The IETF WG on Differentiated Services (diff-serv) has been meeting now for over a year now and has made excellent progress. The framework, architecture, DS Field definition, and format for traffic conditoners document, and most importantly , two PHB definitions have been specified. So what is the problem? The Integrated Services WG (int-serv) (with its related groups such as ISSLL and RSVP and others), has also defined a set of protocols and mechanisms for enhanced services in the Internet. The difference between the int-serv and diff-serv approaches is that int-serv (at least the Guaranteed and Controlled Load services) is amenable to analysis. Effective bandwidth calcualations, and the thesis by Parekh shows how the admission tests for leaky bucket characterised traffic and delay bound for the path can be calculated. The difficulty with IS has been in designing implementations that scale (hence a great deal of work in state aggregation in RSVP, and in fast generalized port specification classifiers, as well asĀ  the work on efficient queue data structures and insert/retrieve algorithms, such as WF2Q and approximations such as SFQ). Indeed, there are network QoS calculi (by Rene Cruz and also by Jean-Yves le Boudec). Meanwhile, with diff-serv, the difference is that there can be no obvious analytic theory of diff-serv. A path can be constructed out of a sequence of hops each with a PHB and some associated SLA. However, the service (and provisioning of service) necessarily depend on the actual network topology and traffic conditions that prevail. This is a positive aspect of diff-serv, since it gives providers (and router vendors) a lot of design freedom in how they deploy actual services (and associated tarrif structures). However, to evaluate a diff-serv PHB is now a complex task, and requires simulation or measurement. To date, only modest simulations and measurements have been carried out. [Aside: of course the exact same argument applies to figuring out call blocking probabilities in int-serv type networks]