The last time we went through the registration in the IMS. During the registration S-CSCF (the SIP Server handling a particular subscriber) authenticates the subscriber, learns her current point of presence, capabilities of her device, etc. But when it comes to multimedia sessions, S-CSCF can offer only some basic functions such as session routing or session management. For VoLTE or RCS service profiles we want to apply the service logic provided by Application Servers (TAS, RCS, IPSMGW..). The purpose of the Third party registration is to let the ASs know that the user is now connected and ready to communicate.
We ended up in the situation when a registrar (S-CSCF) authenticated the user. After the successful authentication S-CSCF downloads a user’s profile from the HSS. The profile contains information about what Application Servers (ASs) shall be triggered on behalf of the subscriber. The information is stored in a form of initial filter criteria (iFC).