5.21.2.2.2 AMF planned removal procedure without UDSF

An AMF can be taken graciously out of service as follows:

• The AMF can forward registered UE contexts, UE contexts grouped by the same GUAMI value, to target AMF(s) within the same AMF set, including the source AMF name used for redirecting UE's MT transaction. The UE context includes the per AMF Set unique AMF UE NGAP ID. In order for the AMF planned removal procedure to work graciously, 5G-S-TMSI shall be unique per AMF set. If there are ongoing transactions (e.g. N1 procedure) for certain UE(s), AMF forwards the UE context(s) to the target AMF upon completion of an ongoing transaction.
• The AMF deregister itself from NRF indicating due to AMF planned removal.

NOTE 1: It is assumed that the UE contexts from the old AMF include all event subscriptions with peer CP NFs.

NOTE 2: Before removal of AMF the overload control mechanism can be used to reduce the amount of ongoing transaction.

An AMF shall be able to instruct the 5G-AN that it will be unavailable for processing transactions by including GUAMI(s) configured on this AMF and its corresponding target AMF(s). The target AMF shall be able to update the 5G-AN that the UE(s) served by the old GUAMI(s) are now served by target AMF. The target AMF provides the old GUAMI value that the 5G-AN can use to locate UE contexts served by the old AMF. Upon receipt of the indication that an old AMF is unavailable, 5G-AN shall take the following action:

• 5G-AN should mark this AMF as unavailable and not consider the AMF for selection for subsequent N2 transactions until 5G-AN learns that it is available (e.g. as part of discovery results or by configuration). The associated GUAMIs are marked as unavailable.
• During NGAP Setup, the AMF may include an additional indicator that the AMF will rebind or release the NGAP UE-TNLA-binding on per UE-basis. If that indicator is included and the 5G-AN supports timer mechanism, the 5G-AN starts a timer to control the release of NGAP UE-TNLA-binding(s). For the duration of the timer or until the AMF releases or re-binds the NGAP UE-TNLA-binding, the AN does not select a new AMF for subsequent transactions. Upon timer expiry, the 5G-AN releases the NGAP UE-TNLA-binding(s) with the corresponding AMF for the respective UE(s), for subsequent N2 message, the 5G-AN uses GUAMI which points to the target AMF that replaced the old unavailable AMF, to forward the N2 message to the corresponding target AMF(s).

NOTE 3: For UE(s) in CM-CONNECTED state, after indicating that the AMF is unavailable for processing UE transactions and including an indicator that the AMF releases the NGAP UE-TNLA-binding on a per UE-basis, the AMF can either trigger a re-binding of the NGAP UE associations to an available TNLA on a different AMF within the same AMF set or use the NGAP UE-TNLA-binding per UE release procedure defined in TS 23.502 [3] to release the NGAP UE-TNLA-binding on a per UE-basis while requesting the AN to maintain N3 (user plane connectivity) and UE context information.

NOTE 4: The support and the use of timer mechanism in 5G-AN is up to implementation.

If the instruction does not include the indicator, for UE(s) in CM-CONNECTED state, 5G-AN considers this as a request to release the NGAP UE UE-TNLA-binding(s) with the corresponding AMF for the respective UE(s) while maintaining N3 (user plane connectivity) and UE context information. For subsequent N2 message, the 5G-AN uses GUAMI based resolution which points to the target AMF that replaced the old unavailable AMF, to forward the N2 message to the corresponding target AMF(s).

• For UE(s) in CM-IDLE state, when it subsequently returns from CM-IDLE state and the 5G-AN receives an initial NAS message with a 5G S-TMSI or GUAMI, based resolution the 5G-AN uses 5G S-TMSI or GUAMI which points to the target AMF that has replaced the old unavailable AMF and, the 5G-AN forwards N2 message.

An AMF shall be able to instruct other peer CP NFs, subscribed to receive such a notification, that it will be unavailable for processing transactions by including GUAMI(s) configured on this AMF and its corresponding target AMF(s). The target AMF shall update the CP NF that the old GUAMI(s) is now served by target AMF. The old AMF provides the old GUAMI value to target AMF and the target AMF can use to locate UE contexts served by the old AMF. If the CP NFs register with NRF for AMF unavailable notification, then the NRF shall be able to notify the subscribed NFs to receive such a notification (along with the corresponding target AMF(s)) that AMF identified by GUAMI(s) will be unavailable for processing transactions. Upon receipt of the notification that an AMF is unavailable, the other CP NFs shall take the following action:

• Mark this AMF and its associated GUAMI(s) as unavailable while not changing the status of UE(s) associated to this AMF (UE(s) previously served by the corresponding AMF still remain registered in the network) and AMF Set information.
• For the UE(s) that were associated to the corresponding AMF, when the peer CP NF needs to initiate a transaction towards the AMF that is marked unavailable and the old unavailable AMF was replaced by the target AMF, CP NF should forward the transaction together with the old GUAMI to the target AMF(s). If CP NF needs to send a notification to new AMF which is associated with a subscription from the old AMF, the CP NF shall exchange the old AMF information embedded in the Notification Address with the new AMF information and use that Notification Address for subsequent communication.

NOTE 5: If the CP NF does not subscribe to receive AMF unavailable notification (either directly with the AMF or via NRF), the CP NF may attempt forwarding the transaction towards the old AMF and detect that the AMF is unavailable after certain number of attempts. When it detects unavailable, it marks the AMF and its associated GUAMI(s) as unavailable.

The following actions should be performed by the target AMF:

• To allow AMF process ongoing transactions for some UE(s) even after it notifies unavailable status to the target AMF, the target AMF keeps the association of the old GUAMI(s) and the old AMF for a configured time. During that configured period, if target AMF receives the transaction from the peer CP NFs and cannot locate UE context, it rejects the transaction with old AMF name based on that association and the indicated AMF is only used for the ongoing transaction. The peer CP NFs resend the transaction to the indicated AMF only for the ongoing transaction. For subsequent transactions, peer CP NFs should use the target AMF. When the timer is expired, the target AMF deletes that association information.
• When there is a transaction with the UE the target AMF uses SUPI, 5G-GUTI or AMF UE NGAP ID to locate UE contexts and processes the UE transactions accordingly and updates the 5G-GUTI towards the UE, if necessary. For UE(s) in CM-CONNECTED state, it may also update the NGAP UE association with a new AMF UE NGAP ID towards the 5G-AN and replace the GUAMI in the UE context stored at the 5G-AN with the new GUAMI associated with the newly selected AMF if the 5G-GUTI has been updated. The AMF also informs the NG-RAN of the new UE Identity Index Value (derived from the new 5G-GUTI).
• Target AMF shall not use old GUAMI to allocate 5G-GUTI for UE(s) that are being served by Target AMF.