Content for  TS 23.503  Word version:  18.0.0

The Access Network Discovery & Selection policy is an optional policy that may be provided to UE by the network. In this release of the specification, the Access Network Discovery & Selection policy shall contain only rules that aid the UE in selecting a WLAN access network. Rules for selecting other types of non-3GPP access networks are not specified. The WLAN access network selected by the UE with the use of Access Network Discovery & Selection policy may be used for direct traffic offload (i.e. sending traffic to the WLAN outside of a PDU Session) and for registering to 5GC using the non-3GPP access network selection information. If the UE supports non-3GPP access to 5GC, it shall support ANDSP. The procedure for WLAN access network selection is defined in clause 6.6.1.3, the procedure for N3IWF selection is defined in clause 6.3.6.1 of TS 23.501. The Access Network Discovery & Selection policy shall contain one or more WLAN Selection Policy (WLANSP) rules defined in clause 4.8.2.1.6 of TS 23.402. The Access Network Discovery & Selection policy may contain information to select ePDG or N3IWF by the UE as specified in TS 23.501

The UE, when not operating in SNPN access mode, may be provisioned with multiple valid WLANSP rules (by the HPLMN and by the VPLMN when the UE is roaming). A WLANSP rule is valid if it meets the validity conditions included in the WLANSP rule (if provided). When the UE is in the home the UE uses the valid WLANSP rules from the home PLMN to select an available WLAN. When the UE is roaming and the UE has valid rules from both HPLMN and VPLMN the UE gives priority to the valid WLANSP rules from the VPLMN. The SNPN-enabled UE may be provisioned with multiple valid WLANSP policy by the subscribed SNPN or by the Credential Holder to be used when operating in SNPN access mode. Based on implementation specific procedure the UE selects the WLANSP corresponding to the Credential Holder or SNPN to which the UE wants to connect to. A WLANSP rule is valid if it meets the validity conditions included in the WLANSP rule (if provided).

The UE shall apply the procedure in this clause when the UE is provisioned with WLANSP rules and:

1. when the UE initiates untrusted non-3GPP access to 5GC and attempts to select a WLAN access network; or
2. when the UE initiates trusted non-3GPP access to 5GC by executing the Access Network Selection Procedure specified in clause 6.3.12.2 of TS 23.501 and attempts to select a WLAN access network.

The procedure in this clause applies to both UE operating in SNPN access mode and UE not operating in SNPN access mode, except where stated otherwise. When the UE has valid 3GPP subscription credentials (i.e. a valid USIM or valid SNPN credentials) and WLANSP rules, the UE shall perform WLAN selection based on these rules, the applicable User Preferences On Non-3GPP Access Selection and the corresponding procedures specified in this document. User Preferences On Non-3GPP Access Selection take precedence over the WLANSP rules. The UE determines the most preferred WLAN access network using WLANSP rules when a WLAN access network cannot be selected based on User Preferences On Non-3GPP Access Selection (e.g. when there are no User Preferences On Non-3GPP Access Selection or when there is no user-preferred WLAN access network available). The UE constructs a prioritized list of the available WLANs by discovering the available WLANs and comparing their attributes / capabilities against the groups of selection criteria in the valid WLANSP rule(s). When there are multiple valid WLANSP rules the UE evaluates the valid WLANSP rules in priority order. The UE evaluates first if an available WLAN access meets the criteria of the highest priority valid WLANSP rule. The UE then evaluates if an available WLAN access meets the selection criteria of the next priority valid WLANSP rule. Within a valid WLANSP rule, the WLAN(s) that match the group of selection criteria with the highest priority are considered as the most preferred WLANs, the WLAN(s) that match the group of selection criteria with the second highest priority are considered as the second most preferred WLANs, etc. When a group of selection criteria includes the HomeNetwork attribute and is set, then the UE (a) shall create a list of available WLANs that directly interwork with the home operator (as specified in clause 4.8.2.1.6 of TS 23.402) and (b) shall apply the group of selection criteria to all the WLANs in this list. Otherwise, when the HomeNetwork attribute is not set or is not present, the UE shall apply the group of selection criteria to all available WLANs. The UE may need to perform ANQP procedures (as specified in the HS2.0 Rel-2 specification [ref]) or other procedures in order to discover the attributes / capabilities of the available WLANs. When the UE is roaming (this implies that it is not operating in SNPN access mode) the UE may have valid WLANSP rules from both the VPLMN and the HPLMN. In such a case the UE gives priority to the valid WLANSP rules from the VPLMN. The UE constructs a prioritised list of the available WLANs when the available WLAN accesses meet the selection criteria of the valid rules from the VPLMN and the valid rules from the HPLMN. The prioritised WLAN accesses based on the WLANSP rules from the HPLMN will have lower priority from the prioritised list of WLAN access based on the WLANSP rules of the VPLMN.

