Abstract

We use an event-driven process model to specify a version of the High-Level Data Link Control (HDLC) protocol between two communicating protocol entities. The protocol is verified using the method of projections. The verification serves as a rigorous exercise to demonstrate the applicability of this method to the analysis of real-life communication protocols. The HDLC protocol has two characteristics found in most real-life communication protocols. First, the HDLC protocol operates under real-time constraints that are important not only for its performance but also for its correct logical behavior. We specify this real-time behavior using time variables and time events. Second, the HDLC protocol has three distinguishable functions: connection management, and one-way data transfers between the protocol entities. For each of these functions, we construct an image protocol using the method of projections. With each image protocol we obtain inductively complete invariant assertions that state various desirable logical safety properties. From the properties of image protocols it follows that these safety properties as proved for the image protocols are also satisfied by the HDLC protocol presented herein. We also suggest a minor modification to HDLC that will make it well-structured.

Citations