docs: networking: convert l2tp.txt to ReST

- add SPDX header;
- add a document title;
- mark tables as such;
- adjust identation, whitespaces and blank lines;
- add to networking/index.rst.

Signed-off-by: Mauro Carvalho Chehab <[e-mail]>
Signed-off-by: David S. Miller <[e-mail]>

Documentation/networking/index.rst

···
    ipvlan
    ipvs-sysctl
    kcm
+   l2tp
 
 .. only::  subproject and html
 

Documentation/networking/l2tp.txt Documentation/networking/l2tp.rst

···
+.. 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
···
 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.
+=========   ===========================================================
+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.
···
 
 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.
+debugfs filesystem must first be mounted::
 
-# mount -t debugfs debugfs /debug
+	# mount -t debugfs debugfs /debug
 
-Files under the l2tp directory can then be accessed.
+Files under the l2tp directory can then be accessed::
 
-# cat /debug/l2tp/tunnels
+	# 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
···
 
 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:-
+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
+	# 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
···
 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.
+data over the network. e.g.::
 
-# ping 10.5.1.1
+	# ping 10.5.1.1
 
 
 Sample Userspace Code
 =====================
 
-1. Create tunnel management PPPoX socket
+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;
+	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 */
+		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;
-                }
-        }
+		if(connect(kernel_fd, (struct sockaddr *)&sax, sizeof(sax) ) < 0 ) {
+			perror("connect failed");
+			result = -errno;
+			goto err;
+		}
+	}
 
-2. Create session PPPoX data socket
+2. Create session PPPoX data socket::
 
-        struct sockaddr_pppol2tp sax;
-        int fd;
+	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;
+	/* 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;
+	/* 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
 =======================