5.34.6.1 General

This clause applies only in the case of non-roaming or LBO roaming as control of UL CL/Branching Point in VPLMN is not supported in HR case. It applies for the architectures described in clauses 5.34.4 and 5.34.5

If the local offloading management is applied for the PDU session, see clause 6.10 of TS 23.548 [130] and clause 5.34.11.

When the I-SMF is inserted into a PDU Session, e.g. during PDU Session establishment or due to UE mobility, the I-SMF may provide the DNAI list it supports to the SMF. Based on the DNAI list information received from I-SMF, the SMF may provide the DNAI(s) of interest for this PDU Session for local traffic steering and Local Offloading Management Policy received from PCF that indicates IP range(s) and/or FQDN(s) allowed to be routed to the local part of a DN to the I-SMF e.g. immediately or when a new or updated or removed PCC rule(s) is/are received. The DNAI(s) of interest is derived from PCC rules. The Local Offloading Management Policy received from PCF is sent via SMF to the I-SMF.

The I-SMF is responsible for the insertion, modification and removal of UPF(s) to ensure local traffic steering. The SMF does not need to have access to local configuration or NRF output related with UPF(s) controlled by I-SMF. Based on the DNAI(s) of interest for this PDU Session for local traffic steering and UE location the I-SMF determines which DNAI(s) are to be selected, selects UPF(s) acting as UL CL/BP and/or PDU Session Anchor based on selected DNAI and insert these UPF(s) into the data path of the PDU Session. If the Indication of considering N6 delay is received the I-SMF triggers N6 delay measurement by considering the N6 delay measurement assistance information as part of EAS Deployment Information as described in clause 6.2.3.4 of TS 23.548 [130].

When a UL CL/BP has been inserted, changed or removed, the I-SMF indicates to the SMF that traffic offload have been inserted, updated or removed for a DNAI, providing also the IPv6 prefix that has been allocated if a new IPv6 prefix has been allocated for the PDU Session.

From now on the SMF and I-SMF interactions entail:

• Notifying the SMF with the new Prefix (multi-Homing case): the SMF is responsible of issuing Router Advertisement message. The SMF constructs a link-local address as the source IP address. The Router Advertisement message includes the IPv6 multi-homed routing rules provided to the UE to select the source IPv6 prefix among the prefixes related with the PDU Session according to RFC 4191 [8]. The SMF sends the Router Advertisement message to the UE via the PSA UPF controlled by the SMF.
• N4 interactions related with traffic offloading. The SMF provide N4 information to the I-SMF for how the traffic shall be detected, enforced, monitored in UPF(s) controlled by the I-SMF: the SMF issues requests to the I-SMF containing N4 information to be used for creating / updating /removing PDR, FAR, QER, URR, etc. The N4 information for local traffic offload provided by the SMF to the I-SMF are described in clause 5.34.6.2.
• Receiving N4 notifications related with traffic usage reporting: the I-SMF forwards to the SMF N4 information corresponding to UPF notifications related with traffic usage reporting; the SMF aggregates and constructs usage reports towards PCF/CHF.

NOTE: How the SMF decides what traffic steering and enforcement actions are enforced in UPF(s) controlled by I-SMF is left for implementation.

The I-SMF is responsible of the N4 interface towards the local UPF(s) including:

• the usage of AN Tunnel Info received from the 5G AN via the AMF in order to build PDR and FAR;
• requesting the allocation of the CN Tunnel Info between local UPFs (if needed);
• to control UPF actions when the UP of the PDU Session becomes INACTIVE.
• provide Trace Requirements on the N4 interface towards the UPF(s) it is controlling, using Trace Requirements received from AMF.