = " ({}".format(length) + " is < _NTP_PACKET_MIN_SIZE "
              err += "({})).".format(_NTP_PACKET_MIN_SIZE)
              raise _NTPInvalidDataException(err)
          return s
docs: Check that the payload is long enough to build a NTP packet.