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 86