After the UE has obtained IP connectivity, it can perform the IM registration. To do so, the UE sends the Register information flow to the proxy (Public User Identity, Private User Identity, home network domain name, UE IP address, Instance Identifier, GRUU Support Indication).
Upon receipt of the register information flow, the P-CSCF shall examine the "home domain name" to discover the entry point to the home network (i.e. the I-CSCF). The proxy shall send the Register information flow to the I-CSCF (P-CSCF address/name, Public User Identity, Private User Identity, P-CSCF network identifier, UE IP address). A name-address resolution mechanism is utilised in order to determine the address of the home network from the home domain name. The P-CSCF network identifier is a string that identifies at the home network, the network where the P-CSCF is located (e.g., the P-CSCF network identifier may be the domain name of the P-CSCF network).
The I-CSCF shall send the Cx-Query/Cx-Select-Pull information flow to the HSS (Public User Identity, Private User Identity, P-CSCF network identifier).
The HSS shall check whether the user is registered already. The HSS shall indicate whether the user is allowed to register in that P-CSCF network (identified by the P-CSCF network identifier) according to the User subscription and operator limitations/restrictions if any.
Cx-Query Resp/Cx-Select-Pull Resp is sent from the HSS to the I-CSCF. It shall contain the S-CSCF name, if it is known by the HSS, or the S-CSCF capabilities, if it is necessary to select a new S-CSCF. When capabilities are returned, the I-CSCF shall construct a name from the capabilities returned.
If the checking in HSS was not successful the Cx-Query Resp shall reject the registration attempt.
The I-CSCF, using the name of the S-CSCF, shall determine the address of the S-CSCF through a name-address resolution mechanism. The name-address resolution mechanism is allowed to take the load information of the S-CSCFs (e.g. obtained using network management procedures) into consideration when deciding the address of the S-CSCF. The I-CSCF also determines the name of a suitable home network contact point, possibly based on information received from the HSS. I-CSCF shall then send the register information flow (P-CSCF address/name, Public User Identity, Private User Identity, P-CSCF network identifier, UE IP address to the selected S-CSCF. The home network contact point will be used by the P-CSCF to forward session initiation signalling to the home network.
The S-CSCF shall reject the registration if the number of registered contact addresses for a Public User Identity from the same UE exceeds the limit of simultaneous registrations configured at the S-CSCF. The S-CSCF shall also reject the registration from separate UEs if the allowed number of simultaneous registrations according to the S-CSCF configuration or per subscribed value for a Public User Identity received from the HSS exceeds the limit of simultaneous registrations. The S-CSCF shall store the P-CSCF address/name, as supplied by the visited network. This represents the address/name that the home network forwards the subsequent terminating session signalling to the UE. The S-CSCF shall store the P-CSCF Network ID information.
The S-CSCF shall send Cx-Put/Cx-Pull (Public User Identity, Private User Identity, S-CSCF name) to the HSS.
The HSS shall store the S-CSCF name for that user and return the information flow Cx-Put Resp/Cx-Pull Resp (user information) to the S-CSCF. The user information passed from the HSS to the S-CSCF shall include one or more names/addresses information which can be used to access the platform(s) used for service control while the user is registered at this S-CSCF. The S-CSCF shall store the information for the indicated user. In addition to the names/addresses information, security information may also be sent for use within the S-CSCF.
Based on the filter criteria, the S-CSCF shall send register information to the service control platform and perform whatever service control procedures are appropriate.
The S-CSCF shall return the 200 OK information flow (home network contact information, a GRUU set) to the I-CSCF.
The I-CSCF shall send information flow 200 OK (home network contact information, a GRUU set) to the P-CSCF. The I-CSCF shall release all registration information after sending information flow 200 OK.
The P-CSCF shall store the home network contact information, and shall send information flow 200 OK (a GRUU set) to the UE. The P-CSCF may subscribe to notifications of the status of the IMS Signalling connectivity from PCRF/PCF (see TS 23.203
and TS 23.503
for more details).
If the S-CSCF receives the priority information of the MPS subscribed-UE as a part of user profile from the HSS, the S-CSCF provides the priority information to the P-CSCF and the P-CSCF stores this information for the MPS-subscribed UE.