docs: def DESCRIBE(self):
          """Request description of what services RTSP server make available."""
          message = "DESCRIBE " + self.session.url + " RTSP/1.0\r\n"
          message += self.sequence
          message += self.authentication
          message += self.user_agent
          message += "Accept: application/sdp\r\n"
          message += '\r\n'
          return message