 to watch videos that are multi- cast before they join the group and leaving clients are not able to continue watching multicast videos after they have left the group. Service Delivery Platforms 9.2.6 Service Orchestration Module The main purpose of this module is to enhance the basic functions provided in the current IMS's S-CSCF (the S-CSCF 'sits' on the path of all signaling messages, and can inspect every message). Under this new arrangement, the SIP-based service orchestration module is the first module to be contacted by the I-CSCF after it is contacted by the user terminal. I-CSCF will also provide the relevant user-related information. It decides both the order and to which application server(s) (both IMS-based and Web Service (WS)-based) messages will be forwarded in order to provide the relevant services. It accomplishes this by sending the messages via the S-CSCF module. There are two objectives to be achieved by this new service orchestration module: 1. Chaining of SIP-based services: This function allows for the setting up of a SIP session that involves two or more application servers (AS). For example, a "Do Not Disturb (DND)" service can be added and placed in front of the "Press to Call (PoC)" service SO that before a PoC service is engaged, an instance of the corresponding session description protocol will be generated. This session description protocol will then be used to direct the setting up of a PoC service with the requirement to first consult the DND service. Depending on the outcome of initializing DND, a PoC may be set up subse- quently. To accomplish this function, the service orchestration component is expected to interact with the S-CSCF according to dynamically generated session description protocol information. 2. SIP- and WS-based service chaining: In contrast to (1), the chaining of services now involves some of those services offered at WS level and others at SIP level. For example, to realize a "blended" service that allows a viewer to answer his call while watching IPTV (Intern