g no call-ID, from, or to header fields must not break because of the missing information. Ideally, it should respond with a 400 bad request error. 8.5 Conclusions and Future Directions The investigation conducted on SIP testing methods and tools detects some common issues for SIP attendants. The aim of interoperability is addressed on the one hand from the production of technical standards and reports, on the other hand from the development of standardized test specifications: conformance test specifications and interoperability test specifications. SIP testing standardization is focusing on the creation of effective test suites fol- lowing well-defined and transparent procedures in which all interested par- ties can participate. Standardization bodies are directing their efforts toward C 2009 by Taylor & Francis Group, LLC Compliance and Interoperability Testing of SIP Devices the development and application of testing methodologies, architecture, tech- niques, and languages designed according to the SIP features that can guar- antee a certain degree of conformance and interoperability. From a commercial point of view, production houses are addressing their attention toward the creation of SIP testing instruments and tools that are easy to use, sometimes embedded in the system, and that can allow users with a high configurability and programmability in order to emulate every particular situation. References [1] Rosenberg, J., H. Schulzrinne, G. Camarillo, A. Johnston, J. Peterson, R. Sparks, R. Handley, M. Schooler, and E. Schooler. 2002. SIP: Session Initiation Protocol. RFC 3261, 1-269. [2] Rosenberg, J., and H. Schulzrinne. 2002. Reliability of provisional responses in the Session Initiation Protocol (SIP). RFC 3262, 1-14. [3] Rosenberg, J., and H. Schulzrinne. 2002. The Session Initiation Protocol (SIP) UPDATE Method. RFC 3264, 1-25. [4] Roach, A.B. 2002. Session Initiation Protocol (SIP)-specific event noti- fication. RFC 3265, 1-38. [5] Sparks, R. 2003. The Session Initiation Protocol (SIP) refer method. 