5.21.2.3 Procedure for AMF Auto-recovery

In order to try and handle AMF failure in a graceful manner (i.e. without impacting the UE), AMF can either back up the UE contexts in UDSF, or per GUAMI granularity in other AMFs (serving as backup AMF for the indicated GUAMI).

NOTE 1: Frequency of backup is left to implementation.

For deployments without UDSF, for each GUAMI the backup AMF information (in association to the GUAMI) is configured in the AMF. The AMF sends this information to 5G-AN and other CP NFs during the N2 setup procedure or the first (per NF) interaction with other CP NFs.

In the case that an AMF fails and the 5G-AN/peer CP NFs detect that the AMF has failed, or the 5G-AN/peer CP NFs receives notification from another AMF in the same AMF set that this AMF has failed, following actions are taken:

• The OAM deregister the AMF from NRF indicating due to AMF failure.
• 5G-AN marks this AMF as failed and not consider the AMF for selection until explicitly notified.
• For UE(s) in CM-CONNECTED state, 5G-AN considers failure detection or failure notification as a trigger to release the NGAP UE-TNLA-binding(s) with the corresponding AMF for the respective UE(s) while maintaining N3 (user plane connectivity) and other UE context information. For subsequent N2 message, if the backup AMF information of the corresponding failed AMF is not available the 5G-AN should select a different AMF (as in clause 6.3.5) from the same AMF set when the subsequent N2 message needs to be sent for the UE(s). If no other AMF from the AMF set is available, then it can select an AMF (implementation dependent) from the same AMF Region as in clause 6.3.5. If backup AMF information of the corresponding failed AMF is available, the 5G-AN forwards the N2 message to the backup AMF.

NOTE 2: One AMF in the AMF set may be configured to send this failure notification message.

• 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 S-TMSI or GUAMI pointing to an AMF that is marked failed, if the backup AMF information of the corresponding failed AMF is not available the 5G-AN should select a different AMF from the same AMF set and forward the initial NAS message. If no other AMF from the AMF set is available, then it can select an AMF (implementation dependent) from the same AMF Region as in clause 6.3.5. If backup AMF information of the corresponding failed AMF is available, the 5G-AN forwards the N2 message to the backup AMF.
• Peer CP NFs consider this AMF as unavailable while retaining the UE context.
• For the UE(s) that were associated to the corresponding AMF, when the peer CP NF needs to initiate a transaction towards the AMF, if backup AMF information of the corresponding failed AMF is not available, CP NF should select another AMF from the same AMF set and forward the transaction together with the old GUAMI. If neither the backup AMF nor any other AMF from the AMF set is available, then CP NF can select an AMF from the same AMF Region as in clause 6.3.5. If backup AMF information of the corresponding failed AMF is available, the CP NF forwards transaction to the backup AMF. 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.
• When the 5G-AN or CP NFs need to select a different AMF from the same AMF set,
• For deployments with UDSF, any AMF from the same AMF set can be selected.
• For deployments without UDSF, the backup AMF is determined based on the GUAMI of the failed AMF.

Following actions should be taken by the newly selected AMF:

• For deployments with UDSF, when there is a transaction with the UE the newly selected AMF retrieves the UE context from the UDSF using SUPI, 5G-GUTI or AMF UE NGAP ID and it processes the UE message accordingly and updates the 5G-GUTI towards the UE, if necessary.
• For deployments without UDSF, backup AMF (the newly selected AMF), based on the failure detection of the old AMF, instructs peer CP NFs and 5G-AN that the UE contexts corresponding to the GUAMI of the failed AMF is now served by this newly selected AMF. The backup AMF shall not use old GUAMI to allocate 5G-GUTI for UE(s) that are being served by Target AMF. The backup AMF uses the GUAMI to locate the respective UE Context(s).
• When there is a transaction with the UE, the new AMF updates the peer NFs (that subscribed to receive AMF unavailability notification from old AMF) with the new AMF information.
• If the new AMF is aware of a different AMF serving the UE (by implementation specific means) it redirects the uplink N2 signalling to that AMF, or reject the transaction from the peer CP NFs with a cause to indicate that new AMF has been selected. The peer CP NFs may wait for the update from the new AMF and resend the transaction to the new AMF.

NOTE 3: This bullet above addresses situations where 5G-AN node selects an AMF and other CP NFs select an AMF for the UE concurrently. It also addresses the situation where CP NFs select an AMF for the UE concurrently.

NOTE 4: It is assumed that the UE contexts from the old AMF include all event subscriptions with peer CP NFs.

• If the UE is in CM-IDLE state and the new AMF does not have access to the UE context, the new AMF selects one available AMF from the old AMF set as described in clause 6.3.5. The selected AMF retrieves the UE context from the UDSF and provides the UE context to the new AMF. If the new AMF doesn't receive the UE context then the AMF may force the UE to perform Initial Registration.
• If the UE is in CM-CONNECTED state, the new AMF 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.

NOTE 5: The above N2 TNL association selection and AMF management is applied to the selected PLMN.