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86 lines
3.6 KiB
C
86 lines
3.6 KiB
C
/*
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* Definitions for tcp compression routines.
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*
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* Copyright (c) 1989, 1990, 1992, 1993 Regents of the University of
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* California. All rights reserved.
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*
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* Redistribution and use in source and binary forms are permitted
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* provided that the above copyright notice and this paragraph are
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* duplicated in all such forms and that any documentation,
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* advertising materials, and other materials related to such
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* distribution and use acknowledge that the software was developed
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* by the University of California, Berkeley. The name of the
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* University may not be used to endorse or promote products derived
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* from this software without specific prior written permission.
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* THIS SOFTWARE IS PROVIDED ``AS IS'' AND WITHOUT ANY EXPRESS OR
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* IMPLIED WARRANTIES, INCLUDING, WITHOUT LIMITATION, THE IMPLIED
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* WARRANTIES OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE.
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*
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* Van Jacobson (van@ee.lbl.gov), Dec 31, 1989:
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* - Initial distribution.
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*/
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/*
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* Compressed packet format:
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*
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* The first octet contains the packet type (top 3 bits), TCP
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* 'push' bit, and flags that indicate which of the 4 TCP sequence
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* numbers have changed (bottom 5 bits). The next octet is a
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* conversation number that associates a saved IP/TCP header with
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* the compressed packet. The next two octets are the TCP checksum
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* from the original datagram. The next 0 to 15 octets are
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* sequence number changes, one change per bit set in the header
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* (there may be no changes and there are two special cases where
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* the receiver implicitly knows what changed -- see below).
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*
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* There are 5 numbers which can change (they are always inserted
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* in the following order): TCP urgent pointer, window,
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* acknowlegement, sequence number and IP ID. (The urgent pointer
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* is different from the others in that its value is sent, not the
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* change in value.) Since typical use of SLIP links is biased
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* toward small packets (see comments on MTU/MSS below), changes
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* use a variable length coding with one octet for numbers in the
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* range 1 - 255 and 3 octets (0, MSB, LSB) for numbers in the
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* range 256 - 65535 or 0. (If the change in sequence number or
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* ack is more than 65535, an uncompressed packet is sent.)
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*/
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/*
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* Packet types (must not conflict with IP protocol version)
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*
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* The top nibble of the first octet is the packet type. There are
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* three possible types: IP (not proto TCP or tcp with one of the
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* control flags set); uncompressed TCP (a normal IP/TCP packet but
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* with the 8-bit protocol field replaced by an 8-bit connection id --
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* this type of packet syncs the sender & receiver); and compressed
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* TCP (described above).
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*
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* LSB of 4-bit field is TCP "PUSH" bit (a worthless anachronism) and
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* is logically part of the 4-bit "changes" field that follows. Top
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* three bits are actual packet type. For backward compatibility
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* and in the interest of conserving bits, numbers are chosen so the
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* IP protocol version number (4) which normally appears in this nibble
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* means "IP packet".
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*/
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/* packet types */
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#define TYPE_IP 0x40
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#define TYPE_UNCOMPRESSED_TCP 0x70
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#define TYPE_COMPRESSED_TCP 0x80
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#define TYPE_ERROR 0x00
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/* Bits in first octet of compressed packet */
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#define NEW_C 0x40 /* flag bits for what changed in a packet */
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#define NEW_I 0x20
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#define NEW_S 0x08
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#define NEW_A 0x04
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#define NEW_W 0x02
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#define NEW_U 0x01
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/* reserved, special-case values of above */
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#define SPECIAL_I (NEW_S|NEW_W|NEW_U) /* echoed interactive traffic */
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#define SPECIAL_D (NEW_S|NEW_A|NEW_W|NEW_U) /* unidirectional data */
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#define SPECIALS_MASK (NEW_S|NEW_A|NEW_W|NEW_U)
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#define TCP_PUSH_BIT 0x10
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