Documentation/networking/l2tp.rst - linux - Git at Google

 .. SPDX-License-Identifier: GPL-2.0

 ====
 L2TP
 ====

 This document describes how to use the kernel's L2TP drivers to
 provide L2TP functionality. L2TP is a protocol that tunnels one or
 more sessions over an IP tunnel. It is commonly used for VPNs
 (L2TP/IPSec) and by ISPs to tunnel subscriber PPP sessions over an IP
 network infrastructure. With L2TPv3, it is also useful as a Layer-2
 tunneling infrastructure.

 Features
 ========

 L2TPv2 (PPP over L2TP (UDP tunnels)).
 L2TPv3 ethernet pseudowires.
 L2TPv3 PPP pseudowires.
 L2TPv3 IP encapsulation.
 Netlink sockets for L2TPv3 configuration management.

 History
 =======

 The original pppol2tp driver was introduced in 2.6.23 and provided
 L2TPv2 functionality (rfc2661). L2TPv2 is used to tunnel one or more PPP
 sessions over a UDP tunnel.

 L2TPv3 (rfc3931) changes the protocol to allow different frame types
 to be passed over an L2TP tunnel by moving the PPP-specific parts of
 the protocol out of the core L2TP packet headers. Each frame type is
 known as a pseudowire type. Ethernet, PPP, HDLC, Frame Relay and ATM
 pseudowires for L2TP are defined in separate RFC standards. Another
 change for L2TPv3 is that it can be carried directly over IP with no
 UDP header (UDP is optional). It is also possible to create static
 unmanaged L2TPv3 tunnels manually without a control protocol
 (userspace daemon) to manage them.

 To support L2TPv3, the original pppol2tp driver was split up to
 separate the L2TP and PPP functionality. Existing L2TPv2 userspace
 apps should be unaffected as the original pppol2tp sockets API is
 retained. L2TPv3, however, uses netlink to manage L2TPv3 tunnels and
 sessions.

 Design
 ======

 The L2TP protocol separates control and data frames.  The L2TP kernel
 drivers handle only L2TP data frames; control frames are always
 handled by userspace. 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.

 Each L2TP tunnel is implemented using a UDP or L2TPIP socket; L2TPIP
 provides L2TPv3 IP encapsulation (no UDP) and is implemented using a
 new l2tpip socket family. The tunnel socket is typically created by
 userspace, though for unmanaged L2TPv3 tunnels, the socket can also be
 created by the kernel. Each L2TP session (pseudowire) gets a network
 interface instance. In the case of PPP, these interfaces are created
 indirectly by pppd using a pppol2tp socket. In the case of ethernet,
 the netdevice is created upon a netlink request to create an L2TPv3
 ethernet pseudowire.

 For PPP, the PPPoL2TP driver, net/l2tp/l2tp_ppp.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 kernel's PPP
 subsystem arranges for PPP control frames to be delivered to pppd,
 while data frames are forwarded as usual.

 For ethernet, the L2TPETH driver, net/l2tp/l2tp_eth.c, implements a
 netdevice driver, managing virtual ethernet devices, one per
 pseudowire. These interfaces can be managed using standard Linux tools
 such as "ip" and "ifconfig". If only IP frames are passed over the
 tunnel, the interface can be given an IP addresses of itself and its
 peer. If non-IP frames are to be passed over the tunnel, the interface
 can be added to a bridge using brctl. All L2TP datapath protocol
 functions are handled by the L2TP core driver.

 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. (Actually, in L2TPv3, the tunnel_id isn't
 present in data frames - it is inferred from the IP connection on
 which the packet was received.) The L2TP driver uses the ids to lookup
 internal tunnel and/or session contexts to determine how to handle the
 packet. 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 reference to the tunnel UDP or L2TPIP
 socket. The session context holds data that lets the driver interface
 to the kernel's network frame type subsystems, i.e. PPP, ethernet.

 Userspace Programming
 =====================

 For L2TPv2, 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_pppol2tp.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.

 For L2TPv3, userspace must use the netlink API defined in
 include/linux/l2tp.h to manage tunnel and session contexts. The
 general procedure to create a new L2TP tunnel with one session is:-

 1. Open a GENL socket using L2TP_GENL_NAME for configuring the kernel
    using netlink.

 2. Create a UDP or L2TPIP socket for the tunnel.

 3. Create a new L2TP tunnel using a L2TP_CMD_TUNNEL_CREATE
    request. Set attributes according to desired tunnel parameters,
    referencing the UDP or L2TPIP socket created in the previous step.

