 SCIM with the limited scope of its definition falls short in realizing the full potential of service control in IMS networks. 1.3.1 Limitations in IMS Service Control The primitive service control provided by the Initial Filter Criteria (IFC) in the Call Session Control Function (CSCF) server and the application chaining with SIP AS is fairly a rigid vertical because of the following reasons. The IFC can only match The reason behind the choice of SIP: First, SIP is based on a simple request-response interac- tion model that allows developers to interact with individual protocol messages. Second, SIP can start/manage/tear-down sessions for any media type, be it voice, video, or application sharing. Service Composition and Control initial requests in a session such as INVITE, REGISTER, SUBSCRIBE, MESSAGE, and SO forth. Subsequent messages (e.g., BYE, NOTIFY, re-INVITE) are not checked and filtered by the IFC. This means that IFC alone is not adequate to model and monitor the entire life cycle of a service thus making it incapable of providing greater control to the services. Besides, changing the IFCs is not trivial, hence not flexible enough. The SIP AS chaining is also quite inflexible as the con- trol logic developed in such a chaining application is not easy to change and the interaction with the external capabilities are often hard wired rendering them inflexible in the face of any change in the interaction with the external world. These inflexibilities make the primitive service control in IMS unfit for the next- generation Telecom service and application development. 1.3.2 Service Control Layer Architecture In contrast to the traditional one-size-fits-all model of service control, Service Control Layer (SCL) is a Telecom middleware technology that employs an innova- tive decoupled approach to service delivery. SCL enriches traditional service con- trol on two fronts, namely (1) flexible, dynamic SIP service modeling and (2) transparent, flexible interface with external capabilities or business services. To 