hese filters would never be examined on the termination of a session due to a SIP BYE request. The SCSCF retrieves the end user's profile from the HSS when the user registers on the IMS network. This user profile contains the iFCs. It is not necessary to dis- cuss the technical structure of the IFCs in detail; it shall suffice to mention one important field. This is the priority field, which indicates the order in which the iFCs are evaluated. This directly controls the order in which services can be used by the end user. It is possible to have more than one matching iFC for any particular SIP request. The matching iFC with the highest priority determines which AS receives the request. If the first AS does not change the SIP Request-URI or termi- nate the dialog, the request can be forwarded on to the AS with the next highest priority matching iFC. While these priorities can be modified by the network operator, which would require the HSS to send the changed iFCs to the SCSCF, they are considered to be largely static. For example, consider the following scenario. A service chain could include a Destination Barring service, an Advertising service, and another service such as a Video on Demand (VoD) service. The Destination Barring service would block all sessions to a particular destination or service (e.g., block all calls to a black list of numbers). The Advertising service could play an advertisement to the end user, and then finally the end user's request would be served by the VoD service (Figure 8.2). It is clear that the order in which services are triggered is critically important. For example, if a particular AS terminates SIP dialogues and has a very high prior- ity, it will prevent any of the other services being triggered. As the number of iFCs increase (i.e., as the number of services offered by the network or a third party increases) the delay incurred in the establishment of a session increases, as the SIP request may be passed through a chain of services. 8.2.3 Limitations The current mechanisms fo