NAME
ng_vjc - Van Jacobson compression netgraph node type
SYNOPSIS
#include <sys/types.h>
#include <netinet/in.h>
#include <netinet/in_systm.h>
#include <netinet/ip.h>
#include <net/slcompress.h>
#include <netgraph/ng_vjc.h>
DESCRIPTION
The 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 ip hook represents the uncompressed side of the node, while
the vjcomp, vjuncomp, and vjip hooks represent the compressed side of the
node. Packets received on the 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 “always pass
through” mode in either direction.
Van Jacobson compression only applies to TCP packets. Only “normal”
(i.e., common case) TCP packets are actually compressed. These are
output on the vjcomp hook. Other TCP packets are run through the state
machine but not compressed; these appear on the vjuncomp hook. Other
non-TCP IP packets are forwarded unchanged to vjip.
When connecting to a ng_ppp(4) node, the ip, vjuncomp, vjcomp, and vjip
hooks should be connected to the ng_ppp(4) node’s vjc_ip, vjc_vjcomp,
vjc_vjuncomp, and vjc_ip hooks, respectively.
HOOKS
This node type supports the following hooks:
ip Upstream (uncompressed) IP packets.
vjcomp Downstream compressed TCP packets.
vjuncomp Downstream uncompressed TCP packets.
vjip Downstream uncompressed IP packets.
CONTROL MESSAGES
This node type supports the generic control messages, plus the following:
NGM_VJC_SET_CONFIG
This command resets the compression state and configures it
according to the supplied struct ngm_vjc_config argument. This
structure contains the following fields:
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 */
};
When enableComp is set to zero, all packets received on the ip hook
are forwarded unchanged out the vjip hook. Similarly, when
enableDecomp is set to zero, all packets received on the vjip hook
are forwarded unchanged out the ip hook, and packets are not
accepted on the vjcomp and vjuncomp hooks. When a node is first
created, both compression and decompression are disabled and the
node is therefore operating in bi-directional “pass through” mode.
When enabling compression, maxChannel should be set to the number of
outgoing compression channels minus one, and is a value between 3
and 15, inclusive. The 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 NGM_VJC_RECV_ERROR below).
NGM_VJC_GET_STATE
This command returns the node’s current state described by the
struct slcompress structure, which is defined in
NGM_VJC_CLR_STATS
Clears the node statistics counters. Statistics are also cleared
whenever the enableComp or enableDecomp fields are changed from zero
to one by a NGM_VJC_SET_CONFIG control message.
NGM_VJC_RECV_ERROR
When the peer has CID header field compression enabled, this message
must be sent to the local 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.
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_iface(4), ng_ppp(4), ngctl(8)
V. Jacobson, Compressing TCP/IP Headers, RFC 1144.
G. McGregor, The PPP Internet Control Protocol (IPCP), RFC 1332.
HISTORY
The ng_vjc node type was implemented in FreeBSD 4.0.
AUTHORS
Archie Cobbs 〈archie@FreeBSD.org〉
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.
When built as a loadable kernel module, this module includes the file
net/slcompress.c. Although loading the module should fail if
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.