The Proxy-CSCF discovery shall be performed using one of the following mechanisms:

• As part of the establishment of connectivity towards the IP-Connectivity Access Network, if the IP-Connectivity Access Network provides such means.
• Alternatively, the P-CSCF discovery may be performed after the IP connectivity has been established. To enable P-CSCF discovery after the establishment of IP connectivity, the IP-Connectivity Access Network shall provide the following P-CSCF discovery option to the UE:
  • Use of DHCP to provide the UE with the domain name and/or IP address of a Proxy-CSCF and the address of a Domain Name Server (DNS) that is capable of resolving the Proxy-CSCF name, as described below in clause 5.1.1.1.
• The UE may be configured (e.g. during initial provisioning or via a 3GPP IMS Management Object (MO), TS 24.167 or in the ISIM, TS 31.103) to know the fully qualified domain name (FQDN) of the P-CSCF or its IP address. If the domain name is known, DNS resolution is used to obtain the IP address.

If DNS is used to obtain the IP address of the P-CSCF, the name-address resolution mechanism is allowed to take the load information of the P-CSCFs (e.g. obtained using network management procedures) into consideration when deciding the address of the P-CSCF for the UE. In the case where UE is aware of more than one P-CSCF address, the selection shall be based on home operator configured policy to select the P-CSCF.

The DHCP relay agent within the IP-Connectivity Access Network relays DHCP messages between UE and the DHCP server.

After reception of domain name and IP address of a P-CSCF the UE may initiate communication towards the IM subsystem.

When a UE attaches and makes itself available for access to IMS services by explicitly registering in the IMS, a S-CSCF shall be assigned to serve the UE. The assignment of an S-CSCF is performed in the I-CSCF. The following information is needed in the selection of the S-CSCF:

1. Required capabilities for user services
   This information is provided by the HSS.
2. Operator preference on a per-user basis
   This information is provided by the HSS.
3. Capabilities of individual S-CSCFs in the home network
   This is internal information within the operator's network. This information may be used in the S-CSCF selection. This information is obtained by the I-CSCF by methods not standardised in this release.
4. Topological (i.e. P-CSCF) information of where the user is located
   This is internal information within the operator's network. This information may be used in the S-CSCF selection. The P-CSCF name is received in the registration request. The topological information of the P-CSCF is obtained by the I-CSCF by methods not standardised in this Release.
5. Topological information of where the S-CSCF is located
   This is internal information within the operator's network. This information may be used in the S-CSCF selection. This information is obtained by the I-CSCF by methods not standardised in this release.
6. Availability of S-CSCFs

This is internal information within the operator's network. This information may be used in the S-CSCF selection. This information is obtained by the I-CSCF by methods not standardised in this release. In order to support the S-CSCF selection described above and to allow the S-CSCF to perform its tasks, it is required that the following types of information be transferred between the CSCF and the HSS:

1. The Cx reference point shall support the transfer of CSCF-UE security parameters from HSS to CSCF.
   • This allows the CSCF and the UE to communicate in a trusted and secure way (there is no a priori trust relationship between a UE and a CSCF)
   • The security parameters can be for example pre-calculated challenge-response pairs, or keys for an authentication algorithm, etc.
2. The Cx reference point shall support the transfer of service parameters of the subscriber from HSS to CSCF.
   • This may include e.g. service parameters, Application Server address, triggers, information on subscribed media etc. The information on subscribed media is provided in the form of a profile identifier; details of the allowed media parameters associated with the profile identifier are configured in the S-CSCF.
3. The Cx reference point shall support the transfer of CSCF capability information from HSS to CSCF.
   • This may include e.g. supported service set, protocol version numbers etc.
4. The Cx reference point shall support the transfer of session signalling transport parameters from CSCF to HSS. The HSS stores the signalling transport parameters and they are used for routing mobile terminated sessions to the Serving-CSCF.
   • The parameters may include e.g. IP-address and port number of CSCF, transport protocol etc.

The information mentioned in items 1 - 4 above shall be transferred before the CSCF is able to serve the user. It shall also be possible to update this information while the CSCF is serving the user, for example if new services are activated for the user.

Cancellation of the assigned Serving-CSCF is either:

• Initiated from the Serving-CSCF itself, e.g. due to timeout of the registration
• Performed as a result of an explicit deactivation/de-registration from the IMS. This is triggered by the UE.
• Performed due to a request from the HSS over the Cx interface, e.g. due to changes in the subscription.

Whenever a modification has occurred in the subscription data that constitutes the data used by the S-CSCF, the complete subscription data set shall be sent to the S-CSCF by the HSS. HSS shall use the Push model for downloading the subscription data to the S-CSCF.

This clause provides the information flows for subscription data updating procedure.