/* set ip checksum of a given ip header*/
void compute_ip_checksum(struct iphdr *iphdrp)
{
	iphdrp->check = 0;
	iphdrp->check = compute_checksum((unsigned short *)iphdrp, iphdrp->ihl << 2);
}
 
/* Compute checksum for count bytes starting at addr, using one's complement of one's complement sum*/
static unsigned short compute_checksum(unsigned short *addr, unsigned int count)
{
	register unsigned long sum = 0;
 
	while (count > 1) {
		sum += * addr++;
		count -= 2;
	}
 
	//if any bytes left, pad the bytes and add
 
	if (count > 0)
		sum += ((*addr)&htons(0xFF00));
 
	//Fold sum to 16 bits: add carrier to result
	while (sum >> 16)
		sum = (sum & 0xffff) + (sum >> 16);
 
	//one's complement
	sum = ~sum;
	return ((unsigned short)sum);
}