def PLAY(self):
        """RTSP session is ready to send data."""
        message = "PLAY " + self.session.url + " RTSP/1.0\r\n"
        message += self.sequence
        message += self.authentication
        message += self.user_agent
        message += self.session_id
        message += '\r\n'
        return message
RTSP session is ready to send data.