The concept 'session' modelled from below

Traditionally Pi-calculus and its related families of theories pretend to be descriptive to the domain of discourse, by giving as small a set of primitives as possible to model as much as possible. We take the tools from Milner's last book and related theories in use to analyse the imprecise concept 'session'. This is done by looking at its interpretation in the various systems at hand to do what we call 'conceptual archeology' starting with the simplest system of Communicating Automatas, and then proceed to the richer systems. The concept 'session' is choosen as a benchmark to decide the fruitfulness of applying Pi-calculus as a means for doing conceptual analysis in a precise manner, rather than the common imprecise way done in Software Engineering.