.. SPDX-License-Identifier: GPL-2.0
  
  ====
  L2TP
  ====

  Layer 2 Tunneling Protocol (L2TP) allows L2 frames to be tunneled over
  an IP network.
  
  This document covers the kernel's L2TP subsystem. It documents kernel
  APIs for application developers who want to use the L2TP subsystem and
  it provides some technical details about the internal implementation
  which may be useful to kernel developers and maintainers.
  
  Overview

  ========

  The kernel's L2TP subsystem implements the datapath for L2TPv2 and
  L2TPv3. L2TPv2 is carried over UDP. L2TPv3 is carried over UDP or
  directly over IP (protocol 115).
  
  The L2TP RFCs define two basic kinds of L2TP packets: control packets
  (the "control plane"), and data packets (the "data plane"). The kernel
  deals only with data packets. The more complex control packets are
  handled by user space.
  
  An L2TP tunnel carries one or more L2TP sessions. Each tunnel is
  associated with a socket. Each session is associated with a virtual
  netdevice, e.g. ``pppN``, ``l2tpethN``, through which data frames pass
  to/from L2TP. Fields in the L2TP header identify the tunnel or session
  and whether it is a control or data packet. When tunnels and sessions
  are set up using the Linux kernel API, we're just setting up the L2TP
  data path. All aspects of the control protocol are to be handled by
  user space.
  
  This split in responsibilities leads to a natural sequence of
  operations when establishing tunnels and sessions. The procedure looks
  like this:
  
      1) Create a tunnel socket. Exchange L2TP control protocol messages
         with the peer over that socket in order to establish a tunnel.
  
      2) Create a tunnel context in the kernel, using information
         obtained from the peer using the control protocol messages.
  
      3) Exchange L2TP control protocol messages with the peer over the
         tunnel socket in order to establish a session.
  
      4) Create a session context in the kernel using information
         obtained from the peer using the control protocol messages.
  
  L2TP APIs
  =========
  
  This section documents each userspace API of the L2TP subsystem.
  
  Tunnel Sockets
  --------------
  
  L2TPv2 always uses UDP. L2TPv3 may use UDP or IP encapsulation.
  
  To create a tunnel socket for use by L2TP, the standard POSIX
  socket API is used.
  
  For example, for a tunnel using IPv4 addresses and UDP encapsulation::
  
      int sockfd = socket(AF_INET, SOCK_DGRAM, IPPROTO_UDP);
  
  Or for a tunnel using IPv6 addresses and IP encapsulation::
  
      int sockfd = socket(AF_INET6, SOCK_DGRAM, IPPROTO_L2TP);
  
  UDP socket programming doesn't need to be covered here.
  
  IPPROTO_L2TP is an IP protocol type implemented by the kernel's L2TP
  subsystem. The L2TPIP socket address is defined in struct
  sockaddr_l2tpip and struct sockaddr_l2tpip6 at
  `include/uapi/linux/l2tp.h`_. The address includes the L2TP tunnel
  (connection) id. To use L2TP IP encapsulation, an L2TPv3 application
  should bind the L2TPIP socket using the locally assigned
  tunnel id. When the peer's tunnel id and IP address is known, a
  connect must be done.
  
  If the L2TP application needs to handle L2TPv3 tunnel setup requests
  from peers using L2TPIP, it must open a dedicated L2TPIP
  socket to listen for those requests and bind the socket using tunnel
  id 0 since tunnel setup requests are addressed to tunnel id 0.
  
  An L2TP tunnel and all of its sessions are automatically closed when
  its tunnel socket is closed.
  
  Netlink API
  -----------
  
  L2TP applications use netlink to manage L2TP tunnel and session
  instances in the kernel. The L2TP netlink API is defined in
  `include/uapi/linux/l2tp.h`_.
  
  L2TP uses `Generic Netlink`_ (GENL). Several commands are defined:
  Create, Delete, Modify and Get for tunnel and session
  instances, e.g. ``L2TP_CMD_TUNNEL_CREATE``. The API header lists the
  netlink attribute types that can be used with each command.
  
