l2tp: Update documentation

This patch adds documentation about the L2TPv3 functionality.
Signed-off-by: James Chapman <jchapman@katalix.com>
Reviewed-by: Randy Dunlap <randy.dunlap@oracle.com>
Signed-off-by: David S. Miller <davem@davemloft.net>

Documentation/networking/l2tp.txt

-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
+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.
+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 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
+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. 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.
+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.
 
@@ -86,6 +137,35 @@ 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
 =========
 
@@ -102,6 +182,69 @@ PPPOL2TP_MSG_CONTROL  userspace - kernel interface
 PPPOL2TP_MSG_SEQ      sequence numbers handling
 PPPOL2TP_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 compaibility 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:-
+
+# modprobe l2tp_eth
+# modprobe l2tp_netlink
+
+# 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
+# ifconfig -a
+# ip addr add 10.5.1.2/32 peer 10.5.1.1/32 dev l2tpeth0
+# ifconfig 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
 =====================
 
@@ -158,12 +301,48 @@ Sample Userspace Code
         }
         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 PPPoL2TP driver was developed as part of the OpenL2TP project by
+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://openl2tp.sourceforge.net.
+http://www.openl2tp.org.