10.13. Re-registration and re-authentication
10.13.1. User-initiated re-registration
Tobias's UE can at any time perform re-registration by sending a new REGISTER request (see Section 10.4) to the network (Figure 10.13). This happens when, say, the registration needs to be refreshed due to expiry of the registration time getting close. As the re-registration is handled in the same way as an initial SIP registration procedure, this is not further described here.
10.13.2. Network-initiated re-authentication
The IMS UE registers its contact information for a time of 600,000 seconds, which means that the binding of the registered public user identities and the physical IP address is kept for around 7 days in the S-CSCF. As user authentication procedures are directly coupled to registration procedures, this would mean that the S-CSCF has no means of re-authenticating the user within this time period. Certain conditions may, nevertheless, make it necessary for the S-CSCF to re-authenticate the UE.
To achieve this, the S-CSCF can reduce the expiry time of the user's registration. Let us assume that Tobias has already been registered for 3 hours and his home operator wants to perform a random re-authentication. The S-CSCF assigned to Tobias will reduce the expiry time of Tobias's registration to 600 seconds (exactly 10 minutes). Up to that moment Tobias's UE is unaware of the reduced registration time and would, therefore, not perform a re-registration, which is needed for re-authentication. ...