 so-called Web services. In the case of implementing telephony services, video transfer or real-time communications, called continuous media services, the help of specific protocols is necessary, such as SIP, SDP, RTP, and RTSP. These protocols provide the specific transport and signaling needs of the media content. In this sense, within the New Generation Networks (NGN) scenario, a new set of standards known as IP Multimedia Subsystem (IMS) is gaining popularity. IMS is an evolving architecture for providing voice, video, and other advanced multimedia services to mobile and fixed environments. It promises to have an important impact on the whole of the telecommunications industry, including network providers, applications developers, and end users. Though IMS stan- dards come from 3GPP, they have also been adopted by 3GPP2 and TISPAN (a European organization focused on next generation fixed networks). IMS can be considered the basis for all variants of the next generation network (NGN) envisioned by the traditional telecommunication industry. The key technology behind IMS is SIP, which was chosen as the underlying protocol in many of the important interfaces between elements in an IMS- based network. IMS specification is available from the 3GPP Website [23]. The reason for this choice is that SIP presents several advantages to building new features or services. SIP is simple because it is based on a straightforward request-response interaction model, making life simple and comprehensible for developers. It is extensible because it can set up sessions for any media type. It is also flexible, making it possible to interact with individual protocol messages without breaking anything. Finally, it is well-known because it is based on HTTP and other IETF standards, using many Web-based technologies to develop innovative SIP applications. IMS basically comprises three layers: the Connectivity Layer, the Control Layer, and the Service Layer. Figure 2.12 shows a simplified view of the lay- ered architecture of IMS. The c