dist/tcpdump/slcompress.h

   1 /*      $NetBSD$        */
   2
   3 /*
   4  * Definitions for tcp compression routines.
   5  *
   6  * @(#) Header: /tcpdump/master/tcpdump/slcompress.h,v 1.2 2000/10/09 02:03:44 guy Exp (LBL)
   7  *
   8  * Copyright (c) 1989, 1990, 1992, 1993 Regents of the University of
   9  * California. All rights reserved.
  10  *
  11  * Redistribution and use in source and binary forms are permitted
  12  * provided that the above copyright notice and this paragraph are
  13  * duplicated in all such forms and that any documentation,
  14  * advertising materials, and other materials related to such
  15  * distribution and use acknowledge that the software was developed
  16  * by the University of California, Berkeley.  The name of the
  17  * University may not be used to endorse or promote products derived
  18  * from this software without specific prior written permission.
  19  * THIS SOFTWARE IS PROVIDED ``AS IS'' AND WITHOUT ANY EXPRESS OR
  20  * IMPLIED WARRANTIES, INCLUDING, WITHOUT LIMITATION, THE IMPLIED
  21  * WARRANTIES OF MERCHANTIBILITY AND FITNESS FOR A PARTICULAR PURPOSE.
  22  *
  23  *      Van Jacobson (van@ee.lbl.gov), Dec 31, 1989:
  24  *      - Initial distribution.
  25  */
  26
  27 /*
  28  * Compressed packet format:
  29  *
  30  * The first octet contains the packet type (top 3 bits), TCP
  31  * 'push' bit, and flags that indicate which of the 4 TCP sequence
  32  * numbers have changed (bottom 5 bits).  The next octet is a
  33  * conversation number that associates a saved IP/TCP header with
  34  * the compressed packet.  The next two octets are the TCP checksum
  35  * from the original datagram.  The next 0 to 15 octets are
  36  * sequence number changes, one change per bit set in the header
  37  * (there may be no changes and there are two special cases where
  38  * the receiver implicitly knows what changed -- see below).
  39  *
  40  * There are 5 numbers which can change (they are always inserted
  41  * in the following order): TCP urgent pointer, window,
  42  * acknowlegement, sequence number and IP ID.  (The urgent pointer
  43  * is different from the others in that its value is sent, not the
  44  * change in value.)  Since typical use of SLIP links is biased
  45  * toward small packets (see comments on MTU/MSS below), changes
  46  * use a variable length coding with one octet for numbers in the
  47  * range 1 - 255 and 3 octets (0, MSB, LSB) for numbers in the
  48  * range 256 - 65535 or 0.  (If the change in sequence number or
  49  * ack is more than 65535, an uncompressed packet is sent.)
  50  */
  51
  52 /*
  53  * Packet types (must not conflict with IP protocol version)
  54  *
  55  * The top nibble of the first octet is the packet type.  There are
  56  * three possible types: IP (not proto TCP or tcp with one of the
  57  * control flags set); uncompressed TCP (a normal IP/TCP packet but
  58  * with the 8-bit protocol field replaced by an 8-bit connection id --
  59  * this type of packet syncs the sender & receiver); and compressed
  60  * TCP (described above).
  61  *
  62  * LSB of 4-bit field is TCP "PUSH" bit (a worthless anachronism) and
  63  * is logically part of the 4-bit "changes" field that follows.  Top
  64  * three bits are actual packet type.  For backward compatibility
  65  * and in the interest of conserving bits, numbers are chosen so the
  66  * IP protocol version number (4) which normally appears in this nibble
  67  * means "IP packet".
  68  */
  69
  70 /* packet types */
  71 #define TYPE_IP 0x40
  72 #define TYPE_UNCOMPRESSED_TCP 0x70
  73 #define TYPE_COMPRESSED_TCP 0x80
  74 #define TYPE_ERROR 0x00
  75
  76 /* Bits in first octet of compressed packet */
  77 #define NEW_C   0x40    /* flag bits for what changed in a packet */
  78 #define NEW_I   0x20
  79 #define NEW_S   0x08
  80 #define NEW_A   0x04
  81 #define NEW_W   0x02
  82 #define NEW_U   0x01
  83
  84 /* reserved, special-case values of above */
  85 #define SPECIAL_I (NEW_S|NEW_W|NEW_U)           /* echoed interactive traffic */
  86 #define SPECIAL_D (NEW_S|NEW_A|NEW_W|NEW_U)     /* unidirectional data */
  87 #define SPECIALS_MASK (NEW_S|NEW_A|NEW_W|NEW_U)
  88
  89 #define TCP_PUSH_BIT 0x10