  Tunnel and session instances are identified by a locally unique
  32-bit id.  L2TP tunnel ids are given by ``L2TP_ATTR_CONN_ID`` and
  ``L2TP_ATTR_PEER_CONN_ID`` attributes and L2TP session ids are given
  by ``L2TP_ATTR_SESSION_ID`` and ``L2TP_ATTR_PEER_SESSION_ID``
  attributes. If netlink is used to manage L2TPv2 tunnel and session
  instances, the L2TPv2 16-bit tunnel/session id is cast to a 32-bit
  value in these attributes.
  
  In the ``L2TP_CMD_TUNNEL_CREATE`` command, ``L2TP_ATTR_FD`` tells the
  kernel the tunnel socket fd being used. If not specified, the kernel
  creates a kernel socket for the tunnel, using IP parameters set in
  ``L2TP_ATTR_IP[6]_SADDR``, ``L2TP_ATTR_IP[6]_DADDR``,
  ``L2TP_ATTR_UDP_SPORT``, ``L2TP_ATTR_UDP_DPORT`` attributes. Kernel
  sockets are used to implement unmanaged L2TPv3 tunnels (iproute2's "ip
  l2tp" commands). If ``L2TP_ATTR_FD`` is given, it must be a socket fd
  that is already bound and connected. There is more information about
  unmanaged tunnels later in this document.
  
  ``L2TP_CMD_TUNNEL_CREATE`` attributes:-
  
  ================== ======== ===
  Attribute          Required Use
  ================== ======== ===
  CONN_ID            Y        Sets the tunnel (connection) id.
  PEER_CONN_ID       Y        Sets the peer tunnel (connection) id.
  PROTO_VERSION      Y        Protocol version. 2 or 3.
  ENCAP_TYPE         Y        Encapsulation type: UDP or IP.
  FD                 N        Tunnel socket file descriptor.
  UDP_CSUM           N        Enable IPv4 UDP checksums. Used only if FD is
                              not set.
  UDP_ZERO_CSUM6_TX  N        Zero IPv6 UDP checksum on transmit. Used only
                              if FD is not set.
  UDP_ZERO_CSUM6_RX  N        Zero IPv6 UDP checksum on receive. Used only if
                              FD is not set.
  IP_SADDR           N        IPv4 source address. Used only if FD is not
                              set.
  IP_DADDR           N        IPv4 destination address. Used only if FD is
                              not set.
  UDP_SPORT          N        UDP source port. Used only if FD is not set.
  UDP_DPORT          N        UDP destination port. Used only if FD is not
                              set.
  IP6_SADDR          N        IPv6 source address. Used only if FD is not
                              set.
  IP6_DADDR          N        IPv6 destination address. Used only if FD is
                              not set.
  DEBUG              N        Debug flags.
  ================== ======== ===
  
  ``L2TP_CMD_TUNNEL_DESTROY`` attributes:-
  
  ================== ======== ===
  Attribute          Required Use
  ================== ======== ===
  CONN_ID            Y        Identifies the tunnel id to be destroyed.
  ================== ======== ===
  
  ``L2TP_CMD_TUNNEL_MODIFY`` attributes:-
  
  ================== ======== ===
  Attribute          Required Use
  ================== ======== ===
  CONN_ID            Y        Identifies the tunnel id to be modified.
  DEBUG              N        Debug flags.
  ================== ======== ===
  
  ``L2TP_CMD_TUNNEL_GET`` attributes:-
  
  ================== ======== ===
  Attribute          Required Use
  ================== ======== ===
  CONN_ID            N        Identifies the tunnel id to be queried.
                              Ignored in DUMP requests.
  ================== ======== ===
  
  ``L2TP_CMD_SESSION_CREATE`` attributes:-
  
  ================== ======== ===
  Attribute          Required Use
  ================== ======== ===
  CONN_ID            Y        The parent tunnel id.
  SESSION_ID         Y        Sets the session id.
  PEER_SESSION_ID    Y        Sets the parent session id.
  PW_TYPE            Y        Sets the pseudowire type.
  DEBUG              N        Debug flags.
  RECV_SEQ           N        Enable rx data sequence numbers.
  SEND_SEQ           N        Enable tx data sequence numbers.
  LNS_MODE           N        Enable LNS mode (auto-enable data sequence
                              numbers).
  RECV_TIMEOUT       N        Timeout to wait when reordering received
                              packets.
  L2SPEC_TYPE        N        Sets layer2-specific-sublayer type (L2TPv3
                              only).
  COOKIE             N        Sets optional cookie (L2TPv3 only).
  PEER_COOKIE        N        Sets optional peer cookie (L2TPv3 only).
  IFNAME             N        Sets interface name (L2TPv3 only).
  ================== ======== ===
  
  For Ethernet session types, this will create an l2tpeth virtual
  interface which can then be configured as required. For PPP session
  types, a PPPoL2TP socket must also be opened and connected, mapping it
  onto the new session. This is covered in "PPPoL2TP Sockets" later.
  
