ng_vjc man page on FreeBSD

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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
     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.

     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:

	  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
	  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 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).

	  This command returns the node's current state described by the
	  struct slcompress structure, which is defined in <net/slcompress.h>.

	  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.

	  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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