  We present a type checking algorithm for establishing a session-based
discipline in the pi calculus of Milner, Parrow and Walker. Our session types
are qualified as linear or unrestricted. Linearly typed communication channels
are guaranteed to occur in exactly one thread, possibly multiple times;
afterwards they evolve as unrestricted channels. Session protocols are
described by a type constructor that denotes the two ends of one and the same
communication channel. We ensure the soundness of the algorithm by showing that
processes consuming all linear resources are accepted by a type system
preserving typings during the computation and that type checking is consistent
w.r.t. structural congruence.
