ISO has developed its own set of jargon to describe the concepts in the
model and in the related standards. We have already surreptitiously
introduced some of this 'OSIspeak' and it is used extensively in what
follows. Some of the important terms are summarised below.
A Service definition is a document defining a particular layer service.
Usually it defines a set of several services provided by a layer. Each
service is described in terms of four Service Primitives:
-
Request - the user of a service asks it to carry out an action
-
Indication a service informs a user of some event
-
Response - a service indicates to a user that an answer has been forthcoming
-
Confirm - a service indicates to a user that a request has been carried out
An operation is begun when the local service user generates a request
primitive across the local service interface. This should result in the
generation of a corresponding indication primitive across the remote
service interface. In some cases a reply will be required from the remote
service user. This is triggered by the generation of a response primitive
across the remote service interface which should eventually result in the
generation of a confirm primitive across the local service interface.
By way of example, Table #tbtserv#1437> shows the set of primitives that are defined
for the OSI Connection Oriented Transport Service (Layer 4).
#table1438#
Table: Transport Service Primitives
In general, the primitives mentioned above have parameters associated with
them. For example, a T.CONNECT.request will include the address of the
remote host while a T.DATA.request will include data for transfer across
the network.
Primitive requests cannot be issued in a completely arbitrary way, there
are usually rules governing which primitives may follow which and which are
legal when the system is in some particular state. In the Transport Service
example above, a T.DATA.req cannot be issued in advance of a
T.CONNECT.conf. OSI service definitions include a state table specifying
these temporal orderings.