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33 .\" Author: Archie Cobbs <archie@FreeBSD.org>
36 .\" $Whistle: ng_vjc.8,v 1.4 1999/01/25 23:46:28 archie Exp $
43 .Nd Van Jacobson compression netgraph node type
47 .In netinet/in_systm.h
54 node type performs Van Jacobson compression, which is used
55 over PPP, SLIP, and other point-to-point IP connections to
56 compress TCP packet headers.
59 hook represents the uncompressed side of the node, while the
64 hooks represent the compressed side of the node.
65 Packets received on the
67 will be compressed or passed through as appropriate.
68 Packets received on the other three hooks will be uncompressed as appropriate.
69 This node also supports
70 .Dq always pass through
71 mode in either direction.
73 Van Jacobson compression only applies to TCP packets.
76 (i.e., common case) TCP packets are actually compressed.
77 These are output on the
80 Other TCP packets are run through the state machine but not
81 compressed; these appear on the
84 Other non-TCP IP packets are forwarded unchanged to
95 hooks should be connected to the
105 This node type supports the following hooks:
107 .Bl -tag -width foobarbazi
109 Upstream (uncompressed) IP packets.
111 Downstream compressed TCP packets.
113 Downstream uncompressed TCP packets.
115 Downstream uncompressed IP packets.
118 This node type supports the generic control messages, plus the following:
120 .It Dv NGM_VJC_SET_CONFIG
121 This command resets the compression state and configures it according
123 .Dv "struct ngm_vjc_config"
125 This structure contains the following fields:
126 .Bd -literal -offset 4n
127 struct ngm_vjc_config {
128 u_char enableComp; /* Enable compression */
129 u_char enableDecomp; /* Enable decompression */
130 u_char maxChannel; /* Number of outgoing channels - 1 */
131 u_char compressCID; /* OK to compress outgoing CID's */
137 is set to zero, all packets received on the
139 hook are forwarded unchanged out the
144 is set to zero, all packets received on the
146 hook are forwarded unchanged out the
148 hook, and packets are not accepted on the
153 When a node is first created,
154 both compression and decompression are disabled and the node is
155 therefore operating in bi-directional
159 When enabling compression,
161 should be set to the number of outgoing compression channels minus one,
162 and is a value between 3 and 15, inclusive.
165 field indicates whether it is OK to compress the CID header field for
166 outgoing compressed TCP packets.
167 This value should be zero unless
168 either (a) it is not possible for an outgoing frame to be lost, or
169 (b) lost frames can be reliably detected and immediately
170 reported to the peer's decompression engine (see
171 .Dv NGM_VJC_RECV_ERROR
173 .It Dv NGM_VJC_GET_STATE
174 This command returns the node's current state described by the
175 .Dv "struct slcompress"
176 structure, which is defined in
177 .In net/slcompress.h .
178 .It Dv NGM_VJC_CLR_STATS
179 Clears the node statistics counters.
180 Statistics are also cleared whenever the
184 fields are changed from zero to one by a
185 .Dv NGM_VJC_SET_CONFIG
187 .It Dv NGM_VJC_RECV_ERROR
188 When the peer has CID header field compression enabled,
189 this message must be sent to the local
192 after detecting that a received frame has been lost, due to a bad
193 checksum or for any other reason.
194 Failing to do this can result in corrupted TCP stream data.
197 This node shuts down upon receipt of a
199 control message, or when all hooks have been disconnected.
207 .%T "Compressing TCP/IP Headers"
212 .%T "The PPP Internet Control Protocol (IPCP)"
218 node type was implemented in
221 .An Archie Cobbs Aq archie@FreeBSD.org
223 As the initialization routine in the kernel implementation of
224 Van Jacobson compression initializes both compression and decompression
225 at once, this node does not allow compression and decompression to
226 be enabled in separate operations.
227 In order to enable one when
228 the other is already enabled, first both must be disabled, then
230 This of course resets the node state.
231 This restriction may be lifted in a later version.
233 When built as a loadable kernel module, this module includes the file
234 .Pa net/slcompress.c .
235 Although loading the module should fail if
237 already exists in the kernel, currently it does not, and the duplicate
238 copies of the file do not interfere.
239 However, this may change in the future.