[L2TP]: Add PPPoL2TP in-kernel documentation

Signed-off-by: James Chapman <jchapman@katalix.com>
Signed-off-by: David S. Miller <davem@davemloft.net>

diff --git a/Documentation/networking/l2tp.txt b/Documentation/networking/l2tp.txt
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+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;
+
+Miscellanous
+============
+
+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.