Installing and Administering OSI Transport Services

42 Chapter 1
HP OTS/9000 Resources
Addressing Concepts
NOTE Because of the relationship between SAPs and selectors, the terms are
sometimes used synonymously. If the term SAP (for example, PSAP,
SSAP, TSAP) appears in HP documentation, it should be taken to mean
the selector (that is, P-selector, S-selector, T-selector) representing the
SAP.
Address - An address is a sequence of selectors, plus at least one NSAP
(network service access point), that identifies an entire “conduit” through
a protocol stack. For instance, in the case of FTAM, the address of its
“conduit” includes the presentation, session, transport, and network
layers. Its address is therefore: a P-selector, S-selector, T-selector, and
one or more NSAPs. This address is commonly called a presentation
address (P-address) since it defines a “conduit” whose top-most layer is
presentation. More than one NSAP may be included in a P-address
because the system may be reachable on more than one network (for
example, a system that is accessible via both a CONS network and a
CLNS network). This situation is similar to an ARPA system that is
connected to more than one IP network and therefore has more than one
IP address.
Applications access SAP “conduits” by using a programmatic interface.
An example of a programmatic interface is X/Open’s Transport Interface
(XTI), that gives applications the ability to communicate with remote
systems using the OSI Transport layer. The application tells the
programmatic interface which “conduit” to use by passing an address to
the appropriate interface procedure call. In the case of XTI, the address
of the “conduit” a local application wishes to listen on is passed during
the t_bind() procedure call. If the local application wishes to
communicate with a remote application, it passes the remote
application’s address to XTI during the t_connect() procedure call.