Heterogeneous network protocol systems are integrated together to fulfill complex tasks and their interoperability is a major hurdle for the network reliability and quality of services. We identify a new equivalence relation of states that preserves the integrated system interface behaviors. Based on this state equivalence we study the minimization of the system components with respect to their interfaces and design an efficient polynomial time minimization algorithm. We apply our technique to GMPLS protocols and obtain a significant state space reduction. We discuss integrated protocol system verification and interoperability testing with the minimized state system without resorting to the global state space information.