m or interest group that (1) develop SIP standard or specifications, (2) want to check the coherence of the SIP specifications implemented, (3) need to check progress in using their specification, and (4) want to let their members get useful feedback for quickly and efficiently adapting or improving their specifications. Interoperability events refine both the protocol and its implementations, deter- mine the source of incompatibilities, and, if the specification is at fault, prepare a "fix" of common bugs for the draft revision. A result of these events is an informational document that gives examples of SIP test messages designed to exercise and "torture" a SIP implementation. [22]. 8.2.2.1 ETSI TS 102 027 Technical Specification for SIP IETF RFC 3261 ETSI has over ten years' experience producing compliance test specifications for key telecommunication technologies among which is Voice-over-Internet Protocol (VoIP). These technical specifications (TSs) are used by manufac- turers as a basis for internal development testing. ETSI test specifications are usually developed by a specialist task force, which is a group of experts © 2009 by Taylor & Francis Group, LLC SIP Handbook: Services, Technologies, and Security recruited from the ETSI membership and managed by the Centre for Testing and Interoperability. ETSI TS 102 027 provides TS for testing the compliance of various SIP entities to IETF RFC 3261. In order to evaluate compliance of a particular protocol implementation, a protocol implementation confor- mance statement (PICS) [19], a test suite structure (TSS) and a TP are defined [20]. The PICS for the SIP implementation is in compliance with the relevant requirements specified in IETF RFC 3261. The PICS contains a table for each SIP entity: User sgent, registration server, proxy application server or redirect server. These tables define the services, the service procedures and the related messages (requests and responses) correlated with mandatory (M or m), optional (O or o), not applicable (N/A or n/a) or 