The description here of the Internet services from a QoS perspective will
be useful for reading Chapter 18, “Quality of Service for Real-Time Internet
Communications.”Differentiated Services
The Differentiated Services (DS) [13] model allows only certain classes of service
that are defined in the network. The main properties of differentiated services
are therefore:
■■ DS require no state in the network.
■■ Applications can be fit only within certain classes of service.
■■ The network is not aware of the individual IP packet streams, but only
of classes of service. Accounting for individual users is therefore not
possible.
■■ DS are highly scalable and, therefore, well suited to be used in the core
of the Internet.
■■ Because of their simplicity, DS are also most useful in Internet access
networks.Resource Reservation
At the other end of the IP services spectrum from best-effort service is the
guaranteed-by-per-flow reservation service based on the Resource Reserva-
tion Protocol (RSVP) [14], shown at the right in Figure 5.2. RSVP resembles the
bandwidth and delay qualities of TDM circuits, either for guaranteed service
or to the degree to which TDM circuit properties can be emulated over an
unloaded IP network. RSVP reserves the resources across the IP network asso-
ciated with individual flows. The IP addresses and port numbers of the IP end-
points, and also the transport layer protocol (such as UDP or TCP) characterize
an IP packet flow. RSVP is a form of virtual circuit setup over a packet
switched network.
Following are the main properties of RSVP:
■■ Applications in endpoints can communicate their requirements for QoS
to the network directly and in a flexible manner.
■■ The ownership of the RSVP-supported QoS flows can be clearly distin-
guished. This allows accounting for the use of network resources by
individual users.
■■ Routers in the network have to keep state for each RSVP reservation,
and, as a consequence, RSVP is not scalable to large networks. The use
of RSVP is, therefore, practical in the Internet access part, in the periph-
ery of the network, or in private IP networks of limited size.88 Chapter 5