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>