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 (setconfig)
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' */
uint16_t tunnel_id; /* local tunnel id */
uint16_t peer_id; /* peer's tunnel id */
uint16_t peer_win; /* peer's max recv window size */
uint16_t rexmit_max; /* max retransmits before failure */
uint16_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 (ackfailure) 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 (getconfig)
Returns the current configuration as a struct ng_l2tp_config.

NGM_L2TP_SET_SESS_CONFIG (setsessconfig)
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 {
uint16_t session_id; /* local session id */
uint16_t peer_id; /* peer's session id */
u_char control_dseq; /* whether we control data sequencing */
u_char enable_dseq; /* whether to enable data sequencing */
u_char include_length; /* whether to 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 (getsessconfig)
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 (getstats)
This command returns a struct ng_l2tp_stats containing statistics of the L2TP tunnel.

NGM_L2TP_CLR_STATS (clrstats)
This command clears the statistics for the L2TP tunnel.

NGM_L2TP_GETCLR_STATS (getclrstats)
Same as NGM_L2TP_GET_STATS, but also atomically clears the statistics as well.

NGM_L2TP_GET_SESSION_STATS (getsessstats)
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 (clrsessstats)
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 (getclrsessstats)
Same as NGM_L2TP_GET_SESSION_STATS, but also atomically clears the statistics as well.

NGM_L2TP_SET_SEQ (setsequence)
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>