 4. Create a new L2TP session in the tunnel using a
    L2TP_CMD_SESSION_CREATE request.

 The tunnel and all of its sessions are closed when the tunnel socket
 is closed. The netlink API may also be used to delete sessions and
 tunnels. Configuration and status info may be set or read using netlink.

 The L2TP driver also supports static (unmanaged) L2TPv3 tunnels. These
 are where there is no L2TP control message exchange with the peer to
 setup the tunnel; the tunnel is configured manually at each end of the
 tunnel. 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 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:

 ================  ==============================
 L2TP_MSG_DEBUG    verbose debug (if compiled in)
 L2TP_MSG_CONTROL  userspace - kernel interface
 L2TP_MSG_SEQ      sequence numbers handling
 L2TP_MSG_DATA     data packets
 ================  ==============================

 If enabled, files under a l2tp debugfs directory can be used to dump
 kernel state about L2TP tunnels and sessions. To access it, the
 debugfs filesystem must first be mounted::

 	# mount -t debugfs debugfs /debug

 Files under the l2tp directory can then be accessed::

 	# 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.) Users should use the netlink API.

 /proc/net/pppol2tp is also provided for backwards compatibility with
 the original pppol2tp driver. It lists information about L2TPv2
 tunnels and sessions only. Its use is discouraged.

 Unmanaged L2TPv3 Tunnels
 ========================

 Some commercial L2TP products support unmanaged L2TPv3 ethernet
 tunnels, where there is no L2TP control protocol; tunnels are
 configured at each side manually. New commands are available in
 iproute2's ip utility to support this.

 To create an L2TPv3 ethernet pseudowire between local host 192.168.1.1
 and peer 192.168.1.2, using IP addresses 10.5.1.1 and 10.5.1.2 for the
 tunnel endpoints::

 	# ip l2tp add tunnel tunnel_id 1 peer_tunnel_id 1 udp_sport 5000 \
 	  udp_dport 5000 encap udp local 192.168.1.1 remote 192.168.1.2
 	# ip l2tp add session tunnel_id 1 session_id 1 peer_session_id 1
 	# ip -s -d show dev l2tpeth0
 	# ip addr add 10.5.1.2/32 peer 10.5.1.1/32 dev l2tpeth0
 	# ip li set dev l2tpeth0 up

 Choose IP addresses to be the address of a local IP interface and that
 of the remote system. The IP addresses of the l2tpeth0 interface can be
 anything suitable.

 Repeat the above at the peer, with ports, tunnel/session ids and IP
 addresses reversed.  The tunnel and session IDs can be any non-zero
 32-bit number, but the values must be reversed at the peer.

 ========================       ===================
 Host 1                         Host2
 ========================       ===================
 udp_sport=5000                 udp_sport=5001
 udp_dport=5001                 udp_dport=5000
 tunnel_id=42                   tunnel_id=45
 peer_tunnel_id=45              peer_tunnel_id=42
 session_id=128                 session_id=5196755
 peer_session_id=5196755        peer_session_id=128
 ========================       ===================

 When done at both ends of the tunnel, it should be possible to send
 data over the network. e.g.::

 	# ping 10.5.1.1


 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;

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

 The driver keeps a struct l2tp_tunnel context per L2TP tunnel and a
 struct l2tp_session context for each session. The l2tp_tunnel is
 always associated with a UDP or L2TP/IP socket and keeps a list of
 sessions in the tunnel. The l2tp_session context keeps kernel state
 about the session. It has private data which is used for data specific
 to the session type. With L2TPv2, the session always carried PPP
 traffic. With L2TPv3, the session can also carry ethernet frames
 (ethernet pseudowire) or other data types such as ATM, HDLC or Frame
 Relay.

 When a tunnel is first opened, the reference count on the socket is
 increased using sock_hold(). This ensures that the kernel socket
 cannot be removed while L2TP's data structures reference it.

 Some L2TP sessions also have a socket (PPP pseudowires) while others
 do not (ethernet pseudowires). We can't 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.

 To Do
 =====

 Add L2TP tunnel switching support. This would route tunneled traffic
 from one L2TP tunnel into another. Specified in
 http://tools.ietf.org/html/draft-ietf-l2tpext-tunnel-switching-08

 Add L2TPv3 VLAN pseudowire support.

 Add L2TPv3 IP pseudowire support.

 Add L2TPv3 ATM pseudowire support.

