Flow-based algorithms have the potential to offer good quality of service to one or more flows
because they reserve whatever resources are needed along the route. However, they also have
a downside. They require an advance setup to establish each flow, something that does not
scale well when there are thousands or millions of flows. Also, they maintain internal per-flow
state in the routers, making them vulnerable to router crashes.
Finally, the changes required
to the router code are substantial and involve complex router-to-router exchanges for setting
up the flows. As a consequence, few implementations of RSVP or anything like it exist yet.
For these reasons, IETF has also devised a simpler approach to quality of service, one that can
be largely implemented locally in each router without advance setup and without having the
whole path involved. This approach is known as class-based (as opposed to flow-based)
quality of service. IETF has standardized an architecture for it, called differentiated services,
which is described in RFCs 2474, 2475, and numerous others. We will now describe it.
Differentiated services (DS) can be offered by a set of routers forming an administrative
domain (e.g., an ISP or a telco). The administration defines a set of service classes with
corresponding forwarding rules. If a customer signs up for DS, customer packets entering the
domain may carry a Type of Service field in them, with better service provided to some classes
(e.g., premium service) than to others. Traffic within a class may be required to conform to
some specific shape, such as a leaky bucket with some specified drain rate. An operator with a
nose for business might charge extra for each premium packet transported or might allow up
to N premium packets per month for a fixed additional monthly fee. Note that this scheme
requires no advance setup, no resource reservation, and no time-consuming end-to-end
negotiation for each flow, as with integrated services. This makes DS relatively easy to
implement.
Class-based service also occurs in other industries. For example, package delivery companies often offer overnight, two-day, and three-day service. Airlines offer first class, business class, and cattle class service. Long-distance trains often have multiple service classes. Even the Paris subway has two service classes. For packets, the classes may differ in terms of delay, jitter, and probability of being discarded in the event of congestion, among other possibilities (but probably not roomier Ethernet frames).
To make the difference between flow-based quality of service and class-based quality of
service clearer, consider an example: Internet telephony. With a flow-based scheme, each
telephone call gets its own resources and guarantees. With a class-based scheme, all the
telephone calls together get the resources reserved for the class telephony. These resources cannot be taken away by packets from the file transfer class or other classes, but no telephone call gets any private resources reserved for it alone.
Differentiated Services
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