  ``L2TP_CMD_SESSION_DESTROY`` attributes:-
  
  ================== ======== ===
  Attribute          Required Use
  ================== ======== ===
  CONN_ID            Y        Identifies the parent tunnel id of the session
                              to be destroyed.
  SESSION_ID         Y        Identifies the session id to be destroyed.
  IFNAME             N        Identifies the session by interface name. If
                              set, this overrides any CONN_ID and SESSION_ID
                              attributes. Currently supported for L2TPv3
                              Ethernet sessions only.
  ================== ======== ===
  
  ``L2TP_CMD_SESSION_MODIFY`` attributes:-
  
  ================== ======== ===
  Attribute          Required Use
  ================== ======== ===
  CONN_ID            Y        Identifies the parent tunnel id of the session
                              to be modified.
  SESSION_ID         Y        Identifies the session id to be modified.
  IFNAME             N        Identifies the session by interface name. If
                              set, this overrides any CONN_ID and SESSION_ID
                              attributes. Currently supported for L2TPv3
                              Ethernet sessions only.
  DEBUG              N        Debug flags.
  RECV_SEQ           N        Enable rx data sequence numbers.
  SEND_SEQ           N        Enable tx data sequence numbers.
  LNS_MODE           N        Enable LNS mode (auto-enable data sequence
                              numbers).
  RECV_TIMEOUT       N        Timeout to wait when reordering received
                              packets.
  ================== ======== ===
  
  ``L2TP_CMD_SESSION_GET`` attributes:-
  
  ================== ======== ===
  Attribute          Required Use
  ================== ======== ===
  CONN_ID            N        Identifies the tunnel id to be queried.
                              Ignored for DUMP requests.
  SESSION_ID         N        Identifies the session id to be queried.
                              Ignored for DUMP requests.
  IFNAME             N        Identifies the session by interface name.
                              If set, this overrides any CONN_ID and
                              SESSION_ID attributes. Ignored for DUMP
                              requests. Currently supported for L2TPv3
                              Ethernet sessions only.
  ================== ======== ===
  
  Application developers should refer to `include/uapi/linux/l2tp.h`_ for
  netlink command and attribute definitions.
  
  Sample userspace code using libmnl_:
  
    - Open L2TP netlink socket::
  
          struct nl_sock *nl_sock;
          int l2tp_nl_family_id;
  
          nl_sock = nl_socket_alloc();
          genl_connect(nl_sock);
          genl_id = genl_ctrl_resolve(nl_sock, L2TP_GENL_NAME);
  
    - Create a tunnel::
  
          struct nlmsghdr *nlh;
          struct genlmsghdr *gnlh;
  
          nlh = mnl_nlmsg_put_header(buf);
          nlh->nlmsg_type = genl_id; /* assigned to genl socket */
          nlh->nlmsg_flags = NLM_F_REQUEST | NLM_F_ACK;
          nlh->nlmsg_seq = seq;
  
          gnlh = mnl_nlmsg_put_extra_header(nlh, sizeof(*gnlh));
          gnlh->cmd = L2TP_CMD_TUNNEL_CREATE;
          gnlh->version = L2TP_GENL_VERSION;
          gnlh->reserved = 0;
  
          mnl_attr_put_u32(nlh, L2TP_ATTR_FD, tunl_sock_fd);
          mnl_attr_put_u32(nlh, L2TP_ATTR_CONN_ID, tid);
          mnl_attr_put_u32(nlh, L2TP_ATTR_PEER_CONN_ID, peer_tid);
          mnl_attr_put_u8(nlh, L2TP_ATTR_PROTO_VERSION, protocol_version);
          mnl_attr_put_u16(nlh, L2TP_ATTR_ENCAP_TYPE, encap);
  
