/* set tcp checksum: given IP header and tcp segment */
void compute_tcp_checksum(struct iphdr *pIph, unsigned short *ipPayload)
{
	register unsigned long sum = 0;
	unsigned short tcpLen = ntohs(pIph->tot_len) - (pIph->ihl << 2);
	struct tcphdr *tcphdrp = (struct tcphdr *)(ipPayload);
	//add the pseudo header
	//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: 6
	sum += htons(IPPROTO_TCP);
	//the length
	sum += htons(tcpLen);
	//add the IP payload
	//initialize checksum to 0
	tcphdrp->check = 0;

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

	//if any bytes left, pad the bytes and add
	if (tcpLen > 0) {		
		sum += ((*ipPayload)&htons(0xFF00));
	}

	//Fold 32-bit sum to 16 bits: add carrier to result
	while (sum >> 16)
		sum = (sum & 0xffff) + (sum >> 16);

	sum = ~sum;
	//set computation result
	tcphdrp->check = (unsigned short)sum;
}