The UE Route Selection Policy (URSP) includes a prioritized list of URSP rules.

  The structure of the URSP rules is described in Table 6.6.2.1-2 and Table 6.6.2.1-3.

 

Each URSP rule contains a Traffic descriptor (containing one or more components described in Table 6.6.2.1-2) that determines when the rule is applicable. A URSP rule is determined to be applicable when every component in the Traffic descriptor matches the corresponding information from the application. A URSP rule is determined not to be applicable when for any given component in the Traffic descriptor:

• No corresponding information from the application is available; or
• The corresponding information from the application does not match any of the values in the Traffic descriptor component.

If a URSP rule is provided that contains a Traffic descriptor with two or more components, it is recommended to also provide URSP rule(s) with lower precedence and a Traffic descriptor with less components, in order to increase the likelihood of URSP rule matching for a particular application. Each URSP rule contains a list of Route Selection Descriptors containing one or multiple Route Selection Descriptors each having a different Route Selection Descriptor Precedence value. A Route Selection Descriptor contains one or more of the following components:

• Session and Service Continuity (SSC) Mode: Indicates that the traffic of the matching application shall be routed via a PDU Session supporting the included SSC Mode.
• Network Slice Selection: Indicates that the traffic of the matching application shall be routed via a PDU Session supporting any of the included S-NSSAIs, see clause 5.15.4 in TS 23.501. It includes one or more S-NSSAI(s).
• DNN Selection: Indicates that the traffic of the matching application shall be routed via a PDU Session supporting any of the included DNNs. It includes one or more DNN(s). If a DNN Selection component is provided in the Route Selection Descriptor then the UE shall use any of the DNNs of the DNN Selection component, instead of the DNN requested by the application for the PDU Session that is used to route the traffic of the matching application. If there is no DNN Selection component in the Route Selection Descriptor, then the UE shall use the DNN requested by the application for the PDU Session that is used to route the traffic of the matching application.
• PDU Session Type Selection: Indicates that the traffic of matching application shall be routed via a PDU Session supporting the included PDU Session Type. The possible PDU Session Types are defined in clause 5.6.10 in TS 23.501.
• Non-Seamless Offload indication: Indicates that traffic of the matching application is to be offloaded to non-3GPP access outside of a PDU Session when the rule is applied. If this component is present in a Route Selection Descriptor, no other components shall be included in the Route Selection Descriptor.
• ProSe Layer-3 UE-to-Network Relay Offload indication: Indicates that the traffic of the matching application is to be sent via a ProSe Layer-3 UE-to-Network Relay outside of a PDU Session when the rule is applied. If this indication is absent then the traffic matching of the URSP rule shall not be sent via a ProSe Layer-3 UE-to-Network Relay outside of a PDU Session. If this component is present in a Route Selection Descriptor, no other components shall be included in the Route Selection Descriptor.
• Access Type Preference: If the UE needs to establish a PDU Session when the rule is applied, this indicates the Access Type (3GPP or non-3GPP or multi-access) on which the PDU Session should be established. The type "Multi-Access" indicates that the PDU Session should be established as a MA PDU Session, using both 3GPP access and non-3GPP access.
• PDU Session Pair ID: An indication shared by redundant PDU Sessions as described in clause 5.33.2.1 of TS 23.501.
• RSN: The RSN for redundant PDU Sessions as described in clause 5.33.2.1 of TS 23.501.
• Time Window: The Route Selection Descriptor is not be considered valid unless the UE is in the time window.
• Location Criteria: The Route Selection Descriptor is not be considered valid unless the UE's location matches the Location Criteria.

In the case of network rejection of the PDU Session Establishment Request, the UE may trigger a new PDU Session establishment based on the rejection cause and the URSP policy. When the PCF provisions URSP rules to the UE, one URSP rule with a "match all" Traffic descriptor may be included. The URSP rule with the "match all" Traffic descriptor is used to route the traffic of applications which do not match any other URSP rules and shall therefore be evaluated as the last URSP rule, i.e. with lowest priority. There shall be only one Route Selection Descriptor in this URSP rule. The Route Selection Descriptor in this URSP rule includes at most one value for each Route Selection Component.

