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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.