Data Access Protocol Functional Specification

This document, "DECnet DIGITAL Network Architecture Data Access Protocol (DAP) Functional Specification Version 5.6.0," published by Digital Equipment Corporation in October 1980, details the features, message formats, and operational procedures of the Data Access Protocol (DAP).

Purpose and Context: DAP is an application-level protocol within the DECnet environment, part of the larger DIGITAL Network Architecture (DNA). Its primary goal is to provide standardized formats and procedures for remote file access and data transfer between user processes and file systems across a network, independently of the operating system's I/O structure. It relies on lower DNA layers (Session Control, Network Services) for establishing communication paths.

Key Functions: DAP enables a wide range of remote file operations, including:

• Retrieving and storing files on various input/output devices.
• Providing ASCII file transportability between networked nodes.
• Error recovery and managing multiple data streams over a logical link.
• Executing command files and submitting batch jobs.
• Random and sequential access to records within files.
• File management tasks such as deletion, renaming, and directory listing.

Operational Aspects: DAP communication involves a structured exchange of messages:

• Link Setup: Processes first exchange "Configuration" messages to share system capabilities (buffer size, OS type, protocol version), followed by "Attributes" and "Access" messages to specify file characteristics and desired operations.
• Data Transfer: "Control" messages initiate data streams and operations (like GET or PUT), with "Data" messages handling the actual file content transfer. "Status" messages are used to report operation outcomes or errors.
• Termination: "Access Complete" messages terminate file access or data streams, and a "Disconnect Request" terminates the logical link.
• File Organization and Data Types: DAP supports sequential, relative, and hashed/indexed file organizations, along with various record formats (fixed, variable, ASCII stream). It handles ASCII, Image, and (reserved) EBCDIC data types, including optional compression. However, system-specific data conversions are the responsibility of the user process, not DAP itself.
• Security and Integrity: The protocol incorporates mechanisms for access control (via user identification and existing Session Control authentication) and data integrity (optional 16-bit CRC checksums for data transfers).

The document is primarily aimed at assisting those who implement DAP in understanding its functions within a system, rather than providing specific implementation details.