A logical account of subtyping for session types

We study iso-recursive and equi-recursive subtyping for session types in a logical setting, where session types are propositions of multiplicative/additive linear logic extended with least and greatest fixed points. Both subtyping relations admit a simple characterization that can be roughly spelled out as the following lapalissade: every session type is larger than the smallest session type and smaller than the largest session type. We observe that, because of the logical setting in which they arise, these subtyping relations preserve termination in addition to the usual safety properties of sessions.