1/*	$OpenBSD: iso_cksum.c,v 1.1 2007/10/08 10:44:50 norby Exp $ */
2
3/*
4 * Copyright (c) 2005 Claudio Jeker <claudio@openbsd.org>
5 *
6 * Permission to use, copy, modify, and distribute this software for any
7 * purpose with or without fee is hereby granted, provided that the above
8 * copyright notice and this permission notice appear in all copies.
9 *
10 * THE SOFTWARE IS PROVIDED "AS IS" AND THE AUTHOR DISCLAIMS ALL WARRANTIES
11 * WITH REGARD TO THIS SOFTWARE INCLUDING ALL IMPLIED WARRANTIES OF
12 * MERCHANTABILITY AND FITNESS. IN NO EVENT SHALL THE AUTHOR BE LIABLE FOR
13 * ANY SPECIAL, DIRECT, INDIRECT, OR CONSEQUENTIAL DAMAGES OR ANY DAMAGES
14 * WHATSOEVER RESULTING FROM LOSS OF USE, DATA OR PROFITS, WHETHER IN AN
15 * ACTION OF CONTRACT, NEGLIGENCE OR OTHER TORTIOUS ACTION, ARISING OUT OF
16 * OR IN CONNECTION WITH THE USE OR PERFORMANCE OF THIS SOFTWARE.
17 */
18
19#include <sys/types.h>
20
21#include "ospf6d.h"
22
23/* implementation of Fletcher Checksum -- see RFC 1008 for more info */
24
25/* pos needs to be 0 for verify and 2 <= pos < len for calculation */
26u_int16_t
27iso_cksum(void *buf, u_int16_t len, u_int16_t pos)
28{
29	u_int8_t	*p = buf;
30	int		 c0 = 0, c1 = 0, x;	/* counters */
31	u_int16_t	 sop;
32
33	sop = len - pos - 1;	/* pos is an offset (pos 2 is 3rd element) */
34	p += 2;			/* jump over age field */
35	len -= 2;
36	while (len--) {
37		c0 += *p++;
38		c1 += c0;
39		if ((len & 0xfff) == 0) {
40			/* overflow protection */
41			c0 %= 255;
42			c1 %= 255;
43		}
44	}
45
46	if (pos) {
47		x = ((sop * c0 - c1)) % 255;
48		if (x <= 0)
49			x += 255;
50		c1 = 510 - c0 - x;
51		if (c1 > 255)
52			c1 -= 255;
53		c0 = x;
54	}
55
56	return (c0 << 8 | c1);
57}
58