Obtaining the service language for H.245's multimedia capability exchange signalling protocol: the final step

摘要

The capability exchange signalling (CES) protocol is a subprotocol of ITU-T recommendation H.245, "control protocol for multimedia communication". We are interested in verifying this protocol, including verification of its general properties such as absence of deadlocks and livelocks, and verification of the protocol against its service. In previous work, we have analysed the general properties of the CES protocol. In order to verify the protocol against its service, we need to generate the CES service language, which defines all the possible sequences of user observable events. We firstly create the CES service CPN model and then extract the language from the occurrence graph (OG) of the model. In the case of the CES service, this is challenging because we wish to have a general result for arbitrary capacity of the network over which the CES protocol operates. To tackle this problem we introduced into the CPN model a parameter, l, representing the capacity of the communication channel. We derived a recursive formula for the parameterised OG in terms of l. We treat this OG as a finite state automaton (FSA) by nominating acceptance states and apply FSA reduction algorithms to obtain a deterministic FSA that represents the CES service language. We have discovered a recursive formula in l for the CES service language. This paper introduces the CES service via its CPN model as necessary background and presents the final step of the proof of this finding.