UPF Node Selection
The UPF Selection feature enables the 5GS and EPS core networks to select an UPF for reduced latency on user plane and priority-based serviceability.
The SMF selects an appropriate UPF during the setup of a PDU session. The UPF selection depends on the following query parameters:
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DNN
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Subscriber location
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Network slice information
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PDU session type
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PDU subscription type
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Priority
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Load
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Dual Connectivity with New Radio (DCNR)
When multiple UPFs meet the UPF selection criteria, UPF selection is based on priority and load. For the load metric information, the SMF fetches the Packet Forwarding Control Protocol (PFCP) IE from UPF over N4 interface. If the failure handling support exists and N4 Session Establishment fails, the SMF selects the next least-loaded UPF.
The network operator leverages this functionality for efficient handling of the user plane traffic based on priority, PDU session type, and so on. This functionality is also used for effective load balancing of the user plane connections across multiple UPFs.
In scenarios where multiple UPFs are available for a particular Subscription Permanent Identifier (SUPI), SMF provides the capability to configure multiple UP addresses for each SUPI. The SMF performs UPF selection for a particular PDU session based on the SUPI preferred configuration. For configuration details, see the Configuring UPF Address section.
That is, the SMF checks if any of the configured SUPI values match the current SUPI. If the match is successful, SMF uses information on the available user plane nodes and checks if the IP address matches with any of the values configured for the SUPI. The SMF performs the following validations for UPF selection:
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Check if the UP node is valid and active
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Check if the location-based DNN or the DNN received from service is available in the list of supported DNNs in UP node
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Check if the PDU session type is supported for the configured user plane. For this validation, SMF fetches the UP profile name and UPF group configured within network profile UPF. Then, SMF checks if the UPF group is empty or if the group has the PDU session type that is available in the supported PDU session types.
When all the validations are successful, the SMF skips the existing UPF selection logic involving the query parameters and uses the UPF selected by SUPI. In cases where UPF address is not configured for the SUPI or if the preceding validation checks fail, the SMF uses the default UPF selection mechanism. For co-located UPF selection, the cnSGW-C configuration remains the same as on the SMF.