prohibited (X or x) parameters and capabilities. An example related to a user agent entity and in particular to the REGISTER message request parameters is reported in Figure 8.5. The TSS is a test classification based on SIP entities and their role in the main functionalities: (1) registration, (2) call control, (3) querying for capabilities, and (4) messaging (Figure 8.6). Areas of compliance investigated by TSS are: (1) capability and valid behav- ior to verify that mandatory capabilities and "normal" procedure handling are supported, (2) syntactically invalid behavior to verify that the IUT is able to properly react to receiving a syntactically incorrect message or stimulus, (3) invalid and inopportune behavior to verify that the IUT is able to react properly when a syntactically correct, but unexpected, message is received, and (4) timer expiry to verify the IUT reacts properly. In each main functionality and for each entity, a TP is identified by four keywords: The TP Identifier (TPId) gives a unique identifier to each test purpose. "Status" specifies, when a test purpose is optional, recommended, and mandatory, according to the IETF RFC 3261 [1] PICS document. "Ref" outlines the references in IETF RFC 3261 [1] used to create the test purpose. "Purpose" describes the objective of the test. Some examples are reported, for the case of registration in Figure 8.7, call control in Figure 8.8, and for registrant and originating endpoint, respectively. 8.2.3 SIP Interoperability Test Events SIP interoperability test events (SIPITs) [23], are weeklong events where peo- ple bring their SIP implementations to ensure that they work together. The SIPITs are open to anyone with a working SIP implementation verifying com- pliance reaction of SIP entities to syntactically incorrect or correct but unex- pected messages. During the events, problematic messages were noted and released as an IETF-draft (IETF RFC 4475) [22]. C 2009 by Taylor & Francis Group, LLC Compliance and Interoperability Testing of SIP Devices Prerequisit