Editing IP Multimedia Subsystem (section)

== Session handling ==
One of the most important features of IMS, that of allowing for a SIP application to be dynamically and differentially (based on the user's profile) triggered, is implemented as a filter-and-redirect signalling mechanism in the S-CSCF.

The S-CSCF might apply filter criteria to determine the need to forward SIP requests to AS. It is important to note that services for the originating party will be applied in the originating network, while the services for the terminating party will be applied in the terminating network, all in the respective S-CSCFs.

=== Initial filter criteria ===
An '''initial filter criteria''' (iFC) is an [[XML]]-based format used for describing control logic. iFCs represent a provisioned subscription of a user to an application. They are stored in the HSS as part of the IMS Subscription Profile and are downloaded to the S-CSCF upon user registration (for registered users) or on processing demand (for services, acting as unregistered users). iFCs are valid throughout the registration lifetime or until the User Profile is changed.<ref name="29.228"/>

The iFC is composed of:
* Priority - determines the order of checking the trigger.
* Trigger point - logical condition(s) which is verified against initial dialog creating SIP requests or stand-alone SIP requests.
* Application server URI - specifies the application server to be forwarded to when the trigger point matches.

There are two types of iFCs:
* Shared - When provisioning, only a reference number (the shared iFC number) is assigned to the subscriber. During registration, only the number is sent to the CSCF, not the entire XML description. The complete XML will have previously been stored on the CSCF.
* Non-shared - when provisioning, the entire XML description of the iFC is assigned to the subscriber. During registration, the entire XML description is sent to the CSCF.