Skip site navigation (1)Skip section navigation (2)

FreeBSD Man Pages

Man Page or Keyword Search:
Man Architecture
Apropos Keyword Search (all sections) Output format
home | help
NG_L2TP(4)             FreeBSD Kernel Interfaces Manual             NG_L2TP(4)

     ng_l2tp - L2TP protocol netgraph node type

     #include <netgraph/ng_l2tp.h>

     The ng_l2tp node type implements the encapsulation layer of the L2TP
     protocol 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.

     The ng_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
     session with session ID 0xabcd, the hook is named session_abcd.

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

         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 control
         connection.  The match_id field determines how incoming L2TP packets
         with a tunnel ID field different from tunnel_id are handled.  If
         match_id is non-zero, they will be dropped; otherwise, they will be
         dropped only if the tunnel ID is non-zero.  Typically 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 message.
         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
         retransmission.  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 condition
         is declared.  Once a failure condition is declared, each additional
         retransmission will cause the ng_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.

         Returns the current configuration as a struct ng_l2tp_config.

         This control message configures a single data session.  The
         corresponding 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 ID's, 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 contains
         sequence numbers, enable_dseq is set to one, and conversely if an
         incoming L2TP data packet does not contain sequence numbers,
         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 control of data packet sequencing to the

         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.

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

         This command takes a two byte session ID as an argument and returns a
         struct ng_l2tp_stats containing statistics for the corresponding data
         session.  The corresponding session hook must be connected.

         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.

         Same as NGM_L2TP_GET_STATS, but also atomically clears the statistics
         as well.

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

     netgraph(4), ng_ksocket(4), ng_ppp(4), ng_pptp(4), ngctl(8)

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

     The ng_l2tp node type was developed at Packet Design, LLC.

     Archie Cobbs <>

FreeBSD 11.0-PRERELEASE         April 22, 2002         FreeBSD 11.0-PRERELEASE


Want to link to this manual page? Use this URL:

home | help