unsigned short in_cksum(unsigned short * addr, int len)
{
int nleft = len;
unsigned short *w = addr;
int sum = 0;
unsigned short answer = 0;

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

if (nleft == 1) {
*(BYTE *) (&answer) = *(BYTE *) w;
sum += answer;
}

sum = (sum >> 16) + (sum & 0xffff);
sum += (sum >> 16);
answer = ~sum;

return (answer);
}