Ethernet CRC32 calculation - software vs algorithmic result

Question

I\'m trying to calculate the Frame Check Sequence (FCS) of an Ethernet packet byte by byte. The polynomial is 0x104C11DB7. I did follow the XOR-SHIFT algorithm

Answer 1

There is generally a bit of trial and error required to get CRC calculations to match, because you never end up reading exactly what has to be done. Sometimes you have to bit-reverse the input bytes or the polynomial, sometimes you have to start off with a non-zero value, and so on.

One way to bypass this is to look at the source of a program getting it right, such as http://sourceforge.net/projects/crcmod/files/ (at least it claims to match, and comes with a unit test for this).

Another is to play around with an implementation. For instance, if I use the calculator at http://www.lammertbies.nl/comm/info/crc-calculation.html#intr I can see that giving it 00000000 produces a CRC of 0x2144DF1C, but giving it FFFFFFFF produces FFFFFFFF - so it's not exactly the polynomial division you describe, for which 0 would have checksum 0

From a quick glance at the source code and these results I think you need to start with an CRC of 0xFFFFFFFF - but I could be wrong and you might end up debugging your code side by side with the implementation, using corresponding printfs to find out where the first differ, and fixing the differences one by one.