rformance of SIP 22.3 SIP Security References 22.1 Introduction Session Initiation Protocol (SIP) is a new application level protocol which was developed to support the exchange of information that is required for the establishment of media sessions in IP networks. It supports mobility at the application layer through its capability to establish and tear down sessions. Its main function is for creating, establishing and tearing down media sessions between one or more SIP end points. The end points have valid SIP addresses and form part of the information that is exchanged during SIP signaling. The end points may reside either in IPv4 or IPv6 domains, and Application Level Gateways (ALG) have been developed for the translation of the addresses inside the SIP messages. SIP targets the enabling of emerging IP services including Voice over IP (VoIP), Internet telephony, Presence, IPTV and instant messaging (IM). Although SIP does not carry the media itself, it works closely with other protocols such as Real-Time Transport Protocol (RTP), which is used to transport the media, RTP Control Protocol (RTCP) which is used for the management and control of the data stream, Real-Time Streaming Protocol (RTSP), session description protocol (SDP) is used for specifying the param- eters of the media sessions (equivalent to the H.245 Call Control in H.323 Protocol), TCP and UDP. Securing SIP by itself is not easy since each session that is established has to be secured. In a highly mobile network environment when either the terminal or the network is mobile, the problem is compounded. Securing SIP means securing the dynamic links, signaling and data involved. C 2009 by Taylor & Francis Group, LLC SIP Handbook: Services, Technologies, and Security The traditional PSTN is relatively secure and provides a level of quality of service (QoS) that subscribers are used to. Hence when SIP is used in IP networks, subscribers will demand, as a minimum, the same levels of security and QoS. However, SIP has not matured enough to support that