The x-kernel maintains reference counts for sessions in order to facilitate their destruction. This section discusses the system support for reference counts. We discuss reference counts in the context of sessions (rather than protocols) because protocols are relatively static objects and the issues surrounding their reference counts are not very interesting.
Constructing a protocol that directly manipulates reference counts and does so correctly can be awkward and tedious. The x-kernel has addressed this difficulty by: (1) moving all direct reference count manipulation into the x-kernel infrastructure; (2) explicitly documenting how references to other sessions may be used; and (3) adding system support for session caching and garbage collection.