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The ability to request and reserve per-flow resources makes it possible for the Intserv framework to provide QoS guarantees
to individual flows. As work on Intserv proceeded, howerver, researchers involved began to appreciate some of the difficulties
associated with the Intserv model and per-flow reservation of resources.
- Scalability-Per-flow resources reservation implies the need for a router to process resource reservations and
to maintain per-flow atate of each flow passing through the router.
- Flexible service models-The Intserv framwork lprovides for a small number of pre-specified service classes.
These considerations led to the so-called Diffserv activity within the Internet Engineering Task Force. The Diffserv
architecture aims to provide scalable and flexible service differentiation--that is, the ability to handle different "classes"
of traffic in differen ways within the Internet. The need for scalability arises from the fact that hundreds of thousands
of simultaneous source-destination traffic flows may be present at a backbone router of the Internet.
Differentiated Services: A Simple Scenario
To set the framwork for defining the architectural components of the differentiated service model.
The Diffserv sachitecture consists of two sets of functional elements:
- Edge functions: packet classification and traffic conditioning. At the incoming edge of the network arriving
pacekts are marked.
- Core function: forwarding. When a DS-marked packet arrives at a Diffserv-capable router, the packet is
forwarded onto its next hop according to the so-called per-hop behavior associated with that packet's class.
Per-Hop Behaviors
The second key component of the Diffserv architecture involves the per-hop behavior (PHB) performed
by Diffserv-capable routers. PHB is rather cryptically, but acrefully, defined as "a description of the externally observable
forwarding behavoir of a Diffserv node applied to a particular Diffserv behavior aggregate". Digging a llittle deeper
int this definition, we can see several important considerations embedded within it:
- A PHB can result in different classes of traffic receiving different performance.
- While a PHB defines differences in perfomance among classes, it does not mandate anu particular mechanism for achieving
these behaviors.
- Differences in performance must be observable and hance measurable.
- The expedited forwarding PHB specifies that the departure rate of a class of traffic from a router must
equal or exceed a configured rate.
- The assured forwarding PHB is more complex.
Intserv and Diffserv Retrospective
For the past 20 years there have been numerous unsuccessful attempts to introduce QoS into packet-switched networks.
The various attempts have failed so far more because of economic and legacy reasons than because of technical reasons.
These attempts include end-to-end ATM networks, and TCP/IP networks deploying Intserv or Diffserv.
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