TCP/IP Tutorial and Technical Overview - IBM Redbooks

8.2.1 Service classes
The Integrated Services model uses different classes of service that are defined
by the Integrated Services IETF working group. Depending on the application,
those service classes provide tighter or looser bounds on QoS controls. The
current IS model includes the Guaranteed Service, which is defined in RFC 2212,
and the Controlled Load Service, which is defined in RFC 2211. To understand
these service classes, some terms need to be explained. Because the IS model
provides per-flow reservations, each flow is assigned a flow descriptor. The flow
descriptor defines the traffic and QoS characteristics for a specific flow of data
packets. In the IS specifications, the flow descriptor consists of a filter
specification (filterspec) and a flow specification (flowspec), as illustrated in
Figure 8-2.
Filterspec
Figure 8-2 Flow descriptor
The filterspec identifies the packets that belong to a specific flow with the sender
IP address and source port. The information from the filterspec is used in the
packet classifier. The flowspec contains a set of parameters that are called the
invocation information. The invocation information divides into two groups:
► Traffic Specification (Tspec)
► Service Request Specification (Rspec)
The Tspec describes the traffic characteristics of the requested service. In the IS
model, this Tspec is represented with a token bucket filter. This principle defines
a data-flow control mechanism that adds characters (tokens) in periodical time
intervals into a buffer (bucket) and allows a data packet to leave the sender only if
292 TCP/IP Tutorial and Technical Overview
Flow Descriptor
Tspec
Flowspec
Rspec