/* set tcp checksum: given IP header and its IP payload, ie, UDP header and its payload */
void compute_udp_checksum(struct iphdr *pIph, unsigned short *ipPayload)
{
	register unsigned long sum = 0;
	struct udphdr *udphdrp = (struct udphdr *)(ipPayload);
	unsigned short udpLen = htons(udphdrp->len);
	//the source ip
	sum += (pIph->saddr >> 16) & 0xFFFF;
	sum += (pIph->saddr) & 0xFFFF;
	//the dest ip
	sum += (pIph->daddr >> 16) & 0xFFFF;
	sum += (pIph->daddr) & 0xFFFF;
	//protocol and reserved: 17
	sum += htons(IPPROTO_UDP);
	//the length
	sum += udphdrp->len;
	//add the IP payload
	//printf("add ip payload\n");
	//initialize checksum to 0
	udphdrp->check = 0;

	while (udpLen > 1) {
		sum += * ipPayload++;
		udpLen -= 2;
	}

	//if any bytes left, pad the bytes and add
	if (udpLen > 0) {
		//printf("+++++++++++++++padding: %d\n", udpLen);
		sum += ((*ipPayload)&htons(0xFF00));
	}

	while (sum >> 16)
		sum = (sum & 0xffff) + (sum >> 16);

	sum = ~sum;
	//set computation result
	udphdrp->check = ((unsigned short)sum == 0x0000) ? 0xFFFF : (unsigned short)sum;
}