CODEWITHSUNDEEP
javatcpchecksum
Maricruzz
Maricruzz

Reputation: 405

Questions regarding TCP checksum calculation

I have two questions:

  1. RFC 793 states that the checksum should be computed over a 96-bit pseudo-header, the tcp header and the data. The pseudo-header includes the source and destination IP address. Doesn't this defeat the whole idea of keeping the layers seperate, because now when the network layer starts using addresses of a different size, the transport layer needs to change as well.

  2. In another SO post i found the following java code to calculate the checksum.

     private long computeChecksum( byte[] buf ){
     int length = buf.length;
     int i = 0;
     long sum = 0;
     long data;

     // loop through all 16-bit words unless there's 0 or 1 byte left.
     while( length > 1 ){
         data = ( ((buf[i] << 8) & 0xFF00) | ((buf[i + 1]) & 0xFF));
         sum += data;
         if( (sum & 0xFFFF0000) > 0 ){
             sum = sum & 0xFFFF;
             sum += 1;
         }
         i += 2;
         length -= 2;
     }

     if (length > 0 ){ // ie. there are 8 bits of data remaining.
         sum += (buf[i] << 8 & 0xFF00); // create a 16 bit word where the 8 lsb are 0's and add it to the sum.
         if( (sum & 0xFFFF0000) > 0) {
             sum = sum & 0xFFFF;
             sum += 1;
         }
     }

     sum = ~sum; 
     sum = sum & 0xFFFF;
     return sum;
 }

There are some things of which I dont understand why they are needed in that code. First in:

data = ( ((buf[i] << 8) & 0xFF00) | ((buf[i + 1]) & 0xFF));

what is the need for the binary AND's? I don't understand because buf[i] is a byte but is treated as an int and shifted by 8 bits to the left. Doesn't that already guarantee that the result looks like: 00000000 00000000 ???????? 00000000.

Also why are sum and data declared as long? As I see it, both variables will never use more then 17 bits, so why cant we just use an int? In the end they even do: sum = sum & 0xFFFF which discards anything but the 16 least significant bits.

Thanks!

Upvotes: 1

Views: 1658

Answers (1)

Oliver Charlesworth
Oliver Charlesworth

Reputation: 272447

Doesn't this defeat the whole idea of keeping the layers seperate, because now when the network layer starts using addresses of a different size, the transport layer needs to change as well.

Yes. And indeed, this is why RFC 2460 redefines the pseudo-header for IPv6:

Any transport or other upper-layer protocol that includes the addresses from the IP header in its checksum computation must be modified for use over IPv6, to include the 128-bit IPv6 addresses instead of 32-bit IPv4 addresses. [...]

Upvotes: 2

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