For every newly detected application the UE evaluates the URSP rules in the order of Rule Precedence and determines if the application is matching the Traffic descriptor of any URSP rule. When a URSP rule is determined to be applicable for a given application (see clause 6.6.2.1), the UE shall select a Route Selection Descriptor within this URSP rule in the order of the Route Selection Descriptor Precedence. When a valid Route Selection Descriptor is found, the UE determines if there is an existing PDU Session that matches all components in the selected Route Selection Descriptor. The UE compares the components of the selected Route Selection Descriptor with the existing PDU Session(s) as follows:

• For a component which only contains one value (e.g. SSC mode), the value of the PDU Session has to be identical to the value specified in the Route Selection Descriptor.
• For a component which contains a list of values (e.g. Network Slice Selection), the value of the PDU Session has to be identical to one of the values specified in the Route Selection Descriptor.
• When some component(s) is not present in the Route Selection Descriptor, a PDU Session is considered matching only if it was established without including the missing component(s) in the PDU Session Establishment Request.
• When the Route Selection Descriptor includes a Time Window or a Location Criteria, the PDU Session is considered matching only if the PDU Session is associated with an RSD that has the same Time Window or a Location Criteria Validity Conditions.

When a matching PDU Session exists the UE associates the application to the existing PDU Session, i.e. route the traffic of the detected application on this PDU Session. If the UE determines that there is more than one existing PDU Session which matches (e.g. the selected Route Selection Descriptor only specifies the Network Slice Selection, while there are multiple existing PDU Sessions matching the Network Slice Selection with different DNNs), it is up to UE implementation to select one of them to use. If none of the existing PDU Sessions matches, the UE tries to establish a new PDU Session using the values specified by the selected Route Selection Descriptor. If the PDU Session Establishment Request is accepted, the UE associates the application to this new PDU Session. If the PDU Session Establishment Request is rejected, based on the rejection cause, the UE selects another combination of values in the currently selected Route Selection Descriptor if any other value for the rejected component in the same Route Selection Description can be used. Otherwise, the UE selects the next Route Selection Descriptor, which contains a combination of component value which is not rejected by network, in the order of the Route Selection Descriptor Precedence, if any. If the UE fails to establish a PDU Session with any of the Route Selection Descriptors, it tries other URSP rules in the order of Rule Precedence with matching Traffic descriptors, except the URSP rule with the "match-all" Traffic descriptor, if any. The UE shall not use the UE Local Configuration in this case. The UE receives the updated URSP rules and (re-)evaluates their validities in a timely manner when certain conditions are met, for example:

• the URSP is updated by the PCF;
• the UE moves from EPC to 5GC;
• change of Allowed NSSAI or Configured NSSAI;
• change of LADN DNN availability;
• UE registers over 3GPP or non-3GPP access;
• UE establishes a connection with a ProSe Layer-3 UE-to-Network Relay;
• UE establishes connection to a WLAN access.

Details of the conditions are defined by TS 24.526. The Route Selection Descriptor of a URSP rule shall be only considered valid if all of the following conditions are fulfilled:

• If any S-NSSAI(s) is present, the S-NSSAI(s) is in the Allowed NSSAI for the non-roaming case and in the mapping of the Allowed NSSAI to HPLMN S-NSSAI(s) for the roaming case.
• If any DNN is present and the DNN is an LADN DNN, the UE is in the area of availability of this LADN.
• If Access Type preference is present and set to Multi-Access, the UE supports ATSSS.
• If a Time Window is present and the time matches what is indicated in the Time Window.
• If a Location Criteria is present and the UE location matches what is indicated in the Location Criteria.
• If ProSe Layer-3 UE-to-Network Relay Offload indication is present and the UE supports the ProSe capability of 5G ProSe Layer-3 Remote UE.

If a matching URSP rule has no valid RSD, the UE tries other URSP rules in the order of Rule Precedence with matching Traffic descriptors, except the URSP rule with "match-all" Traffic descriptor. The UE shall not use the UE Local Configuration in this case. When URSP rules are updated or their validity according to the conditions above change, the association of existing applications to PDU Sessions may need to be re-evaluated. The UE may also re-evaluate the application to PDU Session association due to the following reasons:

• periodic re-evaluation based on UE implementation;
• an existing PDU Session that is used for routing traffic of an application based on a URSP rule is released;
• The expiration of Time Window in Route Selection Validation Criteria, i.e. the expiration of Time Window, or UE's location no longer matches the Location Criteria.

If the re-evaluation leads to a change of the application to PDU Session association, e.g. the application is to be associated with another PDU Session or a new PDU Session needs to be established, the UE may enforce such changes in a timely manner based on implementation, e.g. immediately or when UE enters CM-IDLE state. If the selected Route Selection Descriptor contains a Non-Seamless Offload indication and the UE has established a connection to a WLAN access, the UE routes the traffic matching the Traffic descriptor of the URSP rule via the WLAN access outside of a PDU Session. If the selected Route Selection Descriptor contains a ProSe Layer-3 UE-to-Network Relay Offload indication and the UE has established a connection with a ProSe Layer-3 UE-to-Network Relay, the UE routes the traffic matching the Traffic descriptor of the URSP rule (including the URSP rule with the "match-all" Traffic descriptor) via the ProSe Layer-3 UE-to-Network Relay outside of a PDU session.