How it Works
The SMF generates a PDU session ID (pdu-session-id) upon receiving a Create Session Request from the 4G-only UE. The SMF validates if the request has the EPS interworking indication without the PDU session ID in the Protocol Configuration Option. The UDM provides the interworking indication to the SMF per DNN. The SMF does not use this indication for deciding whether the UE is 5G capable.
The SMF generates a pdu-session-id based on Linked EPS Bearer Identity (LBI). For the 4G sessions, the pdu-sesion-id is LBI+64 and for the Wi-Fi sessions, the pdu-session-id is LBI+80.
The SMF allows you to configure the default NSSAI under the profile DNN. The NSSAI is part of sliceInfo IE sent in the Policy Create Request to the PCF during session creation from 4G-only UEs. If the default slice is not configured, then SMF selects one of the configured slices.
When the UE is DCNR capable, and the DCNR is enabled for the session, the SMF considers that the UE is capable of supporting dual connectivity. You can configure DCNR per DNN and other NFs, such as UPF. The S-GW notifies the DCNR support to PGW-C through the UPF Selection Indication Flags IE.