Network Services Protocol Functional Specification

This document is the functional specification for DECnet's Network Services Protocol (NSP) Version 3.2.0, a key component of the DIGITAL Network Architecture (DNA) Phase III.

NSP's primary role is to provide reliable, end-to-end data communication services between network nodes. Its core functions include:

• Logical Link Management: Enabling the creation and destruction of virtual communication channels (logical links) between Session Control modules, identified by unique 32-bit addresses.
• Data Flow Control: Managing the movement of data between transmit and receive buffers, including separate mechanisms for normal and interrupt data, using "on/off" and "request count" controls.
• End-to-End Error Control: Guaranteeing message delivery through an acknowledgment mechanism, sequential numbering of messages (Data Segment and "Other-Data" subchannels), and retransmission upon timeout.
• Message Segmentation and Reassembly: Breaking large user data messages into smaller segments for transmission and reassembling them in the correct order at the destination.

The document describes NSP as a "black box" layer within DNA, interfacing with the Session Control layer above it and the Transport layer below. It details the various messages exchanged between NSP modules, the states of logical links and ports, and the internal functional components such as data bases, buffer pools, and processing modules (e.g., receive/transmit processes, reassembly, segmentation, and transmit allocation).

Key design requirements for NSP include compatibility with previous versions, performance (minimizing delays, avoiding deadlocks), efficiency, extensibility, fairness, and elasticity. An appendix specifically outlines the differences between NSP Version 3.2 and 3.1, particularly in how connection and disconnection messages are handled.