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 SMF initiates the following messages:
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UE-Connection-Management (UE-CM)
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Nudm_UECM_Registration
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Nudm_UECM_DeRegistration
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UE-Subscription-Management (UE-SDM)
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Nudm_SDM_Get
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Nudm_SDM_Subscribe
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Nudm_SDM_Unsubscribe
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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 NF failure profile, the SMF takes one of the following actions:
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Ignore
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Continue
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Terminate
The SMF provides the following actions to attempt the same request to other available UDM servers.
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retry-and-terminate
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retry-and-ignore
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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. |
Scenario |
Service |
Message |
Condition |
Action |
Success Response |
Handling of Failure Response |
||
---|---|---|---|---|---|---|---|---|
Terminate |
Continue |
Ignore |
||||||
PDU Session Creation procedures in 5G, 4G, and Wi-Fi 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, and Wi-Fi |
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, and Wi-Fi |
SDM |
Nudm_SDM _Unsubscribe |
If the subscription fetch is successful and the registration is not done |
Send the message |
No action |
Terminate call |
Continue call |
Continue call |
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Terminate: The SMF terminates the call in any message type.
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Continue: The SMF ignores the current failure and skips the subsequent interaction for the other messages in the same service group.
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Ignore: The SMF ignores failure only for the current interaction and proceeds with the call. The SMF processes the subsequent message interaction.
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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.
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When the UDM failure handling template is not configured, the default failure handling action is 'Terminate'.