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29 * @(#)cksum.c 8.1 (Berkeley) 6/6/93
30 * $FreeBSD: src/usr.sbin/timed/timed/cksum.c,v 1.3 1999/08/28 01:20:17 peter Exp $
31 * $DragonFly: src/usr.sbin/timed/timed/cksum.c,v 1.4 2004/09/05 02:09:24 dillon Exp $
34 #include <sys/types.h>
40 * Checksum routine for Internet Protocol family headers (C Version)
42 * There is no profit in a specialized version of the checksum
43 * function for any machine where int's are 32 bits and shorts are 16.
45 * All timed packets are smaller than 32K shorts, so there is no need to
46 * worry about carries except at the end.
49 in_cksum(u_short *addr, int len)
57 * Our algorithm is simple, using a 32 bit accumulator (sum),
58 * we add sequential 16 bit words to it, and at the end, fold
59 * back all the carry bits from the top 16 bits into the lower
67 /* mop up an odd byte, if necessary */
69 sum += (*(u_char *)w) << 8;
72 * add back carry outs from top 16 bits to low 16 bits
74 sum = (sum >> 16) + (sum & 0xffff); /* add hi 16 to low 16 */
75 sum += (sum >> 16); /* add carry */
76 answer = ~sum; /* truncate to 16 bits */