ol for SIP developers intended for testing SIP imple- mentation against IETF RFC 3261. The current version can check only server implementations (proxies, user agent servers, redirect servers, and registrars). This program is distributed under terms of GPL. 8.3.2.5 Compliance Engine As part of the requirements for attending the SIPIT interoperability test- ing events, participants are expected to have already run a set of published test messages. The SIP Center's SIP Messenger tool [33] gives developers the facility to do this. C 2009 by Taylor & Francis Group, LLC SIP Handbook: Services, Technologies, and Security SIP Messenger is a Java software that allows the operator to send SIP test messages from text files over UDP to the SIP implementation and, option- ally, listen for responses. The messages can be sent using a command line utility (messenger), suitable for invocation by automated scripting, or via a GUI (MessengerGui). Developers can use this software to construct their own SIP messages that can be pushed onto SIP servers or user agents (possibly in conjunction with the SIP Center's own SIP resources-the SIP network server and UA). This tool is especially useful for stress testing products with sce- narios that are otherwise difficult to reproduce. This software has been made available by Avaya Inc., founder of the SIP Center [34]. 8.3.2.6 SIPSAK SIPSAK [35] is a small command line tool for developers and administrators of SIP applications. It can be used for some simple tests on SIP applications and devices. 8.3.2.7 Spirent Protocol Tester The Spirent Protocol Tester (SPT) [36] is an advanced integrated network diagnostic and test platform. It is specifically designed to address VoIP sig- naling protocol test needs of equipment and service developers with signaling protocol testing requirements. SPT meets the challenges of VoIP network testing and the demands of high levels of automation, reusability, and scal- ability. SPT provides the development or test engineer with a rich set of signaling protocol