5G Terminal Multiple PDU Session Anchor Definition (2)

5G terminals support the simultaneous establishment of multiple PDU sessions; regarding the uplink in these sessions, 3GPP defines the following in TS23.501:

I. Uplink Classifier: For IPv4, IPv6, IPv4v6, or Ethernet type PDU sessions, the SMF can decide to insert an UL CL (Uplink Classifier) in the data path of the PDU session;

• The UL CL is a function supported by the UPF, designed to locally offload part of the traffic based on traffic filters provided by the SMF.
• UL CL insertion and removal are decided by the SMF and controlled by the SMF using generic N4 and UPF functions.

II. The SMF can decide to insert a UPF supporting UL CL functionality into the PDU session data path during or after PDU session establishment, and can also decide to remove a UPF supporting UL CL functionality from the PDU session data path after PDU session establishment.

• The SMF can include multiple UPFs supporting UL CL functionality in the PDU session data path.
• The UE is unaware of the traffic offloading caused by the UL CL and does not participate in the insertion and removal of the UL CL.

III. UE Handling

• For IPv4, IPv6, or IPv4v6 type PDU sessions, the UE associates the PDU session with a single IPv4 address, a single IPv6 prefix, or both, assigned by the network. When the UL CL function is inserted into the data path of the PDU session, the PDU session will have multiple PDU session anchors. These PDU session anchors provide different access methods to the same DN.
• For IPv4, IPv6, or IPv4v6 type PDU sessions, the UE only obtains one IPv4 address and/or IPv6 prefix. The SMF can configure local policies for certain (DNN, S-NSSAI) combinations so that the PDU session is released when the IPv4 address assigned to the UE is associated with a PSA and that PSA has been removed.

IV. UL CL Application: The current version only supports terminals (UEs) using one IPv4 address and/or IPv6 prefix and configuring multiple PDU session anchors, provided that appropriate mechanisms are deployed to correctly forward packets at the N6 reference point when needed. The R18 specification does not cover the mechanism for packet forwarding between the local access PDU session anchor and the DN over the N6 reference point; where:

• The UL CL provides forwarding of UL traffic to different PDU session anchors and merging of DL traffic to the UE, i.e., merging traffic from different PDU session anchors on the link to the UE. This is based on traffic detection and forwarding rules provided by the SMF.
• The UL CL applies filtering rules (e.g., checking the destination IP address/prefix of UL IP packets sent by the UE) and determines how the packets are routed. The UPF supporting the UL CL can also be controlled by the SMF to support charging traffic measurement, LI traffic replication, and bitrate enforcement (per PDU session AMBR).