A time-triggered communication protocol that derives its control information (e.g., when to send a message) from the progression of time offers unique features that meet the stringent demands of automotive class C control applications. The interface to a dedicated network controller executing such a protocol can be viewed as a system wide interface and is thus a perfect starting point for the system's decomposition into autonomous, non-interfering subsystems. This paper presents such a network controller interface that allows applications to transparently access the services of the underlying communication protocol. The interface is designed as a data-sharing interface, i.e., the flow of control information across the interface is restricted to the necessary minimum. With this, the chance of control error propagation is reduced considerably. The interface acts as a temporal firewall facilitating independent subsystem development and validation.