5.6.9.3 SSC mode selection

SSC mode selection is done by the SMF based on the allowed SSC modes -including the default SSC mode) in the user subscription as well as the PDU Session type and if present, the SSC mode requested by the UE.

The operator may provision a SSC mode selection policy (SSCMSP) to the UE as part of the URSP rule -see clause 6.6.2 of TS 23.503 [45]). The UE shall use the SSCMSP to determine the type of session and service continuity mode associated with an application or group of applications for the UE as described in clause 6.6.2.3 of TS 23.503 [45]. If the UE does not have SSCMSP, the UE can select a SSC mode based on UE Local Configuration as described in TS 23.503 [45], if applicable. If the UE cannot select a SSC mode, the UE requests the PDU Session without providing the SSC mode.

NOTE 1: The UE can use the SSC Mode Selection component of the URSP rule with match-all traffic descriptor if there is no SSC mode in the UE local configuration.

The SSC mode selection policy rules provided to the UE can be updated by the operator by updating the URSP rule.

The SMF receives from the UDM the list of allowed SSC modes and the default SSC mode per DNN per S-NSSAI as part of the subscription information.

If a UE provides an SSC mode when requesting a new PDU Session, the SMF selects the SSC mode by either accepting the requested SSC mode or rejecting the PDU Session Establishment Request message with the cause value and the SSC mode(s) allowed to be used back to UE based on the PDU Session type, subscription and/or local configuration. Based on that cause value and the SSC mode(s) allowed to be used, the UE may re-attempt to request the establishment of that PDU Session with the SSC mode allowed to be used or using another URSP rule.

If a UE does not provide an SSC mode when requesting a new PDU Session, then the SMF selects the default SSC mode for the data network listed in the subscription or applies local configuration to select the SSC mode.

SSC mode 1 shall be assigned to the PDU Session when static IP address/prefix is allocated to the PDU Session based on the static IP address/prefix subscription for the DNN and S-NSSAI. The SMF shall inform the UE of the selected SSC mode for a PDU Session.

The UE shall not request and the network shall not assign SSC mode 3 for the PDU Session of Unstructured type or Ethernet type.

NOTE 2: To avoid issues for UEs not supporting all SSC modes, the operator can, in the subscription data and local configuration, include at least SSC mode 1 in the allowed SSC modes and set default SSC mode to 1 (since all UEs supporting PDU sessions are mandated to support SSC mode 1). Still the 5GC can trigger PDU session release with a cause code indicating reactivation due to, e.g. restoration or user plane path optimization purposes, though this may cause interruption of the service.