Ubuntu Manpage: ng_l2tp — L2TP protocol netgraph node type

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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 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.

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 session 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 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
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 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 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 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 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 session 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 statistics 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 processing to the node.

SHUTDOWN

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

HISTORY

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

AUTHORS

       Archie Cobbs <archie@packetdesign.com>

Debian                                           August 2, 2004                                       NG_L2TP(4)