Installing and Administering OSI Transport Services

40 Chapter 1

HP OTS/9000 Resources

Addressing Concepts

SAP - A SAP, or Service Access Point , is a “pipe” between two OSI

layers that allows one layer to obtain a set of services from another layer.

The services obtained vary from layer to layer, but are usually

management functions, such as connection establishment and

termination, and data transfer functions. For instance, a SAP between

an application and the OSI Transport is a “pipe” the application uses to

open connections, for example, send/receive data using the OSI

Transport service.

A SAP is similar to a Berkeley socket. A BSD (Berkeley Software

Distribution) socket is a “pipe” between an application and TCP or UDP

used to create connections and send/receive data.

SAPs may be created between any adjacent layers in the ISO protocol

suite. These SAP “pipes” can then be connected together to form a whole

“conduit” through the protocol stack that allows a user to send data to,

and receive data from, a remote system.

Figure 1-3

In the ARPA protocol suite, “conduits” through a protocol stack are

relatively simple. They consist of a layer 4 (TCP or UDP) SAP (socket),

and an IP SAP. The situation is more complex in ISO because “conduits”

can extend from layer 3 up to layer 7, so several SAPs may be required to

build a complete “conduit” for an ISO application.

SAP

Application

XTI

ISO Transport

Data