Section: 4.22.9.4 Network Triggered Service Request
Content: The signalling flow for a Network Triggered Service Request is based on the signalling flow in Figure 4.2.3.3-1 with the following differences and clarifications: - In step 2a, the SMF determines over which access (3GPP access or non-3GPP access or both accesses) the user plane resources need to be activated for the MA PDU Session. The SMF may consider Steering Mode to determine the target access. - In step 3a, the SMF indicates to the AMF the access type (3GPP access or non-3GPP access) over which the user plane resources are to be activated for the MA PDU Session. NOTE 1: If the SMF determines to activate both accesses, the SMF performs this step two times, i.e. one for 3GPP access and the other one for non-3GPP access. In the case of DN-AAA or SMF initiated Secondary re-authentication procedure, when the SMF invokes the Namf_Communication_N1N2MessageTransfer service operation, SMF may indicate the target access type of sending N1 NAS message to the UE. - In step 4, the AMF considers the MA PDU Session is associated with the access type the SMF has indicated in step 3a. The AMF determines the access type of which to send the N1 NAS message to the UE based on the target access type value if received from the SMF in step 3a. If the AMF does not receive a target access type value and the UE is CM-CONNECTED in both accesses, the AMF determines the target access type. - In step 5, if the SMF requested to re-activate user-plane resources over 3GPP access and the AMF has determined the UE is unreachable over 3GPP access (e.g. the AMF receives no response from the UE to the Paging), the AMF shall notify that the UE is unreachable. The (H-) SMF shall indicate the Anchor UPF that the user-plane resources on 3GPP access is unavailable by triggering N4 Session Modification procedure. Further action by the UPF is implementation dependent. If the SMF requested to re-activate the user-plane resources over non-3GPP access and the AMF has determined the UE is unreachable over non-3GPP access (e.g. the UE is in CM-IDLE on non-3GPP access), the AMF shall reject the request from the SMF. The (H-) SMF shall indicate the Anchor UPF that the user-plane resources on non-3GPP access is unavailable by triggering N4 Session Modification procedure. Further action by the UPF is implementation dependent. If this procedure is triggered for Secondary Re-authentication and UE and SMF+PGW-C supports for DN authentication and authorization over EPC as described in clause 5.4.4b, SMF+PGW-C selects one access type from non-3GPP access connected to 5GC or 3GPP access connected to EPC in step 3a. If the SMF+PGW-C receives failure indication from the AMF or MME that UE is unreachable then SMF+PGW-C retries by sending it to the other access type. Only when the failure is received from both AMF and MME, then SMF+PGW-C informs the DN-AAA Server that UE is not reachable for re-authentication according to clauses 4.3.2.3 and H.2.1. NOTE 2: The provision of access availability/unavailability reports via user plane specified in clause 5.32.5.3 is UE implementation dependent. Such reporting by UE to UPF, can assist Anchor UPF to decide on handling DL traffic for the UE.