There are hunters and there are gatherers. My wife she is a gatherer. When she goes shopping, she is gathering (and bet that it requires some time 🙂 ). I hate it, I’m hunting. I need to have a clear goal. A list of things I have to buy. It doesn’t need to be on paper, but I need to know. As a gatherer I’m lost and I always forget something.
In IMS most of the network elements work as state machines. Some of their logic is based on the configuration, some is stored in databases (HSS, DNS). This is also the case of Initial Filter Criteria. Simply said the S-CSCF has a list which tells it what messages forward to what application servers.
IFCs are described in the 3GPP TS 29.228, Annex B. As you probably know (Registration, 3rd Party Registration, HSS-FE), during the registration S-CSCF retrieves in the SAA message from HSS the Subscriber profile. The profile itself is an XML document which contains information about the user’s identities and also a set of routing rules – IFCs. Note it is also possible to configure iFCs directly on S-CSCF and transfer only id (Shared IFC).