This paper discusses SNA/LEN, a possible extension of Systems Network Architecture intended to allow peer, dynamic, and easy to use networking functions for a variety of node sizes down to and including the new generation of personal computers. After reviewing the special requirements posed by small systems and the need for including them as equal partners in networks, the LEN architecture is described. This is done by following a sequence of steps that begin when a logical resource at some node that may not yet be part of the connectivity of an existing network requests a session with a remote logical resource of unknown location. After connectivity with the preexisiting network is established, directory services locates the remote object, route selection services determines the preferred path, a session is activated, and deadlock-free flow control assures a useful flow of data. Preliminary quantitative results from an LEN prototype are presented.

SNA is both an architecture and a set of products built in conformance with the architecture (1,2,3). The architecture is layered and precisely defined; it is both evolutionary and cost effective for implementing products. Perhaps the largest component of cost effectiveness is performance: transaction throughput and response times. For SNA, this involves data link control protocols (for SDLC and S/370 channel DLC's), routing algorithms, protocols used on the sessions that connect logical units (LU-LU session protocols), and interactions among them.

The definition of IBM's Systems Network Architecture (SNA) has evolved into a specification of a node in the form of a metaimplementation using formal, state-oriented descriptive techniques. This evolution is traced here, and the different formal techniques are described. The culmination of this process has been the development of a PL/I-based programming language, Format and Protocol Language (FAPL), as a descriptive tool. Using FAPL, the architects now define SNA by a programmed meta-implementation of a node. In this form, it is precise, readily accessible to the implementing product designers and programmers, and structurally close to the implementations. The essential features of the meta-implementation and of FAPL are described, along with the implications and advantages of describing the architecture in an executable form. One major benefit, already being realized, is the capability to test the logical consistency and completeness of the executable description itself. The current status of the validation of the executable description and sample results obtained are described.

The services that can be provided by an SNA network are discussed from a user's (e.g., terminal operator's, application programmer's, network system programmer's) point of view. No details of the network's internal behavior are given, but a short review of SNA introduces the paper.

This paper discusses the services provided by a systems network architecture (SNA) network and design aspects related to these services. Both the basic transmission services and higher level network services are discussed. The first section describes the structure of SNA. The second section describes SNA's transmission services and sketches in the other aspects of SNA's structure. The next section describes services provided to users and managers of the network and the distribution of these services throughout the various nodes of the network. A concluding section discusses several potential extensions.