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 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 re

ceived 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 re

ceived 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 retransmis

sion 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 de

clared, 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 ses

sion. 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 se

quencing? */
u_char include_length; /* include length

field? */

};

The session_id and peer_id fields configure the lo

cal 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 in

coming 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 se

quence 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 pack

ets. 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 argu

ment and
returns the current configuration for the corre

sponding data session as a struct ng_l2tp_sess_config. The corre

sponding session
hook must be connected.

NGM_L2TP_GET_STATS

This command returns a struct ng_l2tp_stats contain

ing statistics
of the L2TP tunnel.

NGM_L2TP_CLR_STATS

This command clears the statistics for the L2TP tun

nel.

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 argu

ment 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 argu

ment 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 atomi

cally 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.

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