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 (setconfig)
            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 (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 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.

Debian                                          January 19, 1999                                       NG_VJC(4)