Network functions like load balancers and stateful firewalls which traditionally have been packaged in a single proprietary device are now being virtuahzed in software across multiple physical devices networked together to achieve greater flexibility and scale. A virtualization can become very complex. Separating its definition from the software that implements it allows this complexity to be managed more easily. This paper describes some elementary behaviors that can be rigorously combined to produce modular definitions of new virtualizations. Behaviors are expressed using a new type of symbolic finite automaton called a λ-SFA. These automata can be formally analyzed and serve as a guide for synthesizing efficient code. As behaviors are combined, proofs of invariants for the result can leverage proofs of invariants for the elementary behaviors.