DECnet DIGITAL Network Architecture General Description

This document, "DECnet DIGITAL Network Architecture (Phase III) General Description," published in October 1980, provides a comprehensive overview of Digital Equipment Corporation's (DIGITAL) network architecture (DNA). It serves as a design model for Phase III DECnet implementations, detailing their functions, protocol messages, and operational aspects.

Key Concepts and Design Principles: DNA is based on a layered hierarchy where each layer contains modules performing defined functions. It specifies:

• Interfaces: Relationships between different modules, typically within the same node, for service exchange between layers.
• Protocols: Relationships between equivalent modules in different nodes, defining message formats and exchange rules.

Design Goals: DNA aims to:

• Provide a common user interface across diverse implementations.
• Support a broad spectrum of applications (e.g., resource sharing, distributed computation, remote communication).
• Accommodate a wide range of communication facilities.
• Be cost-effective, highly available, extensible, and easily implementable across various DIGITAL systems.
• Enable distributed network control and user-level management functions.
• Allow for multi-level security.

Architectural Layers (from highest to lowest):

1. User Layer: Contains user-supplied functions and the Network Control Program (NCP) for network management.
2. Network Management Layer: Provides control and access to operational parameters, counters, down-line loading, up-line dumping, and testing.
3. Network Application Layer: Offers generic services like remote file access (DAP) and logical link testing (Loopback Mirror Protocol).
4. Session Control Layer: Defines system-dependent aspects of logical link communication, including name-to-address translation and process activation.
5. Network Services Layer: Manages system-independent aspects of logical links, including data flow control, error control, and message segmentation/reassembly (NSP).
6. Transport Layer: Routes user data (packets) to its destination, potentially through intermediate nodes, and handles congestion control and packet lifetime (Transport Protocol).
7. Data Link Layer: Creates communication paths between adjacent nodes, framing messages, checking integrity, and ensuring correct sequencing (DDCMP, MOP).
8. Physical Link Layer: Manages physical data transmission over a channel (e.g., EIA RS-232C, CCITT V.24).

Protocols: DNA defines specific protocols for inter-node communication at various layers:

• Data Link: Digital Data Communications Message Protocol (DDCMP) for reliable data transfer, and Maintenance Operations Protocol (MOP) for remote system functions.
• Transport: Transport Protocol for routing and congestion control.
• Network Services: Network Services Protocol (NSP) for logical link management.
• Session Control: Session Control Protocol for link data and control.
• Network Application: Data Access Protocol (DAP) for remote file operations, and Loopback Mirror Protocol for logical link testing.
• Network Management: Network Information and Control Exchange (NICE) Protocol for network control and information, and Event Logger Protocol for recording events.

Data Flow: Data originates at the User layer of a source node, is progressively "enveloped" with control information (headers and trailers) as it passes down through each DNA layer, and is then transmitted across physical lines. At each intermediate routing node, the data travels up to the Transport layer for routing decisions before moving back down and being transmitted again. At the final destination node, the data is "un-enveloped" as it moves up through the layers to the destination process.

This document is intended for readers with a knowledge of communications technology who seek an understanding of the overall DNA structure.