def _send_start_ok(self, frame_in):
        """Send Start OK frame.

        :param specification.Connection.Start frame_in: Amqp frame.
        :return:
        """
        mechanisms = try_utf8_decode(frame_in.mechanisms)
        if 'EXTERNAL' in mechanisms:
            mechanism = 'EXTERNAL'
            credentials = '\0\0'
        elif 'PLAIN' in mechanisms:
            mechanism = 'PLAIN'
            credentials = self._plain_credentials()
        else:
            exception = AMQPConnectionError(
                'Unsupported Security Mechanism(s): %s' %
                frame_in.mechanisms
            )
            self._connection.exceptions.append(exception)
            return
        start_ok_frame = specification.Connection.StartOk(
            mechanism=mechanism,
            client_properties=self._client_properties(),
            response=credentials,
            locale=LOCALE
        )
        self._write_frame(start_ok_frame)
Send Start OK frame.

        :param specification.Connection.Start frame_in: Amqp frame.
        :return: