Causal Type System for Ambient Movements

The Ambient Calculus was developed by Cardelli and Gordon as a formal framework to study issues of mobility and migrant code. We present a type system for the calculus, parameterized by security constraints expressing where a given ambient may reside and where it may be dissolved. A subject reduction property then guarantees that a welltyped process never violates these constraints; additionally it ensures that communicating subprocesses agree on their “topic of conversation”. Based on techniques borrowed from finite automata theory, type checking of type-annotated processes is decidable. Under certain quite natural restrictions, type inference is also possible. The type system employs a notion of causality in that processes are assigned “behaviors”. This significantly increases the precision of the analysis and compensates for the lack of “co-capabilities” (an otherwise increasingly popular extension to the ambient calculus); also it allows an ambient to hold multiple topics of conversation.