DLPI Programmer's Guide

Chapter 1 39
Introduction to DLPI
DLPI Services
Figure 1-18 Message Flow: DL_DISCONNECT Indication Arrives after
DL_CONNECT Response is Sent
Data Transfer Service
The connection-mode data transfer service provides for the exchange of
user data in either direction or in both directions simultaneously
between DLS users. Data is transmitted in logical groups called data
link service data units (DLSDUs). The DLS provider preserves both the
sequence and boundaries of DLSDUs as they are transmitted.
Normal data transfer is neither acknowledged nor confirmed. It is up to
the DLS users, if they so choose, to implement a confirmation protocol.
Each DL_DATA_REQ primitive conveys a DLSDU from the local DLS
user to the DLS provider. Similarly, each DL_DATA_IND primitive
conveys a DLSDU from the DLS provider to the remote DLS user. The
normal flow of messages is illustrated in Figure 1-19.
Figure 1-19 Message Flow: Normal Data Transfer
DL_CONNECT
request
DL_OK
request
DL_CONNECT
DL_CONNECT
DL_OK
response
indication
acknowledge
DL_DISCONNECT
indication
DL_DISCONNECT
acknowledge
DL_DATA
request
DL_DATA
indication