Hi All, I have implemented TCP protocol. For calculation of TCP Checksum I have used code from "http://www.netfor2.com/tcpsum.htm". Some times it calculates checksum as zero.
Thanks in advance. Prithvi asked 30 Aug '12, 05:38  prithvi |
2 Answers:
A tcp checksum of 0x0000 is legal. The final ones complement in the algorithm can result in 0x0000. An answer of 0xffff is not legal. That is, prior to the final ones complement in the calculation, the answer can never be 0x0000. This is also true for the IP header checksum, but it is not true for UDP. UDP has a special case where 0x0000 is reserved for "no checksum computed". Thus for UDP, 0x0000 is illegal and when calculated following the standard algorithm, replaced with 0xffff. answered 26 Mar '13, 09:36  alank25 edited 26 Mar '13, 09:44 |
No, the checksum should not be 0x0000, but 0xffff. It all depends on the way you calculate the checksum. When using one-complement, there can be +0 and -0 which are different. For more information see RFC 1624: Computation of the Internet Checksum via Incremental Update answered 30 Aug '12, 08:26  SYN-bit ♦♦ |
I don't think this is correct. RFC 1624 appears to be clarifying a mistake made in RFC 1141 whereby a checksum of
0xFFFFwas computed using incremental update when it should have been0x0000, as would have been computed using the checksum computation from scratch method, presumably the one provided in section 4.1 of RFC 1071.I think alank25's answer is actually the correct one.
By the way, Wireshark considers both
0xFFFFand0x0000as correct IP checksums. I'm not sure this is a good idea. If my (and alank25's) reading of RFC 1624 is correct, then it would seem that Wireshark should be indicating that0xFFFFis not correct, or perhaps only correct according to RFC 1141.You're right! I will accept @alank25's answer on behalf of the original poster.