QoS control requires some number of alternate queues, as well as some form of admission, policing and possibly shaping.
Assuming that admission and policing can be done on small numbers of flows at the ``edge'' (ingress) of the network, then we can police (and shape) aggregate flows as they approach the core of the network (recent results from Roch Guerin's group at TJ Watson show that this in fact gets higher utilisation). Then the problem is that of the performance of queue insertion.