What other QoS schemes exist?
A. RSVP and MPLS are the two most common. Resource reSerVation Protocol is a scheme for getting positive router commitments to handle privileged traffic flow. This is done by issuing setup messages and gaining positive assurance of router capacity before traffic is initiated. RSVP affords all the benefits of diffserv and in addition can provide deterministic service. But RSVP is expensive in terms of scaleability — it adds state back in to an internet that we tried very hard, by inventing IP, to get state out of. RSVP tends to be brittle as well — if a set of RSVP reservations is interrupted by a router or connectivity failure, the “connection” must be reinitiated, much like redialing a failed circuit switched phone call. Consequently, RSVP is viewed as poorly scaleable and unlikely to see implementation beyond a few applications that need determinism.