This brief document describes how to use the kernel's PPPoL2TP driver
to provide L2TP functionality. L2TP is a protocol that tunnels one or
more PPP sessions over a UDP tunnel. It is commonly used for VPNs
(L2TP/IPSec) and by ISPs to tunnel subscriber PPP sessions over an IP
network infrastructure.

Design
======

The PPPoL2TP driver, drivers/net/pppol2tp.c, provides a mechanism by
which PPP frames carried through an L2TP session are passed through
the kernel's PPP subsystem. The standard PPP daemon, pppd, handles all
PPP interaction with the peer. PPP network interfaces are created for
each local PPP endpoint.

The L2TP protocol http://www.faqs.org/rfcs/rfc2661.html defines L2TP
control and data frames. L2TP control frames carry messages between
L2TP clients/servers and are used to setup / teardown tunnels and
sessions. An L2TP client or server is implemented in userspace and
will use a regular UDP socket per tunnel. L2TP data frames carry PPP
frames, which may be PPP control or PPP data. The kernel's PPP
subsystem arranges for PPP control frames to be delivered to pppd,
while data frames are forwarded as usual.

Each tunnel and session within a tunnel is assigned a unique tunnel_id
and session_id. These ids are carried in the L2TP header of every
control and data packet. The pppol2tp driver uses them to lookup
internal tunnel and/or session contexts. Zero tunnel / session ids are
treated specially - zero ids are never assigned to tunnels or sessions
in the network. In the driver, the tunnel context keeps a pointer to
the tunnel UDP socket. The session context keeps a pointer to the
PPPoL2TP socket, as well as other data that lets the driver interface
to the kernel PPP subsystem.

Note that the pppol2tp kernel driver handles only L2TP data frames;
L2TP control frames are simply passed up to userspace in the UDP
tunnel socket. The kernel handles all datapath aspects of the
protocol, including data packet resequencing (if enabled).

There are a number of requirements on the userspace L2TP daemon in
order to use the pppol2tp driver.

1. Use a UDP socket per tunnel.

2. Create a single PPPoL2TP socket per tunnel bound to a special null
   session id. This is used only for communicating with the driver but
   must remain open while the tunnel is active. Opening this tunnel
   management socket causes the driver to mark the tunnel socket as an
   L2TP UDP encapsulation socket and flags it for use by the
   referenced tunnel id. This hooks up the UDP receive path via
   udp_encap_rcv() in net/ipv4/udp.c. PPP data frames are never passed
   in this special PPPoX socket.

3. Create a PPPoL2TP socket per L2TP session. This is typically done
   by starting pppd with the pppol2tp plugin and appropriate
   arguments. A PPPoL2TP tunnel management socket (Step 2) must be
   created before the first PPPoL2TP session socket is created.

When creating PPPoL2TP sockets, the application provides information
to the driver about the socket in a socket connect() call. Source and
destination tunnel and session ids are provided, as well as the file
descriptor of a UDP socket. See struct pppol2tp_addr in
include/linux/if_ppp.h. Note that zero tunnel / session ids are
treated specially. When creating the per-tunnel PPPoL2TP management
socket in Step 2 above, zero source and destination session ids are
specified, which tells the driver to prepare the supplied UDP file
descriptor for use as an L2TP tunnel socket.

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.

Only the DEBUG option is supported by the special tunnel management
PPPoX socket.

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.

Debugging
=========

The driver supports a flexible debug scheme where kernel trace
messages may be optionally enabled per tunnel and per session. Care is
needed when debugging a live system since the messages are not
rate-limited and a busy system could be swamped. Userspace uses
setsockopt on the PPPoX socket to set a debug mask.

The following debug mask bits are available:

PPPOL2TP_MSG_DEBUG    verbose debug (if compiled in)
PPPOL2TP_MSG_CONTROL  userspace - kernel interface
PPPOL2TP_MSG_SEQ      sequence numbers handling
PPPOL2TP_MSG_DATA     data packets

Sample Userspace Code
=====================

1. Create tunnel management PPPoX socket

        kernel_fd = socket(AF_PPPOX, SOCK_DGRAM, PX_PROTO_OL2TP);
        if (kernel_fd >= 0) {
                struct sockaddr_pppol2tp sax;
                struct sockaddr_in const *peer_addr;

                peer_addr = l2tp_tunnel_get_peer_addr(tunnel);
                memset(&sax, 0, sizeof(sax));
                sax.sa_family = AF_PPPOX;
                sax.sa_protocol = PX_PROTO_OL2TP;
                sax.pppol2tp.fd = udp_fd;       /* fd of tunnel UDP socket */
                sax.pppol2tp.addr.sin_addr.s_addr = peer_addr->sin_addr.s_addr;
                sax.pppol2tp.addr.sin_port = peer_addr->sin_port;
                sax.pppol2tp.addr.sin_family = AF_INET;
                sax.pppol2tp.s_tunnel = tunnel_id;
                sax.pppol2tp.s_session = 0;     /* special case: mgmt socket */
                sax.pppol2tp.d_tunnel = 0;
                sax.pppol2tp.d_session = 0;     /* special case: mgmt socket */

                if(connect(kernel_fd, (struct sockaddr *)&sax, sizeof(sax) ) < 0 ) {
                        perror("connect failed");
                        result = -errno;
                        goto err;
                }
        }

2. Create session PPPoX data socket

        struct sockaddr_pppol2tp sax;
        int fd;

        /* Note, the target 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 socket.
         */
        fd = connect(session_fd, (struct sockaddr *)&sax, sizeof(sax));
        if (fd < 0 )    {
                return -errno;
        }
        return 0;

Miscellaneous
============

The PPPoL2TP driver was developed as part of the OpenL2TP project by
Katalix Systems Ltd. OpenL2TP is a full-featured L2TP client / server,
designed from the ground up to have the L2TP datapath in the
kernel. The project also implemented the pppol2tp plugin for pppd
which allows pppd to use the kernel driver. Details can be found at
http://openl2tp.sourceforge.net.