contrib/tcpdump/slcompress.h

   1 /*
   2  * Definitions for tcp compression routines.
   3  *
   4  * @(#) $Header: /tcpdump/master/tcpdump/slcompress.h,v 1.2 2000-10-09 02:03:44 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 /*
  26  * Compressed packet format:
  27  *
  28  * The first octet contains the packet type (top 3 bits), TCP
  29  * 'push' bit, and flags that indicate which of the 4 TCP sequence
  30  * numbers have changed (bottom 5 bits).  The next octet is a
  31  * conversation number that associates a saved IP/TCP header with
  32  * the compressed packet.  The next two octets are the TCP checksum
  33  * from the original datagram.  The next 0 to 15 octets are
  34  * sequence number changes, one change per bit set in the header
  35  * (there may be no changes and there are two special cases where
  36  * the receiver implicitly knows what changed -- see below).
  37  *
  38  * There are 5 numbers which can change (they are always inserted
  39  * in the following order): TCP urgent pointer, window,
  40  * acknowlegement, sequence number and IP ID.  (The urgent pointer
  41  * is different from the others in that its value is sent, not the
  42  * change in value.)  Since typical use of SLIP links is biased
  43  * toward small packets (see comments on MTU/MSS below), changes
  44  * use a variable length coding with one octet for numbers in the
  45  * range 1 - 255 and 3 octets (0, MSB, LSB) for numbers in the
  46  * range 256 - 65535 or 0.  (If the change in sequence number or
  47  * ack is more than 65535, an uncompressed packet is sent.)
  48  */
  49
  50 /*
  51  * Packet types (must not conflict with IP protocol version)
  52  *
  53  * The top nibble of the first octet is the packet type.  There are
  54  * three possible types: IP (not proto TCP or tcp with one of the
  55  * control flags set); uncompressed TCP (a normal IP/TCP packet but
  56  * with the 8-bit protocol field replaced by an 8-bit connection id --
  57  * this type of packet syncs the sender & receiver); and compressed
  58  * TCP (described above).
  59  *
  60  * LSB of 4-bit field is TCP "PUSH" bit (a worthless anachronism) and
  61  * is logically part of the 4-bit "changes" field that follows.  Top
  62  * three bits are actual packet type.  For backward compatibility
  63  * and in the interest of conserving bits, numbers are chosen so the
  64  * IP protocol version number (4) which normally appears in this nibble
  65  * means "IP packet".
  66  */
  67
  68 /* packet types */
  69 #define TYPE_IP 0x40
  70 #define TYPE_UNCOMPRESSED_TCP 0x70
  71 #define TYPE_COMPRESSED_TCP 0x80
  72 #define TYPE_ERROR 0x00
  73
  74 /* Bits in first octet of compressed packet */
  75 #define NEW_C   0x40    /* flag bits for what changed in a packet */
  76 #define NEW_I   0x20
  77 #define NEW_S   0x08
  78 #define NEW_A   0x04
  79 #define NEW_W   0x02
  80 #define NEW_U   0x01
  81
  82 /* reserved, special-case values of above */
  83 #define SPECIAL_I (NEW_S|NEW_W|NEW_U)           /* echoed interactive traffic */
  84 #define SPECIAL_D (NEW_S|NEW_A|NEW_W|NEW_U)     /* unidirectional data */
  85 #define SPECIALS_MASK (NEW_S|NEW_A|NEW_W|NEW_U)
  86
  87 #define TCP_PUSH_BIT 0x10