name: LNS
full_form: L2TP Network Server
context: L2TP may be used between UPF and the DN via N6 to carry traffic of a PDU Session, as defined in TS 23.501[ System architecture for the 5G System (5GS) ]  [2]. The corresponding high level end to end signalling flow is described in this clause and further refined in TS 29.561[ 5G System; Interworking between 5G Network and external Data Networks; Stage 3 ]  [63]. For the procedure described below, it is a prerequisite that the UE is already registered to the 5GC and both SMF and UPF support the L2TP feature. NOTE 1: The scenario where the UE sends actual PPP frames/signalling towards the LAC, which involves back and forth message exchanges between the UE and LAC, for example for LCP Negotiation, is not in the scope of the present document. NOTE 2: The UE does not need to be aware of the L2TP procedure. Figure 4.3.2.4-1: Support of L2TP 1. This step is the same as step 1 in clause 4.3.2.2.1 or step 1 in clause 4.3.2.2.2. The PDU session establishment may include in the PCO information authentication information for PAP and/or CHAP. 2. The SMF may determine that an L2TP session is required for the PDU Session based on local configuration (e.g. related with DNN/S-NSSAI). The SMF may retrieve the L2TP Tunnel parameters from the DN-AAA Server, as described in clause 4.3.2.3, or be configured locally with L2TP Tunnel parameters. The L2TP Tunnel parameters may include information such as the LNS addressing information (e.g. IP address or hostname), as defined in TS 29.561[ 5G System; Interworking between 5G Network and external Data Networks; Stage 3 ]  [63]. 3. This step is the same as step 8 in clause 4.3.2.2.1 with the following additions: If L2TP is required for the PDU Session, the SMF selects a UPF supporting L2TP. 4. This step is the same as step 10a in clause 4.3.2.2.1 with the following additions: The SMF requests the UPF to setup an L2TP Session towards the L2TP server (LNS). The SMF may send to the UPF as part of N4 signalling, L2TP Tunnel Information and L2TP Session Information to setup a L2TP session. The L2TP Session Information includes specific information related to the PDU Session, e.g. a Calling Number which may be set to UE's SUPI, the Called Number for the L2TP Session which may be configured to contain the DNN, PAP/CHAP related parameters if included by the UE in PCO in step 1 etc. This information is defined in TS 29.561[ 5G System; Interworking between 5G Network and external Data Networks; Stage 3 ]  [63]. 5. If needed the UPF may decide to setup a new L2TP Tunnel, as described in TS 29.561[ 5G System; Interworking between 5G Network and external Data Networks; Stage 3 ]  [63]. If the UPF decides to use an already existing L2TP Tunnel for the requested PDU Session from the SMF, it directly proceeds with step 6 below. 6. The UPF proceeds with L2TP Session setup towards the LNS, as described in TS 29.561[ 5G System; Interworking between 5G Network and external Data Networks; Stage 3 ]  [63]. If the SMF has requested the UPF to allocate the UE IP address in step 4, the UPF may retrieve the UE IP address from the LNS. 7. This step is the same as step 10b in clause 4.3.2.2.1 with the following additions: The status of the L2TP Session setup is sent by the UPF to the SMF in a N4 Session Establishment Response. This may indicate information provided by the LNS Server for the UE such as the DNS server address, etc. 8. This step is the same as steps 11 - 13 in clause 4.3.2.2.1.
description: The L2TP Network Server (LNS) is a network function that plays a crucial role in setting up and managing L2TP sessions for PDU Sessions in the 5G system. It is responsible for providing L2TP Tunnel parameters, such as IP address or hostname, to the SMF, which then uses this information to setup an L2TP session towards the LNS. The LNS also provides information such as DNS server addresses to the UPF during the L2TP Session setup process.
explanation: The LNS is a network function that is involved in the procedure for supporting L2TP in the 5G system, as described in clause 4.3.2.4 of TS 29.561. It is responsible for providing L2TP Tunnel parameters to the SMF and setting up L2TP sessions with the UPF.
section_title: 4.3.2.4 Support of L2TP
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