DECnet Digital Network Architecure (Phase IV) General Description

This document, published in May 1982 by Digital Equipment Corporation (DEC), provides a "General Description" of DECnet DIGITAL Network Architecture (DNA) Phase IV.

Key aspects of the document:

1. Purpose of DNA/DECnet:

   • DNA is a layered architectural model defining the structure and functions for network communication.
   • DECnet is a family of hardware and software products that implement DNA, enabling various Digital operating systems and computers to function together in a network, sharing data, resources, programs, and functions.
   • Phase IV introduced new features, including support for Ethernet and larger networks, while maintaining compatibility with Phase III.

2. Design Goals: DNA aims to provide:

   • A common user interface that hides network complexity.
   • Support for a broad range of applications (e.g., remote file access, distributed computation, terminal access).
   • Compatibility with a wide variety of communication facilities (e.g., leased lines, Ethernet local area networks, X.25 packet switching).
   • Cost-effectiveness, high availability, extensibility, distributed functions, user-level network control, and security.

3. Layered Architecture (inspired by the ISO Reference Model): DNA is structured into distinct layers, each performing specific functions and communicating via defined interfaces (within a node) and protocols (between equivalent modules in different nodes). The layers, from lowest to highest, include:

   • Physical Link Layer: Manages physical transmission of data (hardware, modems, lines).
   • Data Link Layer: Creates communication paths between adjacent nodes, ensuring data integrity and proper sequencing (e.g., Digital Data Communications Message Protocol - DDCMP, Ethernet Data Link, X.25 Frame Level).
   • Routing Layer: Routes data "datagrams" (contained in "packets") to their destination across potentially intermediate nodes, handling congestion and packet lifetime control.
   • End Communication Layer: Provides reliable, system-independent process-to-process communication via "logical links" (virtual circuits), managing data flow, error control, and segmentation/reassembly of messages (Network Services Protocol - NSP).
   • Session Control Layer: Handles system-dependent aspects of logical links, including node name-to-address translation, process addressing, and access control.
   • Network Application Layer: Offers generic services to users, such as remote file access (Data Access Protocol - DAP), network virtual terminal services (NVT), and gateway access to non-DNA systems like X.25 or IBM SNA networks.
   • User Layer: Contains user applications and the Network Control Program (NCP) for interactive network management.
   • Network Management Layer: (Overarching) Provides control and monitoring functions across all lower layers, including remote system loading/dumping, parameter management, and link testing.

4. Data Flow Concept: The document explains how user data flows down through the source node's DNA layers, with each layer adding its own control information (enveloping), is transmitted across the network, and then flows up through the destination node's layers, with control information being stripped away at each step until it reaches the destination user process.

In essence, the document serves as a high-level technical introduction to the architecture and operational principles of DECnet Phase IV, detailing its layered design, core protocols, and key functionalities for building robust and adaptable computer networks.