[SOLVED] TCP Checksum function

yaplej · 04-21-2010, 06:14 PM

Hey all,

I have been trying to write my own TCP checksum function. Iv got a description of how the checksum is calculated from here.
http://en.wikipedia.org/wiki/Transmi...cksum_for_IPv4

In short the one complement of each 16-bit word are added up, and then the result of that is the bitwise complemented.

So to sum all the 16-bit words it would be something like this.

Code:

for each word{
      sum += ~word;
}

Then that sum is bitwise complemented.

Code:

sum = ~sum;

So here is my function so far. I have not been able to get a correct checksum out of though. Iv been tweaking things all day, and not figured out what I am doing wrong.

Code:

unsigned short tcp_sum_calc(unsigned short len_tcp, unsigned short *src_addr, unsigned short *dest_addr, unsigned short *buff)
{
      
	unsigned short prot_tcp = 6;
	unsigned short sum;
	int i;
	sum = 0;
	
	/* Check if the tcp length is even or odd.  Add padding if odd. */
	if((len_tcp % 2) == 1){
		buff[len_tcp] = 0;  // Empty space in the ip buffer should be 0 anyway.
      	len_tcp += 1; // incrase length to make even.
	}
	
	/* add the pseudo header */	
	sum += ~src_addr[0];
	sum += ~src_addr[1];
	sum += ~dest_addr[0];
	sum += ~dest_addr[1];
	sum += ~len_tcp;
	sum += ~prot_tcp;
      
	/* 
	 * calculate the checksum for the tcp header and payload
	 * len_tcp represents number of 8-bit bytes, 
	 * we are working with 16-bit words so divide len_tcp by 2. 
	 */
	for(i=0;i<(len_tcp/2);i++){
		sum += buff[i];
	}
    printf("%02x:", (unsigned short)sum);
    
	// keep only the last 16 bits of the 32 bit calculated sum and add the carries
	//sum = (sum >> 16) + (sum & 0xFFFF);
	//printf("%02x:", (unsigned short)sum);
	
	//sum += (sum >> 16);
	//printf("%02x:", (unsigned short)sum);
	
	// Take the bitwise complement of sum
	sum = ~sum;
	printf("%02x:\n", (unsigned short)sum);

	return ((unsigned short) sum);
}

Thank you.

broken · 04-22-2010, 05:10 AM

Mind the endianness of the system.

yaplej · 04-22-2010, 01:45 PM

So a TCP checksum is not the sum of one's complement of all the 16-bit words. Its the 32-bit sum of all the 16-bit words converted to 16-bit adding any carries, folded on itself, then taking the one's complement of that. Well something like that. Here is my working function.

Minding the endianness, and also how the 32-bit sum is converted to 16-bit. I noticed the order I was originally doing it was wrong based on the example given in the RFC.
http://www.faqs.org/rfcs/rfc1071.html

Code:

unsigned short tcp_sum_calc(unsigned short len_tcp, unsigned short *src_addr, unsigned short *dest_addr, unsigned short *buff)
{
      
	unsigned short prot_tcp = 6;
	long sum;
	int i;
	sum = 0;
	
	/* Check if the tcp length is even or odd.  Add padding if odd. */
	if((len_tcp % 2) == 1){
		buff[len_tcp] = 0;  // Empty space in the ip buffer should be 0 anyway.
      	len_tcp += 1; // incrase length to make even.
	}
	
	/* add the pseudo header */	
	sum += ntohs(src_addr[0]);
	sum += ntohs(src_addr[1]);
	sum += ntohs(dest_addr[0]);
	sum += ntohs(dest_addr[1]);
	sum += len_tcp; // already in host format.
	sum += prot_tcp; // already in host format.
      
	/* 
	 * calculate the checksum for the tcp header and payload
	 * len_tcp represents number of 8-bit bytes, 
	 * we are working with 16-bit words so divide len_tcp by 2. 
	 */
	for(i=0;i<(len_tcp/2);i++){
		sum += ntohs(buff[i]);
	}
    
	// keep only the last 16 bits of the 32 bit calculated sum and add the carries
	sum = (sum & 0xFFFF) + (sum >> 16);
	sum += (sum >> 16);
	
	// Take the bitwise complement of sum
	sum = ~sum;

	return htons(((unsigned short) sum));
}