d from the Rules Engine-for exam- ple, the authorized user then initiates session charge. The "to do" list may contain directives to allow list processing to be dynamically changed based on the responses received by the orchestration mechanism from a given business service-for exam- ple, only initiate session charge if the user is authorized to access the service. Responses from calls to business services may be redirected to the Rules Engine for further processing-for example, instruct the SIP protocol adapter to terminate the service session if the user is unauthorized. Scenario (2) provides the SCL with the capability to control a service session either during session establishment (TUA mode) or during its life cycle (B2BUA mode). In the TUA mode, the SCL is able to tear down a service session by responding directly to the originating party-for example, the SCL determines that a user has insufficient credit to establish a media session with another user. In the B2BUA mode, the SCL is able to "transform" a service session into a back-to-back session, effectively acting as a bridging point between service endpoints. This enables the SCL to exercise control over a service session at any stage during its life cycle. For example, the SCL determines that a user who is trying to set up a VoIP session is on a prepaid plan, and SO instructs the SIP protocol adapter to change the service session to B2BUA mode in case the user runs out of credit mid-session. If the user does exhaust her available credit and opts * If the message is not relevant for service control requirement, it is simply forwarded to the destination. Service Composition and Control not to top-up, the SCL is able to generate a SIP BYE message toward all endpoints, effectively tearing down the session. The call to external business capabilities could be complex and a stateful one. Hence, every time the behavior of the external service changes the interaction between the SCL orchestration mechanism and the external service needs to be remodeled and implem