Create a ready-to-transmit authentication request packet.
        Return a RADIUS packet which can be directly transmitted
        to a RADIUS server.

        :return: raw packet
        :rtype:  string

def RequestPacket(self):
        """Create a ready-to-transmit authentication request packet.
        Return a RADIUS packet which can be directly transmitted
        to a RADIUS server.

        :return: raw packet
        :rtype:  string
        """

        attr = self._PktEncodeAttributes()

        if self.id is None:
            self.id = self.CreateID()

        header = struct.pack('!BBH', self.code, self.id, (20 + len(attr)))
        self.authenticator = md5_constructor(header[0:4] + 16 * six.b('\x00') + attr
            + self.secret).digest()
        return header + self.authenticator + attr