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NG_L2TP(4)		 BSD Kernel Interfaces Manual		    NG_L2TP(4)

NAME
     ng_l2tp — L2TP protocol netgraph node type

SYNOPSIS
     #include <sys/types.h>
     #include <netgraph/ng_l2tp.h>

DESCRIPTION
     The l2tp node type implements the encapsulation layer of the L2TP proto‐
     col as described in RFC 2661.  This includes adding the L2TP packet
     header for outgoing packets and verifying and removing it for incoming
     packets.  The node maintains the L2TP sequence number state and handles
     control session packet acknowledgment and retransmission.

HOOKS
     The l2tp node type supports the following hooks:

	   lower	 L2TP frames.
	   ctrl		 Control packets.
	   session_hhhh	 Session 0xhhhh data packets.

     L2TP control and data packets are transmitted to, and received from, the
     L2TP peer via the lower hook.  Typically this hook would be connected to
     the inet/dgram/udp hook of an ng_ksocket(4) node for L2TP over UDP.

     The ctrl hook connects to the local L2TP management entity.  L2TP control
     messages (without any L2TP headers) are transmitted and received on this
     hook.  Messages written to this hook are guaranteed to be delivered to
     the peer reliably, in order, and without duplicates.

     Packets written to the ctrl hook must contain a two byte session ID
     prepended to the frame (in network order).	 This session ID is copied to
     the outgoing L2TP header.	Similarly, packets read from the ctrl hook
     will have the received session ID prepended.

     Once an L2TP session has been created, the corresponding session hook may
     be used to transmit and receive the session's data frames: for the ses‐
     sion with session ID 0xabcd, the hook is named session_abcd.

CONTROL MESSAGES
     This node type supports the generic control messages, plus the following:

     NGM_L2TP_SET_CONFIG
	     This command updates the configuration of the node.  It takes a
	     struct ng_l2tp_config as an argument:

	     /* Configuration for a node */
	     struct ng_l2tp_config {
		 u_char	     enabled;	     /* enables traffic flow */
		 u_char	     match_id;	     /* tunnel id must match 'tunnel_id' */
		 u_int16_t   tunnel_id;	     /* local tunnel id */
		 u_int16_t   peer_id;	     /* peer's tunnel id */
		 u_int16_t   peer_win;	     /* peer's max recv window size */
		 u_int16_t   rexmit_max;     /* max retransmits before failure */
		 u_int16_t   rexmit_max_to;  /* max delay between retransmits */
	     };

	     The enabled field enables packet processing.  Each time this
	     field is changed back to zero the sequence number state is reset.
	     In this way, reuse of a node is possible.

	     The tunnel_id field configures the local tunnel ID for the con‐
	     trol connection.  The match_id field determines how incoming L2TP
	     packets with a tunnel ID field different from tunnel_id are han‐
	     dled.  If match_id is non-zero, they will be dropped; otherwise,
	     they will be dropped only if the tunnel ID is non-zero.  Typi‐
	     cally tunnel_id is set to the local tunnel ID as soon as it is
	     known and match_id is set to non-zero after receipt of the SCCRP
	     or SCCCN control message.

	     The peer's tunnel ID should be set in peer_id as soon as it is
	     learned, typically after receipt of a SCCRQ or SCCRP control mes‐
	     sage.  This value is copied into the L2TP header for outgoing
	     packets.

	     The peer_win field should be set from the “Receive Window Size”
	     AVP received from the peer.  The default value for this field is
	     one; zero is an invalid value.  As long as enabled is non-zero,
	     this value may not be decreased.

	     The rexmit_max and rexmit_max_to fields configure packet retrans‐
	     mission.  rexmit_max_to is the maximum retransmission delay
	     between packets, in seconds.  The retransmit delay will start at
	     a small value and increase exponentially up to this limit.	 The
	     rexmit_max sets the maximum number of times a packet will be
	     retransmitted without being acknowledged before a failure condi‐
	     tion is declared.	Once a failure condition is declared, each
	     additional retransmission will cause the l2tp node to send a
	     NGM_L2TP_ACK_FAILURE control message back to the node that sent
	     the last NGM_L2TP_SET_CONFIG.  Appropriate action should then be
	     taken to shutdown the control connection.

     NGM_L2TP_GET_CONFIG
	     Returns the current configuration as a struct ng_l2tp_config.

