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1 office 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 MERCHANTIBILITY 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