':class:`QoS` manager for this channel.
Lazily instantiates an object of type :class:`QoS` upon access to
the self.qos attribute.
:return: An already existing, or newly created QoS object
:rtype: :class:`QoS`'

@property
def qos(self):

    if (self._qos is None):
        self._qos = self.QoS(self.transport.session)
    return self._qos
