 authentication: A UA in the registered state sends an un-REGISTER message, obtains an error answer, 401 "Unau- thorized", replies with an un-REGISTER message with authentication, receives a 200 OK SIP answer and moves to the idle state. 8.2.2 SIP Testing In the place of the FSM approach, some alternative approaches [18] for the selection of test cases for the compliance tests of SIP could be obtained by © 2009 by Taylor & Francis Group, LLC Compliance and Interoperability Testing of SIP Devices using test cases provided by either (1) standard test specifications, or (2) inter- operability events: European Telecommunications Standards Institute Test Specifications (ETSI TS 102 027 [19-21]) are designed to concentrate on areas critical to interoperability, including testing an implementation's reaction to an erroneous behavior. The goal is compliance testing for interoperability. This should not be confused with interoperability testing, which is a useful but a different activity. In many cases, development of the test specifications gives valuable feedback to the base standards. The test specification designers make a special effort to ensure that their expe- riences are fed back to the relevant ETSI technical committee. How- ever, development of the base standard and test specifications must be done in a coordinated and timely manner for this input to be effective. A focused set of compliance tests can provide an excellent framework for subsequent interoperability testing. Interoperability events for SIP standards and product validation are open to all companies, organizations, and working and study groups implementing SIP entities. The customers of interoperability events are: - Operators, vendors or equipment manufacturers who (1) are about to place their products on the market, (2) want to be sure of the interoperability of their products, (3) feel comfortable with the technologies but still want to improve their know-how; Standardization bodies (ETSI, Internet Engineering Task Force (IETF), etc.) or any foru