The SIP core contains a number of sub-entities responsible for registration, service selection and routing in the signalling control plane.
The SIP core shall be either:
compliant with TS 23.228, i.e. the SIP core is a 3GPP IP multimedia core network subsystem; or
a SIP core, which internally need not comply with the architecture of TS 23.228, but with the reference points that are defined in subclause 7.5.3 (if exposed), compliant to the reference points defined in TS 23.002.
The data related to the functions of the SIP core, e.g. for data for application service selection, the identity of the serving registrar or authentication related information may be provided by the PLMN operator responsible for the bearer plane. In this case, the SIP database that is the source of the data may be part of the HSS. Alternatively, this data may be provided by the MC service provider. In this case, the source of the data may be the MC service provider's SIP database.
The registrar finder functional entity is responsible for:
Identifying the serving registrar / application service selection functional entity. The serving registrar / application service selection functional entity is identified using information provided either by the PLMN operator's own SIP database or the MC service provider's SIP database, and optionally using the PLMN operator's internal information e.g. network topology, registrar availability.
Registrar finder and registrar in the MC service provider domain: registrar finder in the MC service provider's domain uses the information from the MC service provider's SIP database to identify the serving registrar in the MC service provider domain.
Registrar finder and registrar in the PLMN operator domain: registrar finder uses information from PLMN operator's SIP database to identify the serving registrar in the PLMN operator domain.
Registrar finder in PLMN operator domain and registrar in MC service provider domain: registrar finder uses information from the MC service provider's SIP database to identify the serving registrar in the MC service provider domain.
Providing discovery and address resolution, including E.164 numbers.
The registrar / application service selection functional entity provides the following functions:
Registrar function (with integral provision of a location server) and also acts as an inbound proxy (with access to the integral location server), and outbound proxy for all SIP transactions where application service selection is required. It registers the user and maintains the association of the location and identity of the user in a location service. It provides notifications of the registration states.
Supports authentication for identities provided within SIP signalling. Both the registrar (with integral location server) and authentication functions are supported by access either to the public network's own SIP database or the MC service provider's SIP database.
Can provide the application service selection for all SIP transactions, possibly based on application service selection information stored by either the public network's own SIP database or the MC service provider's SIP database.
This functional entity acts as a proxy agent for Diameter messaging as specified in RFC 6733.
The Diameter proxy, when used on the AAA-2 interface, is collocated with the migration management server.
Other instances of the Diameter proxy may also be present in the SIP core / IMS.
The SIP database contains information concerning the SIP subscriptions and corresponding identity and authentication information required by the SIP core, and such information as application service selection.
In deployment scenarios where the PLMN operator provides the SIP core, this database is provided by the HSS.
In deployment scenarios where the MC service provider provides the SIP core, the SIP database may be provided by the MC service provider.
Access to the data residing in the SIP database is restricted to the SIP core entities that are specifically serving the subscriber/user whose data are stored, i.e. registrars and registrar finders can access SIP databases only when they are part of the same trust domain for the data being provided.
The SIP database is responsible for storing the following user related information:
signalling plane user identities: Numbering and addressing information;
signalling plane security information: SIP core access control information for authentication and authorization;
MC service UE Location information at inter-system level: the SIP database supports the user registration, and stores inter-system location information, etc.; and
The SIP database also generates signalling plane security information for mutual authentication, communication integrity check and ciphering.
Based on this information, the SIP database is also responsible to support the call control and session management entities of the SIP core.
The SIP database consists of the following functionalities:
support for control functions of the SIP core such as the Registrar and Registrar finder. This is needed to enable subscriber usage of the SIP core services. This functionality is independent of the access network used to access the SIP core; and
authentication functionality required by the SIP core to authenticate the MC service UE.
The SIP database provides the following logical functions:
provides the UE mobility through the SIP core.
registrar assignment support;
provides to the registrar finder the required capabilities for MC services based on MC service provider requirements on a per-user basis, (e.g. whether a particular registrar within the PLMN operator's network (e.g. a registrar reserved for MC service use or a registrar in a secure location) or a registrar within the MC service provider network is assigned.
call and/or session establishment support;
provides the call and/or session establishment procedures in the SIP core. For terminating traffic, it provides information on which registrar currently hosts the user.
user security information generation;
provides generation of user authentication, integrity and ciphering data for the SIP core.
signalling plane security support;
provides authentication procedures to access MC services by storing the generated data for authentication, integrity and ciphering at the signalling plane and by providing these data to the appropriate registrar.
user identification handling;
provides the appropriate relations among all the identifiers uniquely determining the signalling plane identities in the SIP core e.g. IMS public identities.
access authorisation; and
provides authorisation of the user for mobile access when requested by the registrar e.g. by checking that the user is allowed to roam to that visited network.
service authorisation support.
provides basic authorisation for terminating call/session establishment and service invocation. The SIP database may update the registrar with filter criteria to trigger the MC service server(s).
This functional entity acts as a proxy for hypertext transactions between the HTTP client and one or more HTTP servers. The HTTP proxy terminates a TLS session on HTTP-1 with the HTTP client of the MC service UE allowing the HTTP client to establish a single TLS session for hypertext transactions with multiple HTTP servers that are reachable by the HTTP proxy.
The HTTP proxy terminates the HTTP-3 reference point that lies between different HTTP proxies. It may provide a topology hiding function from HTTP entities outside the trust domain of the MC system.
The HTTP proxy is not used as part of CSC-1 interface. CSC-1 is a direct HTTP interface between the IdM client in the UE and the IdM server as specified in TS 33.180.
According to TS 33.180, either an HTTP proxy or a direct HTTP interface is used between the key management server and the key management client in the UE for CSC-8.
The HTTP proxy shall be in the same trust domain as the HTTP clients and HTTP servers that are located within a MC service provider's network. There can be multiple instances of an HTTP proxy e.g. one per trust domain.