DLPI Programmer's Guide

Chapter 1 21
Introduction to DLPI
The Data Link Layer
Connectionless-mode Service
The connectionless mode service does not use the connection
establishment and release phases of the connection mode service. The
local management phase is still required to initialize a stream. Once
initialized, however, the connectionless data transfer phase is
immediately entered. Because there is no established connection,
however, the connectionless data transfer phase requires the DLS user to
identify the destination of each data unit to be transferred. The
destination DLS user is identified by the address associated with that
user.
DLPI Addressing
Each user of DLPI must establish an identity to communicate with other
data link users. This identity consists of two pieces. First, the DLS user
must somehow identify the physical medium over which it will
communicate. This is particularly evident on systems that are attached
to multiple physical media. Second, the DLS user must register itself
with the DLS provider so that the provider can deliver protocol data
units destined for that user. Figure 1-2 illustrates the components of this
identification approach, which are explained below.
Figure 1-2 Data Link Addressing Components
Physical Attachment Identification
The physical point of attachment (PPA in Figure 1-2) is the point at
which a system attaches itself to a physical communications medium. All
communication on that physical medium funnels through the PPA. On
systems where a DLS provider supports more than one physical medium,
the DLS user must identify which medium it will communicate through.
DLSAP
DLS Users
DLS
PPA
Provider
Physical Media