DECnet Digital Network Architecture Phase IV NSP Functional Specification

This document is the NSP (Network Services Protocol) Functional Specification for DECnet Digital Network Architecture, Phase IV, published in December 1983.

It describes the architecture, functions, interfaces, and messages of NSP, which is the protocol for the End Communications layer within Digital's layered Network Architecture (DNA).

Key aspects of NSP's functionality include:

1. Logical Link Management: Supporting the creation and destruction of virtual communication channels (logical links) between Session Control modules, which can be within the same or different network nodes.
2. Data Transfer: Managing the movement of both normal and interrupt data messages.
3. Segmentation and Reassembly: Breaking down large user messages into smaller segments for transmission and reassembling them correctly at the destination.
4. Error Control: Guaranteeing reliable message delivery through an acknowledgment mechanism and retransmission for data and control messages.
5. Flow Control: Coordinating data flow between transmitting and receiving buffers to prevent congestion and ensure efficient communication.

NSP achieves these functions by defining:

• Its design scope and characteristics (e.g., compatibility, performance, efficiency, and its reliance on the Routing Layer's datagram service).
• Basic concepts like logical links, ports, their states, and data flow.
• Specific message types (Data, Acknowledgment, Control) and their formats used for communication between NSP modules.
• Its interfaces with higher layers (Session Control), lower layers (Routing), and Network Management.
• Internal data bases (e.g., for ports and nodes), buffer pools, and functional modules that implement its operations.
• Detailed algorithms for processes like data segment retransmission, inactivity timing, and confidence testing.

Essentially, the document serves as a comprehensive technical guide for implementing or understanding the NSP component within a DECnet Phase IV network.