contrib/tcpdump/slcompress.h

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