xref: /freebsd/contrib/tcpdump/slcompress.h (revision 9336e0699bda8a301cd2bfa37106b6ec5e32012e)
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
88