The Application, Presentation, and Session layers turn out to be so
closely related that the best thing to do is to have one big state
machine for all three layers rather than implementing a separate state
machine for each.  So, in the upper layers, we feel that the layering
structure of the OSI model is not quite right; I think you'll find
very few people who disagree with that.  Nevertheless, it's something
we have to live with because we realize it would be impossible at this
point to make radical changes to the OSI model.

		-- DECnet Phase V: An OSI Implementation (1992)