#include <sys/types.h>


/*
 *			I N _ C K S U M
 *
 * Checksum routine for Internet Protocol family headers (C Version)
 *   (based upon a version written by Mike Muuss, BRL)
 */
in_cksum(addr, len)
u_short *addr;
int len;
{
	register int nleft = len;
	register u_short *w = addr;
	register int sum = 0;

	/*
	 *  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.
	 */
	while( nleft > 1 )  {
		sum += *w++;
		nleft -= 2;
	}

	/* mop up an odd byte, if necessary */
	if( nleft == 1 )
		sum += *(u_char *)w;

	/*
	 * add back carry outs from top 16 bits to low 16 bits
	 */
    while (sum>>16)
        sum = (sum & 0xffff) + (sum >> 16);     /* add hi 16 to low 16 */
        
	return ( (~sum) & 0xFFFF);
}