6.3.3.1 Overview

The selection and reselection of the UPF for PDU session establishment, UE mobility or UE traffic offloading are performed by the SMF by considering UPF deployment scenarios such as centrally located UPF and distributed UPF located close to or at the Access Network site and/or optionally Energy-Related NF profile attributes of UPF. The selection of the UPF shall also enable deployment of UPF with different capabilities, e.g. UPFs supporting no or a subset of optional functionalities.

The SMF may be made aware of UPF capabilities (e.g. operator configurable UPF capabilities, NAT information exposure functionality, IP or MAC filter-based packet detection functionality) during N4 association setup procedure or N4 association update procedure as described in clause 4.4.3 of TS 23.502 [3], or the SMF may utilize the NRF to discover UPF(s) that supports those capabilities as described in clause 4.17 of TS 23.502 [3]. The SMF may also be aware of Energy-Related NF profile attributes of UPF configured in the NF profile utilizing the NRF as described in clause 5.51.7.

When the UPF selection for PDU session establishment takes place in home routed roaming case, the UPF(s) in home PLMN is selected by SMF(s) in HPLMN and the UPF(s) in the VPLMN is selected by SMF(s) in VPLMN. The exact set of parameters used for the selection mechanism is deployment specific and controlled by the operator configuration.

The UPF selection for PDU session establishment, UE mobility or UE traffic offloading involves:

• A step of SMF Provisioning of available UPF(s) (details are described in clause 6.3.3.2). This step may take place while there is no PDU Session to establish and is followed by N4 Node Level procedures defined in clause 4.4.3 of TS 23.502 [3] where the UPF and the SMF may exchange information such as the support of optional functionalities and capabilities.
• A step of selection of an UPF for a particular PDU Session (details are described in clause 6.3.3.3) which is followed by N4 session management procedures defined in clause 4.4.1 of TS 23.502 [3].

The selection and reselection of the UPF is also performed by an NF (other than the SMF) in order to collect the data from the UPF as defined in clause 5.8.2.17. In this case, the related dedicated UPF is discovered and selected as follows:

• When the NF consumer or SCP directly subscribes to the UPF (if allowed by the conditions defined in clause 5.8.2.17), the NF consumer or SCP queries the NRF including the related discovery parameters. The NRF returns the UPF(s) which meet(s) the discovery request.
• When the NF consumer or SCP shall subscribe via the SMF, the NF consumer gets the serving SMF information from the UDM per SUPI, DNN and S-NSSAI. After that, the NF consumer sends a subscription to the indicated SMF and the SMF identifies the related UPF(s) using the parameters of the subscription (e.g. target flow description, AoI, etc.) and transfers the related event subscription information to the identified UPF(s). If the NF consumer does not know the SUPI but only the UE IP address, it may need to invoke the BSF to get the SUPI corresponding to the triplet (UE IP address, DNN and S-NSSAI).