async def _sasl_start(self, mechanism):
        """ Initiate SASL authentication. """
        # The rest will be handled in on_raw_authenticate()/_sasl_respond().
        await self.rawmsg('AUTHENTICATE', mechanism)
        # create a partial, required for our callback to get the kwarg
        _sasl_partial = partial(self._sasl_abort, timeout=True)
        self._sasl_timer = self.eventloop.call_later(self.SASL_TIMEOUT, _sasl_partial)
Initiate SASL authentication.