6.3.3.3 Selection of an UPF for a particular PDU Session

The following parameter(s) and information may be considered by the SMF for UPF selection and re-selection:

• UPF's dynamic load.
• Analytics (i.e. statistics or predictions) for UPF load, Service Experience analytics and/or DN Performance analytics per UP path (including UPF and/or DNAI and/or AS instance) and UE related analytics (UE mobility, UE communication and expected UE behavioural parameters) as received from NWDAF (see TS 23.288 [86]), if NWDAF is deployed.
• UPF's relative static capacity among UPFs supporting the same DNN.
• UPF location available at the SMF.
• UE location information.
• Capability of the UPF and the functionality required for the particular UE session: An appropriate UPF can be selected by matching the functionality and features required for an UE.
• Data Network Name (DNN).
• PDU Session Type (i.e. IPv4, IPv6, IPv4v6, Ethernet Type or Unstructured Type) and if applicable, the static IP address/prefix.
• SSC mode selected for the PDU Session.
• UE subscription profile in UDM.
• DNAI as included in the PCC Rules and described in clause 5.6.7.
• Local operator policies.
• S-NSSAI.
• Access technology being used by the UE.
• Information related to user plane topology and user plane terminations, that may be deduced from:
• 5G-AN-provided identities (e.g. CellID, TAI), available UPF(s) and DNAI(s);
• Identifiers (i.e. a FQDN and/or IP address(es)) of N3 terminations provided by a W-AGF or a TNGF or a TWIF;

NOTE 1: A W-AGF or a TNGF may provide Identifiers of its N3 terminations when forwarding over N2 uplink NAS signalling to the 5GC. The AMF may relay this information to the SMF, as part of session management signalling for a new PDU Session.

• Information regarding the user plane interfaces of UPF(s). This information may be acquired by the SMF using N4;
• Information regarding the N3 User Plane termination(s) of the AN serving the UE. This may be deduced from 5G-AN-provided identities (e.g. CellID, TAI);
• Information regarding the N9 User Plane termination(s) of UPF(s) if needed;
• Information regarding the User plane termination(s) corresponding to DNAI(s).
• RSN, support for redundant GTP-U path or support for redundant transport path in the transport layer (as in clause 5.33.2) when redundant UP handling is applicable.
• Information regarding the ATSSS Steering Capability of the UE session (e.g. any combination of ATSSS-LL capability, MPTCP capability, MPQUIC-UDP capability, MPQUIC-IP capability and MPQUIC-E capability) and information on the UPF support of RTT measurements without PMF.
• Support for UPF allocation of IP address/prefix.
• Support of the IPUPS functionality, specified in clause 5.8.2.14.
• Support for High latency communication (see clause 5.31.8).
• Support for functionality associated with high data rate low latency services, eXtended Reality (XR) and interactive media services, specified in clause 5.37 (for example, ECN marking for L4S, specified in clause 5.37.3, PDU Set Marking, specified in clause 5.37.5, UE power saving management, specified in clause 5.37.8, Data Burst Size Marking, specified in clause 5.37.10.1, Time To Next Data Burst Marking, specified in clause 5.37.10.2, PDU Set Importance based transport level packet marking, specified in clause 5.8.2.7).
• Support for the On-path N6 signalling methods, e.g. MoQ relay functionality and/or CONNECT-UDP HTTP client functionality and/or UDP option functionality (as described in clause 5.37.9)
• User Plane Latency Requirements within AF request (see clause 5.6.7.1 and clause 6.3.6 of TS 23.548 [130]).
• List of supported Event ID(s) for exposure of UPF-related information via service based interface (see clause 7.2.29 and clause 5.2.26.2 of TS 23.502 [3]).
• Information regarding required and/or preferred UPF functionalities (see clause 5.2.3.3.1 of TS 23.502 [3]). If received from UDM, the SMF selects a PSA UPF supporting all the required UPF functionalities and the best set of preferred functionalities, based on their priorities if received.
• Support of NAT information exposure functionality.
• Support of IP or MAC filter-based packet detection functionality.
• Supported operator configurable UPF capabilities as described in clause 5.8.2.21.
• N6 delay between the candidate UPF(s) and measurement endpoint in the DN as specified in clause 5.8.2.23.
• Delay between the 5G AN and candidate UPF(s) as part of user plane latency (see clause 5.33.3.1).

NOTE 2: Latencies between 5GC AN and UPF(s) can be configured. If QoS monitoring has previously been performed, the delays of such measurements can also be used.

NOTE 3: How the SMF determines information about the user plane network topology from information listed above and what information is considered by the SMF, is based on operator configuration.

NOTE 4: In this release the SMF uses no additional parameters for UPF selection for a PDU Session serving TSC or Deterministic Networking. If a PDU Session needs to connect to a specific UPF hosting a specific TSN 5GS bridge or 5GS router, this can be achieved e.g. by using a dedicated DNN/S-NSSAI combination.

• Energy-Related NF profile attributes of UPF as described in clause 5.51.7.

If there is an existing PDU Session and the SMF receives another PDU Session request to the same DNN and S-NSSAI and if the SMF determines that interworking with EPC is supported for this PDU Session (as specified in clause 4.11.5 of TS 23.502 [3]), the SMF should select the same UPF if it supports all capabilities required for the new PDU Session. Otherwise, if the SMF determines that interworking with EPC is not supported for the new PDU Session or the UPF of the existing PDU Session does not support all capabilities required for the new PDU Session, a different UPF may be selected according to operator policy.

For the same DNN and S-NSSAI if different UPFs are selected at 5GC, when the UE is moved to EPC network, there is no requirement to enforce APN-AMBR. Whether and how to apply APN-AMBR for the PDN Connection associated with this DNN/APN is implementation dependent, e.g. possibly only AMBR enforcement per PDU Session applies.