Origin of the OSI Model

As ubiquitous as the OSI model is in the world of network engineering, it occurred to me recently that I could not recall seeing a standard document defining the model referenced even once in all of the books I've read. Odd. A little digging turned up a formal definition of the model in the form of ITU recommendation X.200, available for free in PDF format.

The document is, as one would expect from an international standards body, extremely dry. But there are a few interesting sections worth reading through. Section 5.2, Principles of Layering, describes the sequential structure of the layers and how each was developed to interface only with the layers directly above and/or below it. Annex A provides a brief discussion of how the layers were chosen. The meat of the standard, a technical definition of each of the seven layers, is found in section 7.