Full abstraction for polymorphic Pi-calculus

The problem of finding a fully abstract model for the polymorphic pi-calculus was stated in Pierce and Sangiorgi's work in 1997 and has remained open since then. In this paper, we show that a slight variant of their language has a direct fully abstract model, which does not depend on type unification or logical relations. This is the first fully abstract model for a polymorphic concurrent language. In addition, we discuss the relationship between our work and Pierce and Sangiorgi's, and show that their conjectured fully abstract model is, in fact, sound but not complete.