Section: 11.2.1.7 Support L2TP for CUPS across SGi
Content: L2TP (described in IETF RFC 2661 [118]) is a standard method for tunneling encapsulated Point-to-Point Protocol (PPP) frames over an IP network. L2TP operates between two L2TP endpoints (LAC and LNS), and tunnels PPP-encapsulated IP traffic between these endpoints. L2TP runs over UDP/IP and was originally defined for systems where PPP is used by an end-device to connect to a network (e.g. via DSL connections, or 2G/3G PPP PDP context). In these cases, a LAC could be deployed in the network (e.g. in a BNG or GGSN/PGW) to tunnel the PPP traffic to a server (LNS) over an IP network. For CUPS with the UE using IP PDN connection, the PPP functionality that is required to use L2TP is instead supported by the PGW-U, as illustrated in below figure. Upon receiving a PDN connection session establishment request from the UE via MME and SGW-C, PGW-C may depend on local L2TP configuration per APN or the received L2TP information from a DN AAA server in Access-Accept message, request the PGW-U to setup L2TP tunnel towards an L2TP network server (LNS) in the external DN and tunnel the PDN Connection user plane traffic in this L2TP tunnel. In this case the PGW-U acts as a L2TP access concentrator (LAC). To enable this, the PGW-C may provide L2TP tunnel information to the PGW-U as LAC, such as LNS IP address as described in 3GPP TS 29.244[ Interface between the Control Plane and the User Plane nodes ]  [114]. This L2TP tunnel information may be configured on the PGW-C as part of the APN configuration or received from the DN-AAA server. Alternatively, the L2TP tunnel parameters may be configured in the PGW-U. The L2TP tunnel parameters include necessary parameters for setting up L2TP tunnel towards the LNS (e.g. LNS address, tunnel password). In addition, the PGW-C may provide PAP/CHAP authentication information to the PGW-U, for use in L2TP session establishment, in case it was received from the UE in the PCO or ePCO IE of the PDN Connection establishment request message. When L2TP is to be used for a PDN Connection, the PGW-C may select a PGW-U and requests the UE IP address to be allocated by LNS according to 3GPP TS 29.244[ Interface between the Control Plane and the User Plane nodes ]  [114], the PGW-U (LAC) may retrieve this IP address from the LNS. Figure 11f: L2TP Tunnel between CUPS and external DN Below figure describes the L2TP connection procedures between CUPS and external DN, upon the UE is accessed in EPS and the PGW-C and PGW-U has been negotiated supporting L2TP feature. Figure 11g: L2TP connection procedures between CUPS and external DN 0. The PGW-C and the PGW-U negotiated supporting L2TP feature as specified in 3GPP TS 29.244[ Interface between the Control Plane and the User Plane nodes ]  [114]. 1. The PGW-C receives a PDN Connection session establishment request from the UE via MME and SGW-C. The UE may include the authentication information for PAP and/or CHAP in PCO or ePCO IE. The PGW-C may locally configure the UE authentication information for a given APN. The PGW-C may determine that an L2TP session is required for the PDN Connection session based on local configured L2TP parameters per APN. 2. The PGW-C may receive the L2TP Tunnel parameters (e.g. LNS IP address or FQDN, tunnel password) from the DN-AAA server in Access-Accept message or Diameter AAA message, or local configured. 3. If L2TP protocol is determined to support the PDN connection, the PGW-C selects a PGW-U supporting L2TP and be configured with the LAC name/addresses and then requests the PGW-U to setup an L2TP tunnel if needed and/or L2TP session towards the L2TP network server (LNS). The PGW-C sends PFCP Session Establishment Request to the PGW-U, which may include L2TP Tunnel Information for setting up a L2TP tunnel and L2TP session information to setup a L2TP session, together with the information for authentication used during L2TP Tunnel setup, as well as for L2TP session. The L2TP Tunnel Information includes LNS IPv4 address or IPv6 address of LNS, Tunnel Password. The L2TP Session Information includes specific information related to the PDN Connection, e.g. a Calling Number which may be set to UE's MSISDN, an indication to instruct that the PGW-U shall request the LNS to allocate an IP address for the PDN session, indications to instruct that the PGW-U shall request the LNS to provide DNS server addresses or NBNS server addresses etc. as specified in 3GPP TS 29.244[ Interface between the Control Plane and the User Plane nodes ]  [114]. 4. The PGW-U checks if any existing L2TP tunnel can be used to serve the PDN Connection according to the information provided in the L2TP Tunnel Information. If the PGW-U decides to setup a new L2TP tunnel, it initiates L2TP Tunnel establishment by sending an SCCRQ (Start-Control-Connection-Request) message towards the LNS, the PGW-U will allocate a Tunnel ID, and it may include a CHAP Challenge to authenticate the LNS. The Challenge and Challenge Response (to be included in SCCCN) is produced by the PGW-U using the Tunnel Password received from the PGW-C. The LNS responds with an SCCRP (Start-Control-Connection-Reply) message, containing its allocated Tunnel ID and a CHAP Challenge Response to the Challenge in SCCRQ. The PGW-U then responds with a Challenge response for tunnel authentication in the SCCCN (Start-Control-Connection-Connected) message. An L2TP tunnel is established after the tunnel authentication is successful, with the reception of the SCCCN message sent by the PGW-U to the LNS. If the PGW-U decides to use an already existing L2TP tunnel for the requested PDN Connection from the PGW-C, it proceeds with step 5 below directly without current step. 5. Once the L2TP Tunnel is established (or already present) between the PGW-U and the LNS for the PDN Connection requested by the UE, the PGW-U proceeds with L2TP session setup towards the LNS. The PGW-U sends an ICRQ (Incoming-Call-Request) message towards the LNS, which contains the Tunnel ID assigned by the LNS, its assigned Session ID, and optionally, the Calling Number and Called Number. The LNS responds with an ICRP (Incoming-Call-Reply) message and provides the Session ID assigned by it to the LAC. The PGW-U then sends an ICCN (Incoming-Call-Connected) message. If proxy LCP and authentication are employed, the ICCN message includes the link control parameters (e.g. MRU) and UE authentication information sent from the PGW-C which was received via PCO or ePCO IE in step 1. In addition, the PGW-U (LAC) will act as a PPP endpoint to use IPCP to request UE IP Address, DNS server address and/or NBNS server address(es). The LCP renegotiation may by triggered by the LNS after receiving the ICCN message. If so, the LAC and LNS will use PPP LCP to communicate link specific control parameter (e.g. MRU), and indicate authentication type, then either PPP PAP/CHAP takes place. The PPP IPCP transactions takes places to retrieve UE IP Address, DNS server address and/or NBNS server address. 6. The status of the L2TP session setup is sent by the PGW-U to the PGW-C in a PFCP Session Establishment Response. 7. The PGW-C sends a PDU Session Establishment Response to the UE and the user data session is initiated, which may contain the DNS and NBNS Server information.