DLPI Programmer's Guide

Chapter 1 19
Introduction to DLPI
The Data Link Layer
endpoint between a DLS user and the DLS provider. After the stream is
created, the DLS user and DLS provider communicate via messages
discussed later.
DLPI is intended to free data link users from specific knowledge of the
characteristics of the data link provider. Specifically, the definition of
DLPI hopes to achieve the goal of allowing a DLS user to be implemented
independent of a specific communications medium. Any data link
provider (supporting any communications medium) that conforms to the
DLPI specification may be substituted beneath the DLS user to provide
the data link services. Support of a new DLS provider should not require
any changes to the implementation of the DLS user.
Modes of Communication
Although DLPI supports three modes of communication,
Hewlett-Packard supports connection and connectionless modes. The
connection mode is circuit-oriented and enables data to be transferred
over a pre-established connection in a sequenced manner. Data may be
lost or corrupted in this service mode due to provider-initiated
resynchronization or connection aborts.
The connectionless mode is message-oriented and supports data transfer
in self-contained units with no logical relationship required between
units. Because there is no acknowledgment of each data unit
transmission, this service mode can be unreliable in the most general
case. However, a specific DLS provider can provide assurance that
messages will not be lost, duplicated, or reordered.
Raw mode interface is also supported. Raw mode allows the DLS user to
send and receive packets with complete LLC and MAC header
information.
Connection-mode Service
The connection-mode service is characterized by four phases of
communication:
• Local Management
• Connection Establishment
• Data Transfer
• Connection Release