     NGM_L2TP_SET_SESS_CONFIG
	     This control message configures a single data session.  The cor‐
	     responding hook must already be connected before sending this
	     command.  The argument is a struct ng_l2tp_sess_config:

	     /* Configuration for a session hook */
	     struct ng_l2tp_sess_config {
		 u_int16_t   session_id;     /* local session id */
		 u_int16_t   peer_id;	     /* peer's session id */
		 u_char	     control_dseq;   /* we control data sequencing? */
		 u_char	     enable_dseq;    /* enable data sequencing? */
		 u_char	     include_length; /* include length field? */
	     };

	     The session_id and peer_id fields configure the local and remote
	     session IDs, respectively.

	     The control_dseq and enable_dseq fields determine whether
	     sequence numbers are used with L2TP data packets.	If enable_dseq
	     is zero, then no sequence numbers are sent and incoming sequence
	     numbers are ignored.  Otherwise, sequence numbers are included on
	     outgoing packets and checked on incoming packets.

	     If control_dseq is non-zero, then the setting of enable_dseq will
	     never change except by another NGM_L2TP_SET_SESS_CONFIG control
	     message.  If control_dseq is zero, then the peer controls whether
	     sequence numbers are used: if an incoming L2TP data packet con‐
	     tains sequence numbers, enable_dseq is set to one, and conversely
	     if an incoming L2TP data packet does not contain sequence num‐
	     bers, enable_dseq is set to zero.	The current value of
	     enable_dseq is always accessible via the NGM_L2TP_GET_SESS_CONFIG
	     control message (see below).  Typically an LNS would set
	     control_dseq to one while a LAC would set control_dseq to zero
	     (if the Sequencing Required AVP were not sent), thus giving con‐
	     trol of data packet sequencing to the LNS.

	     The include_length field determines whether the L2TP header
	     length field is included in outgoing L2TP data packets.  For
	     incoming packets, the L2TP length field is always checked when
	     present.

     NGM_L2TP_GET_SESS_CONFIG
	     This command takes a two byte session ID as an argument and
	     returns the current configuration for the corresponding data ses‐
	     sion as a struct ng_l2tp_sess_config.  The corresponding session
	     hook must be connected.

     NGM_L2TP_GET_STATS
	     This command returns a struct ng_l2tp_stats containing statistics
	     of the L2TP tunnel.

     NGM_L2TP_CLR_STATS
	     This command clears the statistics for the L2TP tunnel.

     NGM_L2TP_GETCLR_STATS
	     Same as NGM_L2TP_GET_STATS, but also atomically clears the sta‐
	     tistics as well.

     NGM_L2TP_GET_SESSION_STATS
	     This command takes a two byte session ID as an argument and
	     returns a struct ng_l2tp_session_stats containing statistics for
	     the corresponding data session.  The corresponding session hook
	     must be connected.

     NGM_L2TP_CLR_SESSION_STATS
	     This command takes a two byte session ID as an argument and
	     clears the statistics for that data session.  The corresponding
	     session hook must be connected.

     NGM_L2TP_GETCLR_SESSION_STATS
	     Same as NGM_L2TP_GET_SESSION_STATS, but also atomically clears
	     the statistics as well.

     NGM_L2TP_SET_SEQ
	     This command sets the sequence numbers of a not yet enabled node.
	     It takes a struct ng_l2tp_seq_config as argument, where xack and
	     nr respectively ns and rack must be the same.  This option is
	     particularly useful if one receives and processes the first
	     packet entirely in userspace and wants to hand over further pro‐
	     cessing to the node.

SHUTDOWN
     This node shuts down upon receipt of a NGM_SHUTDOWN control message, or
     when all hooks have been disconnected.

SEE ALSO
     netgraph(4), ng_ksocket(4), ng_ppp(4), ng_pptpgre(4), ngctl(8)

     W. Townsley, A. Valencia, A. Rubens, G. Pall, G. Zorn, and B. Palter,
     Layer Two Tunneling Protocol L2TP, RFC 2661.

HISTORY
     The l2tp node type was developed at Packet Design, LLC,
     http://www.packetdesign.com/.

AUTHORS
     Archie Cobbs ⟨archie@packetdesign.com⟩

BSD				August 2, 2004				   BSD
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