def METHOD_NAME(self):
    """
    Start SASL authentication exchange.
    """
    self.setMechanism()
    self._deferred = defer.Deferred()
    self.xmlstream.addObserver("/challenge", self.onChallenge)
    self.xmlstream.addOnetimeObserver("/success", self.onSuccess)
    self.xmlstream.addOnetimeObserver("/failure", self.onFailure)
    self.sendAuth(self.mechanism.getInitialResponse())
    return self._deferred