3GPP 23.501 v20.2.0 — the document's own text
5.8.2.16 Support for L2TP tunnelling on N6
Taught in 13. Where the packets actually go (The 5G system architecture, in depth), 5. The connection, and what is promised on it (The 5G system architecture, overview).
If requested by the SMF during N4 Session Establishment, the UPF (PSA) may setup L2TP towards an L2TP network server (LNS) in the DN and tunnel the PDU Session user plane traffic in this L2TP tunnel. In this case the UPF acts as a L2TP access concentrator (LAC).
To enable this, the SMF may provide L2TP information to the UPF, such as LNS IP address and/or LNS host name, as described in TS 29.244 [65]. This L2TP information may be configured on the SMF as part of the DNN configuration or received from the DN-AAA Server during secondary authentication/authorization, as described in clause 5.6.6. Alternatively, the L2TP tunnel parameters may be configured in the UPF Function. The L2TP tunnel parameters include necessary parameters for setting up L2TP tunnel towards the LNS (e.g. LNS address, tunnel password, etc.).
In addition, the SMF may provide PAP/CHAP authentication information to the UPF, for use in L2TP session establishment, in case it was received from the UE in the PDU Session Establishment Request.
When L2TP is to be used for a PDU Session, the SMF may select a UPF based upon support of this feature. The SMF determines whether the UPF supports this feature via N4 capability negotiation during N4 Association Setup or via NRF discovery.
If SMF requests the UPF to allocate UE IP address, as described in clause 5.8.2.2.1, the UPF (LAC) may retrieve this IP address from the LNS. In addition, if the SMF requests the UPF to provide DNS address(es), the UPF (LAC) may request the LNS to provide DNS address(es) and report such DNS address(es) to the SMF.