6.3.3 User Plane Function Selection

23.5013GPPRelease 18System architecture for the 5G System (5GS)TS

6.3.3.1 Overview

The selection and reselection of the UPF 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. 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.

For 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 involves:

– a step of SMF Provisioning of available UPF(s). This step may take place while there is no PDU Session to establish and may be 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; it is followed by N4 session management procedures defined in clause 4.4.1 of TS 23.502 [3].

6.3.3.2 SMF Provisioning of available UPF(s)

SMF may be locally configured with the information about the available UPFs, e.g. by OA&M system when UPF is instantiated or removed.

NOTE 1: UPF information can be updated e.g. by OA&M system any time after the initial provisioning, or UPF itself updates its information to the SMF any time after the node level interaction is established.

The UPF selection functionality in the SMF may optionally utilize the NRF to discover UPF instance(s). In this case, the SMF issues a request to the NRF that may include following parameters: DNN, S-NSSAI, SMF Area Identity, ATSSS steering capabilities. In its answer, the NRF provides the NF profile(s) that include(s) the IP address(es) or the FQDN of the N4 interface of corresponding UPF instance(s) to the SMF.

UPFs may be associated with an SMF Area Identity in the NRF. This allows limiting the SMF provisioning of UPF(s) using NRF to those UPF(s) associated with a certain SMF Area Identity. This can e.g. be used in the case that an SMF is only allowed to control UPF(s) configured in NRF as belonging to a certain SMF Area Identity.

The NRF may be configured by OAM with information on the available UPF(s) or the UPF instance(s) may register its/their NF profile(s) in the NRF. This is further defined in clause 4.17 of TS 23.502 [3].

To collect the data from the UPF as defined in clause 5.8.2.17, the related UPF is discovered and selected as following:

– When the NF consumer may directly subscribe to the UPF, the NF consumer queries the NRF including the related discovery parameter. The NRF returns the UPF(s) which meet(s) the discovery request.

– When the NF consumer shall subscribe via the SMF, the NF consumer gets the serving SMF information from the UDM per SUPI, or IP address, DNN and S-NSSAI. After that, the NF consumer sends a subscription to the indicated SMF and the SMF identities 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).

6.3.3.3 Selection of an UPF for a particular PDU Session

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, otherwise, if the SMF determines that interworking with EPC is not supported for the new PDU Session, a different UPF may be selected.

For the same DNN and S-NSSAI if different UPF 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.

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;

– 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 (ATSSS-LL capability, MPTCP capability, or both) 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).

– User Plane Latency Requirements within AF request (see clause 5.6.7.1 and clause 6.3.6 of TS 23.548 [130]).

NOTE 1: 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 2: In this release the SMF uses no additional parameters for UPF selection for a PDU Session serving TSC. If a PDU Session of a specific DS-TT needs to connect to a specific UPF hosting a specific TSN 5GS bridge, this can be achieved e.g. by using a dedicated DNN/S-NSSAI combination.

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.