3GPP 23.501 v20.2.0 — the document's own text

5.6.4.3 Usage of IPv6 multi-homing for a PDU Session

Taught in 12. The connection to a data network (The 5G system architecture, in depth), 5. The connection, and what is promised on it (The 5G system architecture, overview).

A PDU Session may be associated with multiple IPv6 prefixes. This is referred to as multi-homed PDU Session. The multi-homed PDU Session provides access to the Data Network via more than one PDU Session Anchor. The different user plane paths leading to the different PDU Session Anchors branch out at a "common" UPF referred to as a UPF supporting "Branching Point" functionality. The Branching Point provides forwarding of UL traffic towards the different PDU Session Anchors and merge of DL traffic to the UE i.e. merging the traffic from the different PDU Session Anchors on the link towards the UE.

The UPF supporting a Branching Point functionality may also be controlled by the SMF to support traffic measurement for charging, traffic replication for LI and bit rate enforcement (Session-AMBR per PDU Session). The insertion and removal of a UPF supporting Branching Point is decided by the SMF and controlled by the SMF using generic N4 and UPF capabilities. The SMF may decide to insert in the data path of a PDU Session a UPF supporting the Branching Point functionality during or after the PDU Session Establishment, or to remove from the data path of a PDU Session a UPF supporting the Branching Point functionality after the PDU Session Establishment.

Multi homing of a PDU Session applies only for PDU Sessions of IPv6 type. When the UE requests a PDU Session of type "IPv4v6" or "IPv6" the UE also provides an indication to the network whether it supports a Multi-homed IPv6 PDU Session.

The use of multiple IPv6 prefixes in a PDU Session is characterised by the following:

The UPF supporting a Branching Point functionality is configured by the SMF to spread UL traffic between the PDU Session Anchors based on the Source Prefix of the PDU (which may be selected by the UE based on routing information and preferences received from the network).
IETF RFC 4191 [8] is used to configure routing information and preferences into the UE to influence the selection of the source Prefix.

NOTE 1: This corresponds to Scenario 1 defined in IETF RFC 7157 [7] "IPv6 Multi-homing without Network Address Translation". This allows to make the Branching Point unaware of the routing tables in the Data Network and to keep the first hop router function in the PDU Session Anchors.

The multi-homed PDU Session may be used to support make-before-break service continuity to support SSC mode 3. This is illustrated in Figure 5.6.4.3-1.
The multi-homed PDU Session may also be used to support cases where UE needs to access both a local service (e.g. local server) and a central service (e.g. the internet), illustrated in Figure 5.6.4.3-2.
The UE shall use the method specified in clause 4.3.5.3 of TS 23.502 [3] to determine if a multi-homed PDU Session is used to support the service continuity case shown in Figure 5.6.4.3-1, or if it is used to support the local access to DN case shown in Figure 5.6.4.3-2.

Figure 5.6.4.3-1: Multi-homed PDU Session: service continuity case

NOTE 2: It is possible for a given UPF to support both the Branching Point and the PDU Session Anchor functionalities.

Figure 5.6.4.3-2: Multi-homed PDU Session: local access to same DN

NOTE 3: It is possible for a given UPF to support both the Branching Point and the PDU Session Anchor functionalities.