Content for  TS 23.228  Word version:  18.3.0

When a mobile terminated session set-up arrives at an I-CSCF that is authorized to route sessions, the I-CSCF interrogates the HSS for routing information. The mobile terminated sessions for a user shall be routed to a S-CSCF. The Cx reference point shall support retrieval of routing information from HSS to I-CSCF. The resulting routing information is the contact information of S-CSCF.

This clause describes the resolution mechanism, which enables the I-CSCF, the S-CSCF and the AS to find the address of the HSS, that holds the subscriber data for a given user identity when multiple and separately addressable HSSs have been deployed by the network operator. This resolution mechanism is implemented using a Subscription Locator Function (SLF) or a Diameter Proxy Agent that proxies the request to the HSS. This resolution mechanism is not required in networks that utilise a single HSS e.g. optionally, it could be switched off on the I-CSCF and on the S-CSCF and/or on the AS using O&M mechanisms. An example for a single HSS solution is a server farm architecture. By default, the resolution mechanism shall be supported. On REGISTER and on MT INVITEs, the I-CSCF queries the HSS for user's subscription specific data, e. g. the actual location or authentication parameters. This also has to be accomplished by the S-CSCF on REGISTER. In the case when more than one independently addressable HSS is utilized by a network operator, the HSS where user information for a given subscriber is available has to be found. To get the HSS name the I-CSCF and the S-CSCF query the SLF entity or the I-CSCF and the S-CSCF send the query to the HSS via a Diameter Proxy Agent. The SLF is accessed via the Dx interface or via the Dh interface. The Dx interface is the standard interface between the CSCF and the SLF and the Dh interface is the standard interface between the AS and the SLF. The synchronisation between the SLF and the different HSSs is an O&M issue. A way to use the SLF is described in the following. The Dx interface provides:

• an operation to query the SLF from the I-CSCF or from the S-CSCF, respectively.
• a response to provide the HSS name towards the I-CSCF or towards the S-CSCF, respectively.

By sending the Dx-operation DX_SLF_QUERY the I-CSCF or the S-CSCF indicates a user identity of which it is looking for an HSS. By the Dx-operation DX_SLF_RESP the SLF responds with the HSS name. The I-CSCF or the S-CSCF, respectively, continues by querying the selected HSS. The I-CSCF may forward the HSS name towards the S-CSCF. The S-CSCF may use this name to find the subscriber's HSS. Clause 5.8.2 presents the session flows on REGISTER and clause 5.8.3 on INVITE messages. The Dh interface provides:

• an operation to query the SLF from the AS.
• a response to provide the HSS name towards the AS.

By sending the Dh-operation DH_SLF_QUERY the AS indicates a Public User Identity of which it is looking for an HSS. By the Dh-operation DH_SLF_RESP the SLF responds with the HSS name. The AS continues by querying the selected HSS. The AS may store the HSS name for the subsequent Sh-operations. Clause 5.8.4 presents the message flow on the Dh interface.

To prevent an SLF service failure e.g. in the event of a server outage, the SLF could be distributed over multiple servers. Several approaches could be employed to discover these servers. An example is the use of the DNS mechanism in combination with a new DNS SRV record. The specific algorithm for this however does not affect the basic SLF concept and is outside the scope of this document.

The flow shown below is where the AS queries the SLF to identify the HSS to access.