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 */
26 u_int16_t
iso_cksum(void * buf,u_int16_t len,u_int16_t pos)27 iso_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