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 * Definitions for tcp compression routines.
 * @(#) $Header: /cvs/lft/linux-include/net/slcompress.h,v 2003/02/13 20:53:52 nils Exp $ (LBL)
 * Copyright (c) 1989, 1990, 1992, 1993 Regents of the University of
 * California. All rights reserved.
 * Redistribution and use in source and binary forms are permitted
 * provided that the above copyright notice and this paragraph are
 * duplicated in all such forms and that any documentation,
 * advertising materials, and other materials related to such
 * distribution and use acknowledge that the software was developed
 * by the University of California, Berkeley.  The name of the
 * University may not be used to endorse or promote products derived
 * from this software without specific prior written permission.
 *    Van Jacobson (van@ee.lbl.gov), Dec 31, 1989:
 *    - Initial distribution.

#define MAX_STATES 16         /* must be > 2 and < 256 */
#define MAX_HDR MLEN          /* XXX 4bsd-ism: should really be 128 */

 * Compressed packet format:
 * The first octet contains the packet type (top 3 bits), TCP
 * 'push' bit, and flags that indicate which of the 4 TCP sequence
 * numbers have changed (bottom 5 bits).  The next octet is a
 * conversation number that associates a saved IP/TCP header with
 * the compressed packet.  The next two octets are the TCP checksum
 * from the original datagram.  The next 0 to 15 octets are
 * sequence number changes, one change per bit set in the header
 * (there may be no changes and there are two special cases where
 * the receiver implicitly knows what changed -- see below).
 * There are 5 numbers which can change (they are always inserted
 * in the following order): TCP urgent pointer, window,
 * acknowlegement, sequence number and IP ID.  (The urgent pointer
 * is different from the others in that its value is sent, not the
 * change in value.)  Since typical use of SLIP links is biased
 * toward small packets (see comments on MTU/MSS below), changes
 * use a variable length coding with one octet for numbers in the
 * range 1 - 255 and 3 octets (0, MSB, LSB) for numbers in the
 * range 256 - 65535 or 0.  (If the change in sequence number or
 * ack is more than 65535, an uncompressed packet is sent.)

 * Packet types (must not conflict with IP protocol version)
 * The top nibble of the first octet is the packet type.  There are
 * three possible types: IP (not proto TCP or tcp with one of the
 * control flags set); uncompressed TCP (a normal IP/TCP packet but
 * with the 8-bit protocol field replaced by an 8-bit connection id --
 * this type of packet syncs the sender & receiver); and compressed
 * TCP (described above).
 * LSB of 4-bit field is TCP "PUSH" bit (a worthless anachronism) and
 * is logically part of the 4-bit "changes" field that follows.  Top
 * three bits are actual packet type.  For backward compatibility
 * and in the interest of conserving bits, numbers are chosen so the
 * IP protocol version number (4) which normally appears in this nibble
 * means "IP packet".

/* packet types */
#define TYPE_IP 0x40
#define TYPE_ERROR 0x00

/* Bits in first octet of compressed packet */
#define NEW_C     0x40  /* flag bits for what changed in a packet */
#define NEW_I     0x20
#define NEW_S     0x08
#define NEW_A     0x04
#define NEW_W     0x02
#define NEW_U     0x01

/* reserved, special-case values of above */
#define SPECIAL_I (NEW_S|NEW_W|NEW_U)           /* echoed interactive traffic */
#define SPECIAL_D (NEW_S|NEW_A|NEW_W|NEW_U)     /* unidirectional data */

#define TCP_PUSH_BIT 0x10

 * "state" data for each active tcp conversation on the wire.  This is
 * basically a copy of the entire IP/TCP header from the last packet
 * we saw from the conversation together with a small identifier
 * the transmit & receive ends of the line use to locate saved header.
00099 struct cstate {
      struct cstate *cs_next; /* next most recently used cstate (xmit only) */
      u_short cs_hlen;  /* size of hdr (receive only) */
      u_char cs_id;           /* connection # associated with this state */
      u_char cs_filler;
      union {
            char csu_hdr[MAX_HDR];
            struct ip csu_ip; /* ip/tcp hdr from most recent packet */
      } slcs_u;
#define cs_ip slcs_u.csu_ip
#define cs_hdr slcs_u.csu_hdr

 * all the state data for one serial line (we need one of these
 * per line).
00116 struct slcompress {
      struct cstate *last_cs; /* most recently used tstate */
      u_char last_recv; /* last rcvd conn. id */
      u_char last_xmit; /* last sent conn. id */
      u_short flags;
#ifndef SL_NO_STATS
      u_int sls_packets;      /* outbound packets */
      u_int sls_compressed;   /* outbound compressed packets */
      u_int sls_searches;     /* searches for connection state */
      u_int sls_misses; /* times couldn't find conn. state */
      u_int sls_uncompressedin;/* inbound uncompressed packets */
      u_int sls_compressedin; /* inbound compressed packets */
      u_int sls_errorin;      /* inbound unknown type packets */
      u_int sls_tossed; /* inbound packets tossed because of error */
      struct cstate tstate[MAX_STATES];   /* xmit connection states */
      struct cstate rstate[MAX_STATES];   /* receive connection states */
/* flag values */
#define SLF_TOSS 1            /* tossing rcvd frames because of input err */

#ifdef KERNEL
#ifdef __STDC__
extern      void sl_compress_init(struct slcompress *);
extern      u_char sl_compress_tcp(struct mbuf *, struct ip *, struct slcompress *);
extern      int sl_uncompress_tcp(struct mbuf *, int, u_int, struct slcompress *);
extern      void sl_compress_init();
extern      u_char sl_compress_tcp();
extern      int sl_uncompress_tcp();

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