Section: 4.23.9.0 Overview
Content: The procedures in this clause describe the Addition, Removal and Change of PDU Session Anchor (PSA2), Branching Point or UL CL controlled by I-SMF. They all rely on following principles: 1. When a (new) I-SMF is inserted (e.g. as described in clause 4.23.7 or clause 4.23.11), the I-SMF provides the DNAI list it supports to the SMF. This list is assumed to remain constant during the N16a association between the I-SMF and the SMF for a PDU Session. 2. Based on the DNAI list information received from I-SMF, the SMF may then at any time provide or update the list of DNAI(s) of interest for this PDU Session to I-SMF. This may take place e.g. when the I-SMF provides the DNAI list it supports or when new or updated or removed PCC rule(s) is/are received by the SMF as defined in clause 4.23.6. This list of DNAI(s) of interest for this PDU Session indicates to the I-SMF the list of DNAI(s) candidate for local traffic steering within the PDU Session. An indication of whether Multi-homing is possible is also provided to the I-SMF and the I-SMF uses this information to decide whether multi-homing can be used for the PDU Session. 3. Whenever the I-SMF has inserted or removed or changed a local offload capability the I-SMF invokes a Nsmf_PDUSession_Update Request to indicate to the SMF the list of corresponding DNAI(s). Based on this indication the SMF invokes a Nsmf_PDUSession_Update Request to send the corresponding N4 information to the I-SMF. 4. Then SMF may then at any time invoke a Nsmf_PDUSession_Update Request to send N4 information to the I-SMF. 5. The I-SMF may at any time send a Nsmf_PDUSession_Update Request to forward to the SMF N4 events received from a local UPF; this may e.g. correspond to traffic reporting. 6. When source I-SMF is to be removed from a PDU Session (e.g. at I-SMF change or removal), the SMF issues the N4 information targeting the UL CL/BP (s) and L-PSA(s) controlled by this I-SMF, including requests to release the corresponding N4 Sessions. If the N9 forwarding tunnel to support the EAS session continuity controlled by I-SMF(s) is not established between the source ULCL and the target UL CL, when the Source I-SMF receives a Nsmf_PDUSession_ReleaseSMContext Request from AMF, it initiates a data forwarding timer (if indirect data forwarding applies) before releasing the resources of the PDU Session. When the Source I-SMF has received N4 release from SMF, it releases the UL CL/BP (s) and L-PSA(s) resources either directly if no data forwarding timer is started, or at the expiry of the data forwarding timer. Otherwise if the N9 forwarding tunnel to support the EAS session continuity controlled by I-SMF(s) is established between the source UL CL and target UL CL, the source I-SMF releases the UL CL/BP (s) and L-PSA(s) after no active traffic over the N9 forwarding tunnel. When IPv6 multi-homing is used and an I-SMF is removed, the SMF re-configure the UE to not use the original IP prefix @L-PSA(s).