Introduction to DECnet (Phase III)

This document, "Introduction to DECnet (Phase III)" (AA-AV51A-TK), published in May 1982, serves as an overview of the concepts and capabilities of DECnet networks. It defines key terms and describes the network functions performed by various DECnet implementations, assuming the reader is familiar with DIGITAL operating systems.

The manual outlines DECnet's major functions, which include:

• Task-to-Task Communication: Enabling programs to exchange data over logical links.
• Remote File Access: Allowing terminal users and programs to access and manipulate files on remote nodes.
• Terminal Facilities: Providing terminal-to-terminal communication (e.g., TLK utility, VAX/VMS PHONE) and direct access to remote node operating systems.
• Network System Management: Facilities for generating, defining, monitoring, and controlling network nodes (e.g., Network Control Program - NCP).
• Down-line Loading and Up-line Dumping: Loading system images to diskless nodes and dumping memory images from crashed nodes.
• Loopback Testing: Diagnostic procedures to test network hardware and software.

The document elaborates on the DIGITAL Network Architecture (DNA), the layered model underpinning DECnet, and its various protocols (e.g., NSP, DAP, DDCMP). It highlights the differences between DECnet Phase II and Phase III, noting Phase III's advancements such as the addition of a Network Management Layer, a more sophisticated Transport Layer with full routing capabilities (point-to-point in Phase II vs. full routing in Phase III), and a Session Control Layer.

DECnet Routing is detailed, explaining concepts like node addresses (numeric for Phase III, alphanumeric for Phase II), path length, path cost, congestion control, and packet lifetime control. It also covers routing via Public Packet Switching Networks (PPSNs) using the Data Link Mapping (DLM) interface, specifically for RSX DECnet/PSI nodes, adhering to CCITT X.25 recommendations for virtual circuits.

The manual also covers the establishment and management of Logical Links between programs, the calls and addressing schemes involved in Task-to-Task Communication, and the specifics of Remote File Access, including file system capabilities and access control.

The document serves as an introductory guide for system managers, designers, and users, helping them understand DECnet's diverse functionalities and their support across different DIGITAL operating systems like RSX, IAS, VAX, RT, and TOPS-20.