code: def _send_start_ok(self, frame_in):
          
          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)
docs: Send Start OK frame.
  
          :param specification.Connection.Start frame_in: Amqp frame.
          :return: