Methods for Designing SIP Services in SDL with Fewer Feature Interactions

This paper describes methods for implementing telephony services in SIP with fewer traditional feature interactions. A formal SDL model of SIP and its services has been derived from published SIP specifications for verification and validation. It is known that the SIP RFC describes only the protocol specification. The specifications of SIP services and additional service features are informal and can only be found in various IETF drafts. Nevertheless, the service designers are still faced with new feature interaction problems. These new feature interactions are unique to SIP because SIP has flexible signaling features, such as request forking and dynamic assignment of contact addresses, which have both cooperative and adversarial side effects on each other. This paper also describes an extension to the classical feature interaction taxonomy, which is used to associate the causes, effects/symptoms with the preventive measures of the new and traditional feature interactions. Finally, SIP services can be designed and implemented without certain feature interactions by following certain design rules which are based on the knowledge deduced from the verification.