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

 The L2TP drivers were 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://www.openl2tp.org.
	.. SPDX-License-Identifier: GPL-2.0

	====
	L2TP
	====

	This document describes how to use the kernel's L2TP drivers to
	provide L2TP functionality. L2TP is a protocol that tunnels one or
	more sessions over an IP tunnel. It is commonly used for VPNs
	(L2TP/IPSec) and by ISPs to tunnel subscriber PPP sessions over an IP
	network infrastructure. With L2TPv3, it is also useful as a Layer-2
	tunneling infrastructure.

	Features
	========

	L2TPv2 (PPP over L2TP (UDP tunnels)).
	L2TPv3 ethernet pseudowires.
	L2TPv3 PPP pseudowires.
	L2TPv3 IP encapsulation.
	Netlink sockets for L2TPv3 configuration management.

	History
	=======

	The original pppol2tp driver was introduced in 2.6.23 and provided
	L2TPv2 functionality (rfc2661). L2TPv2 is used to tunnel one or more PPP
	sessions over a UDP tunnel.

	L2TPv3 (rfc3931) changes the protocol to allow different frame types
	to be passed over an L2TP tunnel by moving the PPP-specific parts of
	the protocol out of the core L2TP packet headers. Each frame type is
	known as a pseudowire type. Ethernet, PPP, HDLC, Frame Relay and ATM
	pseudowires for L2TP are defined in separate RFC standards. Another
	change for L2TPv3 is that it can be carried directly over IP with no
	UDP header (UDP is optional). It is also possible to create static
	unmanaged L2TPv3 tunnels manually without a control protocol
	(userspace daemon) to manage them.

	To support L2TPv3, the original pppol2tp driver was split up to
	separate the L2TP and PPP functionality. Existing L2TPv2 userspace
	apps should be unaffected as the original pppol2tp sockets API is
	retained. L2TPv3, however, uses netlink to manage L2TPv3 tunnels and
	sessions.

	Design
	======

	The L2TP protocol separates control and data frames. The L2TP kernel
	drivers handle only L2TP data frames; control frames are always
	handled by userspace. 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.

	Each L2TP tunnel is implemented using a UDP or L2TPIP socket; L2TPIP
	provides L2TPv3 IP encapsulation (no UDP) and is implemented using a
	new l2tpip socket family. The tunnel socket is typically created by
	userspace, though for unmanaged L2TPv3 tunnels, the socket can also be
	created by the kernel. Each L2TP session (pseudowire) gets a network
	interface instance. In the case of PPP, these interfaces are created
	indirectly by pppd using a pppol2tp socket. In the case of ethernet,
	the netdevice is created upon a netlink request to create an L2TPv3
	ethernet pseudowire.

	For PPP, the PPPoL2TP driver, net/l2tp/l2tp_ppp.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 kernel's PPP
	subsystem arranges for PPP control frames to be delivered to pppd,
	while data frames are forwarded as usual.

	For ethernet, the L2TPETH driver, net/l2tp/l2tp_eth.c, implements a
	netdevice driver, managing virtual ethernet devices, one per
	pseudowire. These interfaces can be managed using standard Linux tools
	such as "ip" and "ifconfig". If only IP frames are passed over the
	tunnel, the interface can be given an IP addresses of itself and its
	peer. If non-IP frames are to be passed over the tunnel, the interface
	can be added to a bridge using brctl. All L2TP datapath protocol
	functions are handled by the L2TP core driver.

	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. (Actually, in L2TPv3, the tunnel_id isn't
	present in data frames - it is inferred from the IP connection on
	which the packet was received.) The L2TP driver uses the ids to lookup
	internal tunnel and/or session contexts to determine how to handle the
	packet. 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 reference to the tunnel UDP or L2TPIP
	socket. The session context holds data that lets the driver interface
	to the kernel's network frame type subsystems, i.e. PPP, ethernet.

	Userspace Programming
	=====================

	For L2TPv2, 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_pppol2tp.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.

	For L2TPv3, userspace must use the netlink API defined in
	include/linux/l2tp.h to manage tunnel and session contexts. The
	general procedure to create a new L2TP tunnel with one session is:-

	1. Open a GENL socket using L2TP_GENL_NAME for configuring the kernel
	using netlink.

	2. Create a UDP or L2TPIP socket for the tunnel.

	3. Create a new L2TP tunnel using a L2TP_CMD_TUNNEL_CREATE
	request. Set attributes according to desired tunnel parameters,
	referencing the UDP or L2TPIP socket created in the previous step.

