freebsd/share/man/man4/ng_vjc.4

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.\" Author: Archie Cobbs <archie@whistle.com>
.\"
.\" $FreeBSD$
.\" $Whistle: ng_vjc.8,v 1.4 1999/01/25 23:46:28 archie Exp $
.\"
.Dd January 19, 1999
.Dt NG_VJC 8
.Os FreeBSD 3
.Sh NAME
.Nm ng_vjc
.Nd Van Jacobsen compression netgraph node type
.Sh SYNOPSIS
.Fd #include <net/slcompress.h>
.Fd #include <netgraph/ng_vjc.h>
.Sh DESCRIPTION
The
.Nm vjc
node type performs Van Jacobsen compresion, 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
nodes 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.
.Pp
Van Jacobsen compression only applies to TCP packets.
Only ``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 8
node, the
.Dv vjuncomp ,
.Dv vjcomp , 
and
.Dv vjip
nodes should be connected to the
.Xr ng_ppp 8
node's
.Dv vjcomp ,
.Dv vjuncomp ,
and
.Dv ip
nodes, respectively.
.Sh HOOKS
This node type supports the following hooks:
.Pp
.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.
.Sh CONTROL MESSAGES
This node type supports the generic control messages, plus the following:
.Bl -tag -width foo
.It Dv NGM_VJC_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   enabled;       /* Enable compression/decompression */
  u_char   numChannels;   /* Number of outgoing channels */
  u_char   compressCID;   /* OK to compress outgoing CID's */
};
.Ed
.Pp
When
.Dv enabled
is set to zero, the node operates in ``pass through'' mode, only
accepting packets on the
.Dv ip
and
.Dv vjip
hooks.
.Dv numChannels
should be set to the number of compression channels, and is a value
between 3 and 16, inclusive.
.Pp
The
.Dv compressCID
field indicates whether it is OK to compress the CID field for
outgoing compressed TCP packets. This value should be zero unless
either (a) it not possible for an incoming frame to be lost, or
(b) lost frames can be reliably detected and a
.Dv NGM_VJC_RECV_ERROR
mesages is immediately sent whenever this occurs.
.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
.Dv "net/slcompress.h" .
.It Dv NGM_VJC_CLR_STATS
Clears the node statistics counters. Statistics are also cleared whenever the
.Dv enabled
field is changed from zero to one by a
.Dv NGM_VJC_CONFIG
control message.
.It Dv NGM_VJC_RECV_ERROR
When the
.Dv compressCID
is set to one, this message must be sent to the node immediately
after detecting that a recieved 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.
.Sh SHUTDOWN
This node shuts down upon receipt of a
.Dv NGM_SHUTDOWN
control message, or when all hooks have been disconnected.
.Sh BUGS
When used as a loadable kernel module, this node type requires that the file
.Dv "net/slcompress.c"
was compiled into the kernel. Currently the only way to insure this
is to include the
.Dv slip ,
.Dv ppp ,
or
.Dv i4bipr
pseudo-devices in your kernel compilation. In the future there should
be a kernel option that causes inclusion of this file without requiring
one of these drivers.
.Sh SEE ALSO
.Xr netgraph 4 ,
.Xr ng_ppp 8 ,
.Xr ng_iface 8 ,
.Xr ngctl 8 .
.Rs
.%A V. Jacobsen
.%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 AUTHOR
Archie Cobbs <archie@whistle.com>