1/*
2 * Checksum routine for Internet Protocol family headers (C Version).
3 *
4 * Refer to "Computing the Internet Checksum" by R. Braden, D. Borman and
5 * C. Partridge, Computer Communication Review, Vol. 19, No. 2, April 1989,
6 * pp. 86-101, for additional details on computing this checksum.
7 */
8
9#include <zebra.h>
10
11int				/* return checksum in low-order 16 bits */
12in_cksum(ptr, nbytes)
13register u_short	*ptr;
14register int		nbytes;
15{
16	register long		sum;		/* assumes long == 32 bits */
17	u_short			oddbyte;
18	register u_short	answer;		/* assumes u_short == 16 bits */
19
20	/*
21	 * Our algorithm is simple, using a 32-bit accumulator (sum),
22	 * we add sequential 16-bit words to it, and at the end, fold back
23	 * all the carry bits from the top 16 bits into the lower 16 bits.
24	 */
25
26	sum = 0;
27	while (nbytes > 1)  {
28		sum += *ptr++;
29		nbytes -= 2;
30	}
31
32				/* mop up an odd byte, if necessary */
33	if (nbytes == 1) {
34		oddbyte = 0;		/* make sure top half is zero */
35		*((u_char *) &oddbyte) = *(u_char *)ptr;   /* one byte only */
36		sum += oddbyte;
37	}
38
39	/*
40	 * Add back carry outs from top 16 bits to low 16 bits.
41	 */
42
43	sum  = (sum >> 16) + (sum & 0xffff);	/* add high-16 to low-16 */
44	sum += (sum >> 16);			/* add carry */
45	answer = ~sum;		/* ones-complement, then truncate to 16 bits */
46	return(answer);
47}
48