	4. Create a new L2TP session in the tunnel using a
	L2TP_CMD_SESSION_CREATE request.

	The tunnel and all of its sessions are closed when the tunnel socket
	is closed. The netlink API may also be used to delete sessions and
	tunnels. Configuration and status info may be set or read using netlink.

	The L2TP driver also supports static (unmanaged) L2TPv3 tunnels. These
	are where there is no L2TP control message exchange with the peer to
	setup the tunnel; the tunnel is configured manually at each end of the
	tunnel. 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 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:

	================ ==============================
	L2TP_MSG_DEBUG verbose debug (if compiled in)
	L2TP_MSG_CONTROL userspace - kernel interface
	L2TP_MSG_SEQ sequence numbers handling
	L2TP_MSG_DATA data packets
	================ ==============================

	If enabled, files under a l2tp debugfs directory can be used to dump
	kernel state about L2TP tunnels and sessions. To access it, the
	debugfs filesystem must first be mounted::

	# mount -t debugfs debugfs /debug

	Files under the l2tp directory can then be accessed::

	# 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.) Users should use the netlink API.

	/proc/net/pppol2tp is also provided for backwards compatibility with
	the original pppol2tp driver. It lists information about L2TPv2
	tunnels and sessions only. Its use is discouraged.

	Unmanaged L2TPv3 Tunnels
	========================

	Some commercial L2TP products support unmanaged L2TPv3 ethernet
	tunnels, where there is no L2TP control protocol; tunnels are
	configured at each side manually. New commands are available in
	iproute2's ip utility to support this.

	To create an L2TPv3 ethernet pseudowire between local host 192.168.1.1
	and peer 192.168.1.2, using IP addresses 10.5.1.1 and 10.5.1.2 for the
	tunnel endpoints::

	# ip l2tp add tunnel tunnel_id 1 peer_tunnel_id 1 udp_sport 5000 \
	udp_dport 5000 encap udp local 192.168.1.1 remote 192.168.1.2
	# ip l2tp add session tunnel_id 1 session_id 1 peer_session_id 1
	# ip -s -d show dev l2tpeth0
	# ip addr add 10.5.1.2/32 peer 10.5.1.1/32 dev l2tpeth0
	# ip li set dev l2tpeth0 up

	Choose IP addresses to be the address of a local IP interface and that
	of the remote system. The IP addresses of the l2tpeth0 interface can be
	anything suitable.

	Repeat the above at the peer, with ports, tunnel/session ids and IP
	addresses reversed. The tunnel and session IDs can be any non-zero
	32-bit number, but the values must be reversed at the peer.

	======================== ===================
	Host 1 Host2
	======================== ===================
	udp_sport=5000 udp_sport=5001
	udp_dport=5001 udp_dport=5000
	tunnel_id=42 tunnel_id=45
	peer_tunnel_id=45 peer_tunnel_id=42
	session_id=128 session_id=5196755
	peer_session_id=5196755 peer_session_id=128
	======================== ===================

	When done at both ends of the tunnel, it should be possible to send
	data over the network. e.g.::

	# ping 10.5.1.1


	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;

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

	The driver keeps a struct l2tp_tunnel context per L2TP tunnel and a
	struct l2tp_session context for each session. The l2tp_tunnel is
	always associated with a UDP or L2TP/IP socket and keeps a list of
	sessions in the tunnel. The l2tp_session context keeps kernel state
	about the session. It has private data which is used for data specific
	to the session type. With L2TPv2, the session always carried PPP
	traffic. With L2TPv3, the session can also carry ethernet frames
	(ethernet pseudowire) or other data types such as ATM, HDLC or Frame
	Relay.

	When a tunnel is first opened, the reference count on the socket is
	increased using sock_hold(). This ensures that the kernel socket
	cannot be removed while L2TP's data structures reference it.

	Some L2TP sessions also have a socket (PPP pseudowires) while others
	do not (ethernet pseudowires). We can't 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.

	To Do
	=====

	Add L2TP tunnel switching support. This would route tunneled traffic
	from one L2TP tunnel into another. Specified in
	http://tools.ietf.org/html/draft-ietf-l2tpext-tunnel-switching-08

	Add L2TPv3 VLAN pseudowire support.

	Add L2TPv3 IP pseudowire support.

	Add L2TPv3 ATM pseudowire support.

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

	The L2TP drivers were 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://www.openl2tp.org.