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NG_VJC(4)		 BSD Kernel Interfaces Manual		     NG_VJC(4)

     ng_vjc -- Van Jacobson compression	netgraph node type

     #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>

     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 uncom-
     pressed 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 out-
     put on the	vjcomp hook.  Other TCP	packets	are run	through	the state ma-
     chine 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.

     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.

     This node type supports the generic control messages, plus	the following:

     NGM_VJC_SET_CONFIG	(setconfig)
	  This command resets the compression state and	configures it accord-
	  ing 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 ac-
	  cepted on the	vjcomp and vjuncomp hooks.  When a node	is first cre-
	  ated,	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 pack-
	  ets.	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 (getstate)
	  This command returns the node's current state	described by the
	  struct slcompress structure, which is	defined	in <net/slcompress.h>.

     NGM_VJC_CLR_STATS (clrstats)
	  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	(recverror)
	  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

     This node shuts down upon receipt of a NGM_SHUTDOWN control message, or
     when all hooks have been disconnected.

     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.

     The ng_vjc	node type was implemented in FreeBSD 4.0.

     Archie Cobbs <>

     As	the initialization routine in the kernel implementation	of Van Jacob-
     son 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 ver-

     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.

BSD			       January 19, 1999				   BSD


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