Fair Subtyping for Multi-Party Session Types

The standard subtyping relation used in dyadic session type theories may compromise the liveness of multi-party sessions. In this paper we define a fair subtyping relation for multi-party session types that preserves liveness, we relate it with the standard subtyping relation, and we give algorithms for deciding it. As a side effect, we provide an original and remarkably simple coinductive characterization of the fair testing preorder for nondeterministic, sequential processes consisting of internal choices of outputs and external choices of inputs.