How it Works

The SMF utilizes the NF Failover to achieve the UDM failover support functionality. This section provides information on how the SMF handles message-level failures and the corresponding HTTP status code-based failures.

The UDM failover supports the following messages that are initiated from the SMF.

  • UE-Connection-Management (UE-CM)

    • Nudm_UECM_Registration

    • Nudm_UECM_DeRegistration

  • UE-Subscription-Management (UE-SDM)

    • Nudm_SDM_Get

    • Nudm_SDM_Subscribe

    • Nudm_SDM_Unsubscribe

During the PDU session lifecycle, the SMF exchanges the preceding messages at various stages with the UDM. Depending on the HTTP status code configured in the NRF failure profile, the SMF performs one of the following actions:

  • Ignore

  • Continue

  • Terminate

The SMF provides the following actions to attempt the same request to other available UDM servers.

  • retry-and-terminate

  • retry-and-ignore

  • retry-and-continue

When all the retry attempts fail, the SMF takes the appropriate failure handling action. For example, if the FH action is retry-and-terminate, the SMF terminates the call after all the attempts fail.

Note

The SMF allows dynamic changes to the failure handling template configuration. Any changes to the configuration apply only to the new calls.

Relationship between N10 Messages and Failover Actions

Scenario

Service

Message

Condition

Action

Success Response

Handling of Failure Response

Terminate

Continue

Ignore

PDU Session Creation procedures in 5G, 4G, WiFi

Inter-RAT Handover procedures

UECM

Nudm_

UECM

_Registration

If the Nudm UECM Registration is not done and the access type is not 4G

Send the message

Mark the Registration is successful

Terminate call

Continue call

Continue call

Nudm_UECM

_DeRegistration

If the Nudm UECM Registration is done

Send the message

No action

Terminate call

Terminate call

Terminate call

PDU Session Creation procedures in 5G, 4G, WiFi

SDM

Nudm_SDM_Get

If skipping the subscription fetch config is not enabled

Send the message

Mark the subscription fetch is successful

Terminate call

Continue call

Continue call

Nudm_

SDM

_Subscribe

If the subscription fetch is successful

Send the message

No action

Terminate call

Continue call if the subscription is not done

Continue call if the subscription is not done

PDU Session Release procedures in 5G, 4G, WiFi

SDM

Nudm_SDM

_Unsubscribe

If the subscription fetch is successful and the registration isns

Send the message

No action

Terminate call

Continue call

Continue call

Note
  • Terminate: The SMF terminates the call in any message type.

  • Continue: The SMF ignores the current failure and skips the subsequent interaction for the other messages in the same service group.

  • Ignore: The SMF ignores failure only for the current interaction and proceeds with the call. The SMF processes the subsequent message interaction.

  • Perform UDM subscription fetch only during the session establishment in EPS and NR network.

    If the UDM subscription fetch fails and the FH action is ‘Ignore’ or the configuration to skip subscribe-to-notification is enabled, then the SMF skips the subscribe-to-notification interaction.

  • When the UDM failure handling template is not configured, the default failure handling action is 'Terminate'.