1/*-
2 * Copyright (c) 1985, 1993
3 *	The Regents of the University of California.  All rights reserved.
4 *
5 * Redistribution and use in source and binary forms, with or without
6 * modification, are permitted provided that the following conditions
7 * are met:
8 * 1. Redistributions of source code must retain the above copyright
9 *    notice, this list of conditions and the following disclaimer.
10 * 2. Redistributions in binary form must reproduce the above copyright
11 *    notice, this list of conditions and the following disclaimer in the
12 *    documentation and/or other materials provided with the distribution.
13 * 4. Neither the name of the University nor the names of its contributors
14 *    may be used to endorse or promote products derived from this software
15 *    without specific prior written permission.
16 *
17 * THIS SOFTWARE IS PROVIDED BY THE REGENTS AND CONTRIBUTORS ``AS IS'' AND
18 * ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED TO, THE
19 * IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE
20 * ARE DISCLAIMED.  IN NO EVENT SHALL THE REGENTS OR CONTRIBUTORS BE LIABLE
21 * FOR ANY DIRECT, INDIRECT, INCIDENTAL, SPECIAL, EXEMPLARY, OR CONSEQUENTIAL
22 * DAMAGES (INCLUDING, BUT NOT LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS
23 * OR SERVICES; LOSS OF USE, DATA, OR PROFITS; OR BUSINESS INTERRUPTION)
24 * HOWEVER CAUSED AND ON ANY THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT
25 * LIABILITY, OR TORT (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY
26 * OUT OF THE USE OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF
27 * SUCH DAMAGE.
28 */
29
30#ifndef lint
31#if 0
32static char sccsid[] = "@(#)cksum.c	8.1 (Berkeley) 6/6/93";
33#endif
34static const char rcsid[] =
35  "$FreeBSD$";
36#endif /* not lint */
37
38#include <sys/types.h>
39
40/*
41 *			I N _ C K S U M
42 *
43 * Checksum routine for Internet Protocol family headers (C Version)
44 *
45 * There is no profit in a specialized version of the checksum
46 * function for any machine where int's are 32 bits and shorts are 16.
47 *
48 * All timed packets are smaller than 32K shorts, so there is no need to
49 * worry about carries except at the end.
50 */
51int
52in_cksum(u_short *addr, int len)
53{
54	register int nleft = len;
55	register u_short *w = addr;
56	register u_short answer;
57	register int sum = 0;
58
59	/*
60	 *  Our algorithm is simple, using a 32 bit accumulator (sum),
61	 *  we add sequential 16 bit words to it, and at the end, fold
62	 *  back all the carry bits from the top 16 bits into the lower
63	 *  16 bits.
64	 */
65	while( nleft > 1 )  {
66		sum += *w++;
67		nleft -= 2;
68	}
69
70	/* mop up an odd byte, if necessary */
71	if( nleft == 1 )
72		sum += (*(u_char *)w) << 8;
73
74	/*
75	 * add back carry outs from top 16 bits to low 16 bits
76	 */
77	sum = (sum >> 16) + (sum & 0xffff);	/* add hi 16 to low 16 */
78	sum += (sum >> 16);			/* add carry */
79	answer = ~sum;				/* truncate to 16 bits */
80	return (answer);
81}
82