    - Create a session::
  
          struct nlmsghdr *nlh;
          struct genlmsghdr *gnlh;
  
          nlh = mnl_nlmsg_put_header(buf);
          nlh->nlmsg_type = genl_id; /* assigned to genl socket */
          nlh->nlmsg_flags = NLM_F_REQUEST | NLM_F_ACK;
          nlh->nlmsg_seq = seq;
  
          gnlh = mnl_nlmsg_put_extra_header(nlh, sizeof(*gnlh));
          gnlh->cmd = L2TP_CMD_SESSION_CREATE;
          gnlh->version = L2TP_GENL_VERSION;
          gnlh->reserved = 0;
  
          mnl_attr_put_u32(nlh, L2TP_ATTR_CONN_ID, tid);
          mnl_attr_put_u32(nlh, L2TP_ATTR_PEER_CONN_ID, peer_tid);
          mnl_attr_put_u32(nlh, L2TP_ATTR_SESSION_ID, sid);
          mnl_attr_put_u32(nlh, L2TP_ATTR_PEER_SESSION_ID, peer_sid);
          mnl_attr_put_u16(nlh, L2TP_ATTR_PW_TYPE, pwtype);
          /* there are other session options which can be set using netlink
           * attributes during session creation -- see l2tp.h
           */
  
    - Delete a session::
  
          struct nlmsghdr *nlh;
          struct genlmsghdr *gnlh;
  
          nlh = mnl_nlmsg_put_header(buf);
          nlh->nlmsg_type = genl_id; /* assigned to genl socket */
          nlh->nlmsg_flags = NLM_F_REQUEST | NLM_F_ACK;
          nlh->nlmsg_seq = seq;
  
          gnlh = mnl_nlmsg_put_extra_header(nlh, sizeof(*gnlh));
          gnlh->cmd = L2TP_CMD_SESSION_DELETE;
          gnlh->version = L2TP_GENL_VERSION;
          gnlh->reserved = 0;
  
          mnl_attr_put_u32(nlh, L2TP_ATTR_CONN_ID, tid);
          mnl_attr_put_u32(nlh, L2TP_ATTR_SESSION_ID, sid);
  
    - Delete a tunnel and all of its sessions (if any)::
  
          struct nlmsghdr *nlh;
          struct genlmsghdr *gnlh;
  
          nlh = mnl_nlmsg_put_header(buf);
          nlh->nlmsg_type = genl_id; /* assigned to genl socket */
          nlh->nlmsg_flags = NLM_F_REQUEST | NLM_F_ACK;
          nlh->nlmsg_seq = seq;
  
          gnlh = mnl_nlmsg_put_extra_header(nlh, sizeof(*gnlh));
          gnlh->cmd = L2TP_CMD_TUNNEL_DELETE;
          gnlh->version = L2TP_GENL_VERSION;
          gnlh->reserved = 0;
  
          mnl_attr_put_u32(nlh, L2TP_ATTR_CONN_ID, tid);
  
  PPPoL2TP Session Socket API
  ---------------------------
  
  For PPP session types, a PPPoL2TP socket must be opened and connected
  to the L2TP session.

  
  When creating PPPoL2TP sockets, the application provides information

  to the kernel about the tunnel and session in a socket connect()
  call. Source and destination tunnel and session ids are provided, as
  well as the file descriptor of a UDP or L2TPIP socket. See struct
  pppol2tp_addr in `include/linux/if_pppol2tp.h`_. For historical reasons,
  there are unfortunately slightly different address structures for
  L2TPv2/L2TPv3 IPv4/IPv6 tunnels and userspace must use the appropriate
  structure that matches the tunnel socket type.

  
  Userspace may control behavior of the tunnel or session using
  setsockopt and ioctl on the PPPoX socket. The following socket
  options are supported:-

  =========   ===========================================================
  DEBUG       bitmask of debug message categories. See below.
  SENDSEQ     - 0 => don't send packets with sequence numbers

              - 1 => send packets with sequence numbers

  RECVSEQ     - 0 => receive packet sequence numbers are optional

              - 1 => drop receive packets without sequence numbers

  LNSMODE     - 0 => act as LAC.

              - 1 => act as LNS.

