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 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