The map library provides a facility for maintaining a set of bindings between identifiers. The map library supports operations for adding new bindings to the set, removing bindings from the set, and mapping one identifier into another, relative to a set of bindings (lookup). Protocol implementations use these operations to translate identifiers extracted from message headers (e.g., addresses, port numbers) into capabilities for (pointers to) x-kernel objects (e.g., Protl, Sessn, Enable).