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   1 /*
   2  * Definitions for tcp compression routines.
   3  *
   4  * @(#) $Header: /tcpdump/master/tcpdump/slcompress.h,v 1.1 2000-10-09 01:53:21 guy Exp $ (LBL)
   5  *
   6  * Copyright (c) 1989, 1990, 1992, 1993 Regents of the University of
   7  * California. All rights reserved.
   8  *
   9  * Redistribution and use in source and binary forms are permitted
  10  * provided that the above copyright notice and this paragraph are
  11  * duplicated in all such forms and that any documentation,
  12  * advertising materials, and other materials related to such
  13  * distribution and use acknowledge that the software was developed
  14  * by the University of California, Berkeley.  The name of the
  15  * University may not be used to endorse or promote products derived
  16  * from this software without specific prior written permission.
  17  * THIS SOFTWARE IS PROVIDED ``AS IS'' AND WITHOUT ANY EXPRESS OR
  18  * IMPLIED WARRANTIES, INCLUDING, WITHOUT LIMITATION, THE IMPLIED
  19  * WARRANTIES OF MERCHANTIBILITY AND FITNESS FOR A PARTICULAR PURPOSE.
  20  *
  21  *      Van Jacobson (van@ee.lbl.gov), Dec 31, 1989:
  22  *      - Initial distribution.
  23  */
  24
  25 #define MAX_STATES 16           /* must be > 2 and < 256 */
  26 #define MAX_HDR MLEN            /* XXX 4bsd-ism: should really be 128 */
  27
  28 /*
  29  * Compressed packet format:
  30  *
  31  * The first octet contains the packet type (top 3 bits), TCP
  32  * 'push' bit, and flags that indicate which of the 4 TCP sequence
  33  * numbers have changed (bottom 5 bits).  The next octet is a
  34  * conversation number that associates a saved IP/TCP header with
  35  * the compressed packet.  The next two octets are the TCP checksum
  36  * from the original datagram.  The next 0 to 15 octets are
  37  * sequence number changes, one change per bit set in the header
  38  * (there may be no changes and there are two special cases where
  39  * the receiver implicitly knows what changed -- see below).
  40  *
  41  * There are 5 numbers which can change (they are always inserted
  42  * in the following order): TCP urgent pointer, window,
  43  * acknowlegement, sequence number and IP ID.  (The urgent pointer
  44  * is different from the others in that its value is sent, not the
  45  * change in value.)  Since typical use of SLIP links is biased
  46  * toward small packets (see comments on MTU/MSS below), changes
  47  * use a variable length coding with one octet for numbers in the
  48  * range 1 - 255 and 3 octets (0, MSB, LSB) for numbers in the
  49  * range 256 - 65535 or 0.  (If the change in sequence number or
  50  * ack is more than 65535, an uncompressed packet is sent.)
  51  */
  52
  53 /*
  54  * Packet types (must not conflict with IP protocol version)
  55  *
  56  * The top nibble of the first octet is the packet type.  There are
  57  * three possible types: IP (not proto TCP or tcp with one of the
  58  * control flags set); uncompressed TCP (a normal IP/TCP packet but
  59  * with the 8-bit protocol field replaced by an 8-bit connection id --
  60  * this type of packet syncs the sender & receiver); and compressed
  61  * TCP (described above).
  62  *
  63  * LSB of 4-bit field is TCP "PUSH" bit (a worthless anachronism) and
  64  * is logically part of the 4-bit "changes" field that follows.  Top
  65  * three bits are actual packet type.  For backward compatibility
  66  * and in the interest of conserving bits, numbers are chosen so the
  67  * IP protocol version number (4) which normally appears in this nibble
  68  * means "IP packet".
  69  */
  70
  71 /* packet types */
  72 #define TYPE_IP 0x40
  73 #define TYPE_UNCOMPRESSED_TCP 0x70
  74 #define TYPE_COMPRESSED_TCP 0x80
  75 #define TYPE_ERROR 0x00
  76
  77 /* Bits in first octet of compressed packet */
  78 #define NEW_C   0x40    /* flag bits for what changed in a packet */
  79 #define NEW_I   0x20
  80 #define NEW_S   0x08
  81 #define NEW_A   0x04
  82 #define NEW_W   0x02
  83 #define NEW_U   0x01
  84
  85 /* reserved, special-case values of above */
  86 #define SPECIAL_I (NEW_S|NEW_W|NEW_U)           /* echoed interactive traffic */
  87 #define SPECIAL_D (NEW_S|NEW_A|NEW_W|NEW_U)     /* unidirectional data */
  88 #define SPECIALS_MASK (NEW_S|NEW_A|NEW_W|NEW_U)