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/*
* Checksum routine for Internet Protocol family headers (C Version).
*
* Refer to "Computing the Internet Checksum" by R. Braden, D. Borman and
* C. Partridge, Computer Communication Review, Vol. 19, No. 2, April 1989,
* pp. 86-101, for additional details on computing this checksum.
*/
#include <zebra.h>
#include "checksum.h"
int /* return checksum in low-order 16 bits */
in_cksum(void *parg, int nbytes)
{
u_short *ptr = parg;
register long sum; /* assumes long == 32 bits */
u_short oddbyte;
register u_short answer; /* assumes u_short == 16 bits */
/*
* Our algorithm is simple, using a 32-bit accumulator (sum),
* we add sequential 16-bit words to it, and at the end, fold back
* all the carry bits from the top 16 bits into the lower 16 bits.
*/
sum = 0;
while (nbytes > 1) {
sum += *ptr++;
nbytes -= 2;
}
/* mop up an odd byte, if necessary */
if (nbytes == 1) {
oddbyte = 0; /* make sure top half is zero */
*((u_char *) &oddbyte) = *(u_char *)ptr; /* one byte only */
sum += oddbyte;
}
/*
* Add back carry outs from top 16 bits to low 16 bits.
*/
sum = (sum >> 16) + (sum & 0xffff); /* add high-16 to low-16 */
sum += (sum >> 16); /* add carry */
answer = ~sum; /* ones-complement, then truncate to 16 bits */
return(answer);
}
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