Initial Filter Criteria : IFC

Initial Filter Criteria (IFC) :

Service-triggering information is presented in the form of initial filter criteria. Initial

filter criteria describe when an incoming SIP message is further routed to a specific AS. Service-specific data are represented as initial filter criteria.

The main components for understanding the IFC are trigger point and application server.

Trigger point :

The trigger point describes conditions that should be checked to discover whether the

indicated AS should be contacted. The absence of a trigger point will indicate unconditional

triggering to an AS. Each trigger point contains one to multiple instances of the

Service Point Trigger. Service Point Triggers may be linked by means of logical expressions

(AND, OR, NOT).
Service point triggers have following components.

• Request-URI class defines SPT for the Request-URI. Request-URI contains attribute RequestURI.
• SIP Method class defines SPT for the SIP method. SIP Method contains attribute Method which holds the name of any SIP method.
• SIP Header class defines SPT for the presence or absence of any SIP header or for the content of any SIP header. SIP Header contains attribute Header which identifies the SIP Header, which is the SPT, and the Content attribute defines the value of the SIP Header if required.The absence of the Content attribute and ConditionNegated = TRUE indicates that the SPT is the absence of a determined SIP header.
• Session Case class represents an enumerated type, with possible values "Originating", "Terminating_Registered", "Terminating_Unregistered", "Originating_Unregistered", "Originating_CDIV" indicating whether the filter should be used by the S-CSCF handling the Originating, Terminating for a registered end user, Terminating for an unregistered end user, Originating for an unregistered end user, or Originating after Call Diversion services. An originating case refers to when the S-CSCF is serving the calling user. A terminating case refers to when the S-CSCF is serving the called user.
• Session Description Information class defines SPT for the content of any SDP field within the body of a SIP Method. The Line attribute identifies the line inside the session description. Content is a string defining the content of the line identified by Line.

Application Server (AS) :

The Application Server (AS) defines the AS that is contacted if the trigger points are met. The AS may contain information about the default handling of the session if contact with the AS fails. Default handling will either terminate the session or let the session continue based on the information in the initial filter criteria.
Use case : If the contacted AS does not respond, then the S-CSCF follows the default-handling procedure associated with initial filter criteria: that is, either terminate the session or let the session continue based on the information in the filter criteria. If the initial filter criteria do not contain instructions to the S-CSCF regarding the failure to contact the AS, then the S-CSCF will let the call continue as the default behaviour.