  REORDERTO   reorder timeout (in millisecs). If 0, don't try to reorder.
  =========   ===========================================================

  In addition to the standard PPP ioctls, a PPPIOCGL2TPSTATS is provided
  to retrieve tunnel and session statistics from the kernel using the
  PPPoX socket of the appropriate tunnel or session.

  Sample userspace code:
  
    - Create session PPPoX data socket::
  
          struct sockaddr_pppol2tp sax;
          int fd;
  
          /* Note, the tunnel socket must be bound already, else it
           * will not be ready
           */
          sax.sa_family = AF_PPPOX;
          sax.sa_protocol = PX_PROTO_OL2TP;
          sax.pppol2tp.fd = tunnel_fd;
          sax.pppol2tp.addr.sin_addr.s_addr = addr->sin_addr.s_addr;
          sax.pppol2tp.addr.sin_port = addr->sin_port;
          sax.pppol2tp.addr.sin_family = AF_INET;
          sax.pppol2tp.s_tunnel  = tunnel_id;
          sax.pppol2tp.s_session = session_id;
          sax.pppol2tp.d_tunnel  = peer_tunnel_id;
          sax.pppol2tp.d_session = peer_session_id;
  
          /* session_fd is the fd of the session's PPPoL2TP socket.
           * tunnel_fd is the fd of the tunnel UDP / L2TPIP socket.
           */
          fd = connect(session_fd, (struct sockaddr *)&sax, sizeof(sax));
          if (fd < 0 ) {
                  return -errno;
          }
          return 0;
  
  Old L2TPv2-only API
  -------------------
  
  When L2TP was first added to the Linux kernel in 2.6.23, it
  implemented only L2TPv2 and did not include a netlink API. Instead,
  tunnel and session instances in the kernel were managed directly using
  only PPPoL2TP sockets. The PPPoL2TP socket is used as described in
  section "PPPoL2TP Session Socket API" but tunnel and session instances
  are automatically created on a connect() of the socket instead of
  being created by a separate netlink request:
  
      - Tunnels are managed using a tunnel management socket which is a
        dedicated PPPoL2TP socket, connected to (invalid) session
        id 0. The L2TP tunnel instance is created when the PPPoL2TP
        tunnel management socket is connected and is destroyed when the
        socket is closed.
  
      - Session instances are created in the kernel when a PPPoL2TP
        socket is connected to a non-zero session id. Session parameters
        are set using setsockopt. The L2TP session instance is destroyed
        when the socket is closed.
  
  This API is still supported but its use is discouraged. Instead, new
  L2TPv2 applications should use netlink to first create the tunnel and
  session, then create a PPPoL2TP socket for the session.
  
  Unmanaged L2TPv3 tunnels
  ------------------------
  
  The kernel L2TP subsystem also supports static (unmanaged) L2TPv3
  tunnels. Unmanaged tunnels have no userspace tunnel socket, and
  exchange no control messages with the peer to set up the tunnel; the
  tunnel is configured manually at each end of the tunnel. All
  configuration is done using netlink. There is no need for an L2TP
  userspace application in this case -- the tunnel socket is created by
  the kernel and configured using parameters sent in the
  ``L2TP_CMD_TUNNEL_CREATE`` netlink request. The ``ip`` utility of
  ``iproute2`` has commands for managing static L2TPv3 tunnels; do ``ip
  l2tp help`` for more information.

  Debugging

  ---------

  The L2TP subsystem offers a range of debugging interfaces through the
  debugfs filesystem.

  To access these interfaces, the debugfs filesystem must first be mounted::

      # mount -t debugfs debugfs /debug

  Files under the l2tp directory can then be accessed, providing a summary
  of the current population of tunnel and session contexts existing in the
  kernel::

      # cat /debug/l2tp/tunnels

  
  The debugfs files should not be used by applications to obtain L2TP
  state information because the file format is subject to change. It is
  implemented to provide extra debug information to help diagnose

  problems. Applications should instead use the netlink API.

  In addition the L2TP subsystem implements tracepoints using the standard
  kernel event tracing API.  The available L2TP events can be reviewed as
  follows::
  
      # find /debug/tracing/events/l2tp
  
  Finally, /proc/net/pppol2tp is also provided for backwards compatibility
  with the original pppol2tp code. It lists information about L2TPv2

  tunnels and sessions only. Its use is discouraged.

  Internal Implementation
  =======================

  This section is for kernel developers and maintainers.
  
