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.
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〉