onnectivity layer comprises devices, such as routers or switches, for backbone and network access (multiple types of net- work access are allowed). The control layer comprises network control servers for managing call or session set-up, modification, and release. Several roles of SIP servers, collectively called Call Session Control Function (CSCF) are used to process SIP signaling packets in the IMS. In the security and user subscrip- tion functions, are the Authentication Center (AuC) and the HSS (Home Sub- scriber Server), which communicate with CSCF in order to provide security © 2009 by Taylor & Francis Group, LLC SIP: Advanced Media Integration IMS application server Service layer SIP AS Parlay AS CAMEL AS IMS core network Control layer (AuC) P-CSCF I-CSCF S-CSCF Connectivity layer Wireline Wireless Terminal FIGURE 2.12 IMS architecture overview. management for subscribers. The interconnections with other network opera- tors and/or other types of networks are handled by border gateways (BGCF). Finally, the service layer comprises application and content servers to execute value-added services for the user. Generic service enablers, as defined in the IMS standard, are implemented as services in a SIP application server, which is the native server in IMS. 2.6 SIP and the Future of the Internet There are efforts of an Internet redesign after thirty years of great success. GENI [24], FIND [25], FIRE [26] are among the major initiatives that are set to address the urgent need of defining research programs for achieving this goal. A need for new architectures and protocols is envisioned. However, it is C 2009 by Taylor & Francis Group, LLC SIP Handbook: Services, Technologies, and Security foreseen that SIP and its extensions accomplishes with the major parts of the expected requirements for signaling and management planes. The content for future Internet will be able to be adapted to the users with respect to their preferences, terminal capabilities, and access networks. The nature of this content will be 3D, in