Handles NTPv4 extensions and MAC part (when authentication is used.)

def guess_payload_class(self, payload):
        """
        Handles NTPv4 extensions and MAC part (when authentication is used.)
        """
        plen = len(payload)

        if plen > _NTP_AUTH_MD5_TAIL_SIZE:
            return NTPExtensions
        elif plen == _NTP_AUTH_MD5_TAIL_SIZE:
            return NTPAuthenticator

        return Packet.guess_payload_class(self, payload)