  Sockets
  -------
  
  UDP sockets are implemented by the networking core. When an L2TP
  tunnel is created using a UDP socket, the socket is set up as an
  encapsulated UDP socket by setting encap_rcv and encap_destroy
  callbacks on the UDP socket. l2tp_udp_encap_recv is called when
  packets are received on the socket. l2tp_udp_encap_destroy is called
  when userspace closes the socket.
  
  L2TPIP sockets are implemented in `net/l2tp/l2tp_ip.c`_ and
  `net/l2tp/l2tp_ip6.c`_.
  
  Tunnels
  -------
  
  The kernel keeps a struct l2tp_tunnel context per L2TP tunnel. The
  l2tp_tunnel is always associated with a UDP or L2TP/IP socket and
  keeps a list of sessions in the tunnel. When a tunnel is first
  registered with L2TP core, the reference count on the socket is
  increased. This ensures that the socket cannot be removed while L2TP's
  data structures reference it.
  
  Tunnels are identified by a unique tunnel id. The id is 16-bit for
  L2TPv2 and 32-bit for L2TPv3. Internally, the id is stored as a 32-bit
  value.
  
  Tunnels are kept in a per-net list, indexed by tunnel id. The tunnel
  id namespace is shared by L2TPv2 and L2TPv3. The tunnel context can be
  derived from the socket's sk_user_data.
  
  Handling tunnel socket close is perhaps the most tricky part of the
  L2TP implementation. If userspace closes a tunnel socket, the L2TP
  tunnel and all of its sessions must be closed and destroyed. Since the
  tunnel context holds a ref on the tunnel socket, the socket's
  sk_destruct won't be called until the tunnel sock_put's its
  socket. For UDP sockets, when userspace closes the tunnel socket, the
  socket's encap_destroy handler is invoked, which L2TP uses to initiate
  its tunnel close actions. For L2TPIP sockets, the socket's close
  handler initiates the same tunnel close actions. All sessions are
  first closed. Each session drops its tunnel ref. When the tunnel ref
  reaches zero, the tunnel puts its socket ref. When the socket is
  eventually destroyed, it's sk_destruct finally frees the L2TP tunnel
  context.
  
  Sessions
  --------
  
  The kernel keeps a struct l2tp_session context for each session.  Each
  session has private data which is used for data specific to the
  session type. With L2TPv2, the session always carries PPP
  traffic. With L2TPv3, the session can carry Ethernet frames (Ethernet
  pseudowire) or other data types such as PPP, ATM, HDLC or Frame
  Relay. Linux currently implements only Ethernet and PPP session types.
  
  Some L2TP session types also have a socket (PPP pseudowires) while
  others do not (Ethernet pseudowires). We can't therefore use the
  socket reference count as the reference count for session
  contexts. The L2TP implementation therefore has its own internal
  reference counts on the session contexts.
  
  Like tunnels, L2TP sessions are identified by a unique
  session id. Just as with tunnel ids, the session id is 16-bit for
  L2TPv2 and 32-bit for L2TPv3. Internally, the id is stored as a 32-bit
  value.
  
  Sessions hold a ref on their parent tunnel to ensure that the tunnel
  stays extant while one or more sessions references it.
  
  Sessions are kept in a per-tunnel list, indexed by session id. L2TPv3
  sessions are also kept in a per-net list indexed by session id,
  because L2TPv3 session ids are unique across all tunnels and L2TPv3
  data packets do not contain a tunnel id in the header. This list is
  therefore needed to find the session context associated with a
  received data packet when the tunnel context cannot be derived from
  the tunnel socket.
  
  Although the L2TPv3 RFC specifies that L2TPv3 session ids are not
  scoped by the tunnel, the kernel does not police this for L2TPv3 UDP
  tunnels and does not add sessions of L2TPv3 UDP tunnels into the
  per-net session list. In the UDP receive code, we must trust that the
  tunnel can be identified using the tunnel socket's sk_user_data and
  lookup the session in the tunnel's session list instead of the per-net
  session list.
  
  PPP
  ---
  
  `net/l2tp/l2tp_ppp.c`_ implements the PPPoL2TP socket family. Each PPP
  session has a PPPoL2TP socket.
  
  The PPPoL2TP socket's sk_user_data references the l2tp_session.
  
