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.\" Author: Archie Cobbs <archie@FreeBSD.org>
.\"
.\" $FreeBSD$
.\" $Whistle: ng_vjc.8,v 1.4 1999/01/25 23:46:28 archie Exp $
.\"
.Dd January 19, 1999
.Dt NG_VJC 4
.Os
.Sh NAME
.Nm ng_vjc
.Nd Van Jacobson compression netgraph node type
.Sh SYNOPSIS
.In sys/types.h
.In netinet/in.h
.In netinet/in_systm.h
.In netinet/ip.h
.In net/slcompress.h
.In netgraph/ng_vjc.h
.Sh DESCRIPTION
The
.Nm vjc
node type performs Van Jacobson compression, which is used
over PPP, SLIP, and other point-to-point IP connections to
compress TCP packet headers.
The
.Dv ip
hook represents the uncompressed side of the node, while the
.Dv vjcomp ,
.Dv vjuncomp ,
and
.Dv vjip
hooks represent the compressed side of the node.
Packets received on the
.Dv ip
will be compressed or passed through as appropriate.
Packets received on the other three hooks will be uncompressed as appropriate.
This node also supports
.Dq always pass through
mode in either direction.
.Pp
Van Jacobson compression only applies to TCP packets.
Only
.Dq normal
(i.e., common case) TCP packets are actually compressed.
These are output on the
.Dv vjcomp
hook.
Other TCP packets are run through the state machine but not
compressed; these appear on the
.Dv vjuncomp
hook.
Other non-TCP IP packets are forwarded unchanged to
.Dv vjip .
.Pp
When connecting to a
.Xr ng_ppp 4
node, the
.Dv ip ,
.Dv vjuncomp ,
.Dv vjcomp ,
and
.Dv vjip
hooks should be connected to the
.Xr ng_ppp 4
node's
.Dv vjc_ip ,
.Dv vjc_vjcomp ,
.Dv vjc_vjuncomp ,
and
.Dv vjc_ip
hooks, respectively.
.Sh HOOKS
This node type supports the following hooks:
.Bl -tag -width foobarbazi
.It Dv ip
Upstream (uncompressed) IP packets.
.It Dv vjcomp
Downstream compressed TCP packets.
.It Dv vjuncomp
Downstream uncompressed TCP packets.
.It Dv vjip
Downstream uncompressed IP packets.
.El
.Sh CONTROL MESSAGES
This node type supports the generic control messages, plus the following:
.Bl -tag -width foo
.It Dv NGM_VJC_SET_CONFIG
This command resets the compression state and configures it according
to the supplied
.Dv "struct ngm_vjc_config"
argument.
This structure contains the following fields:
.Bd -literal -offset 4n
struct ngm_vjc_config {
  u_char   enableComp;    /* Enable compression */
  u_char   enableDecomp;  /* Enable decompression */
  u_char   maxChannel;    /* Number of outgoing channels - 1 */
  u_char   compressCID;   /* OK to compress outgoing CID's */
};
.Ed
.Pp
When
.Dv enableComp
is set to zero, all packets received on the
.Dv ip
hook are forwarded unchanged out the
.Dv vjip
hook.
Similarly, when
.Dv enableDecomp
is set to zero, all packets received on the
.Dv vjip
hook are forwarded unchanged out the
.Dv ip
hook, and packets are not accepted on the
.Dv vjcomp
and
.Dv vjuncomp
hooks.
When a node is first created,
both compression and decompression are disabled and the node is
therefore operating in bi-directional
.Dq pass through
mode.
.Pp
When enabling compression,
.Dv maxChannel
should be set to the number of outgoing compression channels minus one,
and is a value between 3 and 15, inclusive.
The
.Dv compressCID
field indicates whether it is OK to compress the CID header field for
outgoing compressed TCP packets.
This value should be zero unless
either (a) it is not possible for an outgoing frame to be lost, or
(b) lost frames can be reliably detected and immediately
reported to the peer's decompression engine (see
.Dv NGM_VJC_RECV_ERROR
below).
.It Dv NGM_VJC_GET_STATE
This command returns the node's current state described by the
.Dv "struct slcompress"
structure, which is defined in
.In net/slcompress.h .
.It Dv NGM_VJC_CLR_STATS
Clears the node statistics counters.
Statistics are also cleared whenever the
.Dv enableComp
or
.Dv enableDecomp
fields are changed from zero to one by a
.Dv NGM_VJC_SET_CONFIG
control message.
.It Dv NGM_VJC_RECV_ERROR
When the peer has CID header field compression enabled,
this message must be sent to the local
.Nm vjc
node immediately
after detecting that a received frame has been lost, due to a bad
checksum or for any other reason.
Failing to do this can result in corrupted TCP stream data.
.El
.Sh SHUTDOWN
This node shuts down upon receipt of a
.Dv NGM_SHUTDOWN
control message, or when all hooks have been disconnected.
.Sh SEE ALSO
.Xr netgraph 4 ,
.Xr ng_iface 4 ,
.Xr ng_ppp 4 ,
.Xr ngctl 8
.Rs
.%A V. Jacobson
.%T "Compressing TCP/IP Headers"
.%O RFC 1144
.Re
.Rs
.%A G. McGregor
.%T "The PPP Internet Control Protocol (IPCP)"
.%O RFC 1332
.Re
.Sh HISTORY
The
.Nm
node type was implemented in
.Fx 4.0 .
.Sh AUTHORS
.An Archie Cobbs Aq archie@FreeBSD.org
.Sh BUGS
As the initialization routine in the kernel implementation of
Van Jacobson compression initializes both compression and decompression
at once, this node does not allow compression and decompression to
be enabled in separate operations.
In order to enable one when
the other is already enabled, first both must be disabled, then
both enabled.
This of course resets the node state.
This restriction may be lifted in a later version.
.Pp
When built as a loadable kernel module, this module includes the file
.Pa net/slcompress.c .
Although loading the module should fail if
.Pa net/slcompress.c
already exists in the kernel, currently it does not, and the duplicate
copies of the file do not interfere.
However, this may change in the future.