SM context transfer is triggered, e.g. by OAM to Old SMF including SUPI, PDU session ID and New SMF ID or SMF set ID. The SMF selection by using SMF set ID not applicable when the IP range is managed by SMF.
[Conditional - depending on current subscription] Old SMF subscribes to events when UE status becomes CM-IDLE or CM-CONNECTED with RRC inactive state (Namf_EventExposure_Subscribe).
[Conditional - depending on the event] The AMF detects the monitored event occurs and sends the event report by means of Namf_EventExposure_Notify message, to Old SMF.
From Old SMF to AMF Nsmf_PDUSession_SMContextStatusNotify (SMF transfer indication, Old SMF ID, New SMF ID or SMF set ID from Step 1, PDU Session ID, SUPI, SM Context ID).
AMF, or SCP if delegated discovery is used, uses New SMF ID or SMF set ID to select New SMF and sends Nsmf_PDUSession_CreateSMContext
request (PDU Session ID, Old SMF ID, SM Context ID in Old SMF, UE location info, Access Type, RAT Type, Operation Type, SMF transfer indication). The same PDU Session ID as received in step 4 is used. If the AMF receives the service request from the UE for the PDU session(s) affected by this procedure the AMF delays the transaction with the SMF until the step 13 completes. If the AMF receives the UE context transfer request from the other AMF due to the UE mobility, the AMF defers the response until the step 13 completes. Also, to void infinite waiting time, the AMF starts a locally configured guard timer upon sending the request to the SMF, and the AMF decides the procedure has failed at expiry of the guard timer.
Either delay or failure of the SM Context transfer may incur timeout or failure in UE procedure(s).
From New SMF to Old SMF SMF Nsmf_PDUSession_ContextRequest request (SM Context type, SM Context ID, SMF transfer indication). If New SMF is not capable to transfer this SM Context (e.g. it is not responsible for the IP range), steps 9 to 12 are skipped.
Old SMF releases the N4 session with the UPF by sending a flag notifying the UPF about the expected re-establishment of the N4 session for the same PDU session. Based on this, if supported, the UPF should delay the release of the N4 session up to step 10.2 to allow for uninterrupted packet handling until the N4 session is re-established by New SMF.
From Old SMF to New SMF Nsmf_PDUSession_ContextRequest response (SM Context or endpoint address where New SMF can retrieve SM Context). The SM Context includes the IP address(es) if the PDU session is of type IPv4, IPv6 or IPv4v6, or the Ethernet MAC address(es) if the PDU session if of type Ethernet as well as the UPF to be selected by New SMF. Old SMF starts a timer to monitor the SMF context transferring process.
[Conditional] If dynamic PCC is used for the PDU Session, New SMF sets up a new policy association towards PCF.
UPF receives a N4 session establishment request for the same PDU session from step 7. The parameters from step 8 and, if applies, step 9 are used.
New SMF performs a full re-establishment of the N4 session, establishing a new N4 session. All information related to the N4 session of Old SMF that is not used by the N4 session of New SMF is removed from UPF if not already done.
New SMF registers to UDM. The information stored at the UDM includes SUPI, SMF identity and the associated DNN and PDU Session ID.
New SMF subscribes to subscription changes for the UE.
From New SMF to AMF: Nsmf_PDUSession_CreateSMContext response. If this response indicates a redirect (e.g. another SMF in the set), the procedure moves to step 5 with the indicated endpoint address as target.
UDM notifies Old SMF that it is deregistered for the PDU Session by sending Nudm_UECM_DeregistrationNotification, optionally including New SMF ID
[Conditional] If 14 was not received and the timer from step 8 expires, Old SMF re-establishes the N4 session. The UPF may for the purpose use the information stored in step 7. In this case, the procedure ends here.
[Conditional] If Nudm_UECM_DeregistrationNotification in step 14 was received, Old SMF removes its policy association with PCF. Any changes to the QoS rules need to be sent to the UE when it becomes active.
Old SMF releases any internal resources corresponding to the indicated PDU session. Subscribers to SMContextStatusNotify for the transferred SM context are notified of the context transfer and optionally of the new location of the transferred SM context.