I’ve just went through a couple of books about IMS. They mostly don’t mention restoration scenarios at all. But for engineers who really work with IMS the rainy-day scenarios are quite important. We have to know ‘what-if’. In this case what happens if the primary S-CSCF is either restarted or it is not available at all. The scenarios come into play quite often because some maintenance downtimes or even occasional failures are unavoidable. But still we should handle them without any real effect on the service. The 3GPP TS 23.380 document specifies a set of standardized procedures for automatic restoration after loss or corruption of data.
Let’s start with the registration procedure. Firstly it can happen the S-CSCF, whose name is returned by HSS (or selected by I-CSCF), is not for some reason available. In that case the I-CSCF sends one more UAR with Authorization Type set to REGISTRATION_AND_CAPABILITIES to the HSS to explicitly request S-CSCF capabilities. After re-assignment of another S-CSCF according to the S-CSCF capabilities, the I-CSCF forwards the REGISTER to the new S-CSCF.