ABSTRACT
Session Initiation Protocol (SIP [1]) has become the quasi-standard for Voiceover-Internet Protocol (VoIP) communications. As an IETF protocol for session signalling in general, SIP is, for instance, adopted for session management in the Third Generation Partnership Project (3GPP) [2], which aims at providing a next generation infrastructure for mobile phone systems. SIP is based on a client-server infrastructure in which user agents represent the end-terminals as clients, proxy servers handle SIP message routing between the user agents, and registrar servers store the client’s contact information into a location service.
