Section: 5.17.5.3 Availability or expected level of a service API
Content: A service related with common north-bound API may become unavailable due to UE being served by a CN node not supporting the service. If the availability or expected level of support of a service API associated with a UE changes, for example due to a mobility between 5GC and EPC, the AF shall be made aware of the change. NOTE 1: If CAPIF is supported and the service APIs become (un)available for the 5GC or EPC network, the AF obtains such information from the CAPIF core function. If the SCEF+NEF receives the subscription request from the AF for the availability or expected level of support of a service API, the SCEF+NEF subscribes a CN Type Change event for the UE or Group of UEs to the HSS+UDM. If the HSS+UDM receives the subscription for CN Type Change event, the HSS+UDM includes the latest CN type for the UE or Group of UEs in the response for the subscription. If the HSS+UDM detects that the UE switches between being served by the MME and the AMF, the CN Type Change event is triggered, and the HSS+UDM notifies the latest CN type for the UE or Group of UEs to the SCEF+NEF. Based on the CN type information, the SCEF+NEF can determine the availability or expected level of support of a given service. The AF will be informed of such information via a subscription/notification service operation. The AF can subscribe for the availability or expected level of support of a service API with report type indicating either One-time report or Continuous report. If there is no CN type information for the UE in the SCEF+NEF, the SCEF+NEF subscribes monitoring event for a new CN Type Change event for the UE or Group of UEs to the HSS+UDM, otherwise, SCEF+NEF determines the CN type locally in the following conditions: - If the AF subscribes with report type indicating One-time report, the SCEF+NEF may consider the Freshness Timer of the latest CN type information for the UE or Group of UEs. The Freshness Timer is a parameter that is configured based on local SCEF+NEF policy. When a subscription request with One-time report type is received the SCEF+NEF checks if there is the latest CN type information received from the HSS+UDM for the indicated UE ID or External Group ID. If the elapsed time for the CN type information since the last reception is less than the Freshness Timer, then the SCEF+NEF may respond to the AF with the latest CN type information in order to avoid repeated query to HSS+UDM. - The SCEF+NEF has established a direct connection with MME or AMF or SMF. When the UE or all members of a Group of UEs are being served by a MME, EPC is determined as CN type. When the UE or all members of a Group of UEs are being served by an AMF, 5GC is determined as CN type. When the UE is registered both in EPC and 5GC, or some members of a Group of UEs are registered in EPC while some members are registered in 5GC, 5GC+EPC is determined as CN type. NOTE 2: If 5GC+EPC is determined as the CN type serving the UE or the group of UEs, the SCEF+NEF determines that service APIs for both 5GC and EPC are available to the UE or the group of UEs.