7.1.2 MB-SMF discovery and selection for multicast/broadcast session

23.2473GPPArchitectural enhancements for 5G multicast-broadcast servicesRelease 18TS

To facilitate the MB-SMF discovery/selection for one multicast/broadcast MBS session, the following mechanism is used:

– The MB-SMF registers its capability related to Multicast/Broadcast Service session management (e.g. S-NSSAI(s) and the associated NSI ID(s) (if available), DNN(s), TMGI range, service area) as part of its profile to the NRF by invoking Nnrf_NFManagement_NFRegister. In addition, when a Multicast MBS session is created and the MBS Session ID is not yet included in the MB-SMF profile, the MB-SMF updates its profile towards the NRF with the MB Session ID (i.e. TMGI or source specific IP multicast address).

NOTE: The operator can preconfigure MB-SMF for specific source IP multicast address range or TMGI range.

– When the UE joins the multicast MBS session via PDU session modification procedures, the SMF serving the PDU session invokes the Nnrf_NFDiscovery_Request Request including the MBS Session ID for multicast provided by the UE and optionally other information, i.e. the S-NSSAI and the associated NSI ID (if available), DNN, etc. to query the NRF for MB-SMF information. Based on the MBS Session ID and other information for query, the NRF decides whether an MB-SMF serving the MBS session exists (i.e. the NRF decides whether the requested MBS Session ID is in the profile of an MB-SMF, as defined in clause 7.1.1.2 or clause 7.1.1.3 or preconfigured by the operator). If so, the NRF provides in Nnrf_NFDiscovery_Request Response the information of the MB-SMF currently serving the MBS session. The SMF serving the PDU session selects the MB-SMF currently serving the multicast MBS session, based on the MB-SMF information provided by the NRF. For local MBS services, the SMF takes MB-SMF service area, UE location into account when selecting the MB-SMF. If no MB-SMF serving the Multicast MBS session exists, the NRF does not provide MB-SMF profiles in the response message to the SMF as defined in TS 29.510 [19].

– When the shared tunnel for shared delivery is established as described in clause 7.2.1.4, the AMF may invoke the Nnrf_NFDiscovery_Request Request including MBS Session ID and Area Session ID (if received from the NG-RAN), to query the NRF for the MB-SMF information. In this case, based on the MBS Session ID and Area Session ID, the NRF provides in Nnrf_NFDiscovery_Request Response the MB-SMF information currently serving the MBS session and the Area Session ID.

– When the Multicast MBS Session Context is deleted from the MB-SMF, e.g. due to MBS session release, the MB-SMF updates its profile towards the NRF (i.e. removing the MBS Session ID which is no longer served by the MB-SMF), if the MB-SMF performed NRF registration at multicast MBS Session Creation.

– During MBS session information provisioning procedures defined in clause 7.1.1.2, unless the MB-SMF information is available by other means, e.g. locally configured in the NEF/MBSF/AF, the NEF/MBSF/AF queries the NRF with information of the Multicast/Broadcast Service session (e.g. S-NSSAI(s) and the associated NSI ID(s) (if available), DNN(s)), and selects the MB-SMF(s) based on the MB-SMF information provided by the NRF. For local MBS services, the NEF/MBSF/AF takes MB-SMF service area into account when selecting the MB-SMF(s).