  Userspace sends and receives PPP packets over L2TP using a PPPoL2TP
  socket. Only PPP control frames pass over this socket: PPP data
  packets are handled entirely by the kernel, passing between the L2TP
  session and its associated ``pppN`` netdev through the PPP channel
  interface of the kernel PPP subsystem.
  
  The L2TP PPP implementation handles the closing of a PPPoL2TP socket
  by closing its corresponding L2TP session. This is complicated because
  it must consider racing with netlink session create/destroy requests
  and pppol2tp_connect trying to reconnect with a session that is in the
  process of being closed. Unlike tunnels, PPP sessions do not hold a
  ref on their associated socket, so code must be careful to sock_hold
  the socket where necessary. For all the details, see commit
  3d609342cc04129ff7568e19316ce3d7451a27e8.
  
  Ethernet
  --------
  
  `net/l2tp/l2tp_eth.c`_ implements L2TPv3 Ethernet pseudowires. It
  manages a netdev for each session.
  
  L2TP Ethernet sessions are created and destroyed by netlink request,
  or are destroyed when the tunnel is destroyed. Unlike PPP sessions,
  Ethernet sessions do not have an associated socket.

  Miscellaneous

  =============

  RFCs
  ----
  
  The kernel code implements the datapath features specified in the
  following RFCs:
  
  ======= =============== ===================================
  RFC2661 L2TPv2          https://tools.ietf.org/html/rfc2661
  RFC3931 L2TPv3          https://tools.ietf.org/html/rfc3931
  RFC4719 L2TPv3 Ethernet https://tools.ietf.org/html/rfc4719
  ======= =============== ===================================
  
  Implementations
  ---------------
  
  A number of open source applications use the L2TP kernel subsystem:
  
  ============ ==============================================
  iproute2     https://github.com/shemminger/iproute2
  go-l2tp      https://github.com/katalix/go-l2tp
  tunneldigger https://github.com/wlanslovenija/tunneldigger
  xl2tpd       https://github.com/xelerance/xl2tpd
  ============ ==============================================
  
  Limitations
  -----------
  
  The current implementation has a number of limitations:
  
    1) Multiple UDP sockets with the same 5-tuple address cannot be
       used. The kernel's tunnel context is identified using private
       data associated with the socket so it is important that each
       socket is uniquely identified by its address.
  
    2) Interfacing with openvswitch is not yet implemented. It may be
       useful to map OVS Ethernet and VLAN ports into L2TPv3 tunnels.
  
    3) VLAN pseudowires are implemented using an ``l2tpethN`` interface
       configured with a VLAN sub-interface. Since L2TPv3 VLAN
       pseudowires carry one and only one VLAN, it may be better to use
       a single netdevice rather than an ``l2tpethN`` and ``l2tpethN``:M
       pair per VLAN session. The netlink attribute
       ``L2TP_ATTR_VLAN_ID`` was added for this, but it was never
       implemented.
  
  Testing
  -------
  
  Unmanaged L2TPv3 Ethernet features are tested by the kernel's built-in
  selftests. See `tools/testing/selftests/net/l2tp.sh`_.
  
  Another test suite, l2tp-ktest_, covers all
  of the L2TP APIs and tunnel/session types. This may be integrated into
  the kernel's built-in L2TP selftests in the future.
  
  .. Links
  .. _Generic Netlink: generic_netlink.html
  .. _libmnl: https://www.netfilter.org/projects/libmnl
  .. _include/uapi/linux/l2tp.h: ../../../include/uapi/linux/l2tp.h
  .. _include/linux/if_pppol2tp.h: ../../../include/linux/if_pppol2tp.h
  .. _net/l2tp/l2tp_ip.c: ../../../net/l2tp/l2tp_ip.c
  .. _net/l2tp/l2tp_ip6.c: ../../../net/l2tp/l2tp_ip6.c
  .. _net/l2tp/l2tp_ppp.c: ../../../net/l2tp/l2tp_ppp.c
  .. _net/l2tp/l2tp_eth.c: ../../../net/l2tp/l2tp_eth.c
  .. _tools/testing/selftests/net/l2tp.sh: ../../../tools/testing/selftests/net/l2tp.sh
  .. _l2tp-ktest: https://github.com/katalix/l2tp-ktest