The (un)revised OSI reference model

In 1988, SC21, the ISO committee responsible for the Open Systems Interconnection (OSI) Reference Model, determined that it was time to undertake revising ISO 7498-1, The Basic OSI Reference Model. Since the model had been published in 1984, this was in accordance with ISO practice of reviewing and revising standards every five years, and there was good reason for considering the task. Over the intervening five years, the groups developing the OSI protocols had raised many questions about the architecture and these had been answered and documented in the Approved Commentaries [1994]. Many of these commentaries contained information that could be usefully incorporated into the Reference Model itself. In addition, the addendum describing connectionless mode, i.e. datagrams, had been completed several years before and needed to be incorporated. There was also considerable demand for interworking connection-mode and connectionless mode communication, something not supported by the Reference Model or any architecture. Also, when the original version of the Reference Model was frozen about 1983 some aspects of OSI such as the upper layers were not well understood and were only described in the most cursory manner. And while connection-mode and connectionless mode had been brought within OSI, there was no indication as to how broadcast and multicast were to be handled. Thus, the revision might be able to provide a more comprehensive description of these areas.This paper describes how the revision was carried out, describes the changes and additions that were made, considers the effect and contribution this revision has made to our understanding, and describes the outstanding issues that were not addressed by this revision.