Frame check sequence calculation

Question

Hello,

I am trying to calculate the FCS for ipv4/ipv6 packets.

I use the following algorithm:

I get the hex string of the whole frame (from ethernet layer to data incapsulated inside tcp/udp)
I transform it into a binary string
I negate the first 32 bits
I reverse the order of the bits in each byte in the whole frame
I xor the first 33 elements of the binary string with 100000100110000010001110110110111 and trim the left zeros
I repeat step 5 until my binary string has a length less or equal to 32
I negate all the bits
I transform the binary string into a hex string

And that is my algorithm. Wireshark says it's wrong, and I'm sure he's right, but I don't know what I did wrong.

I learned about it here: http://cs.nju.edu.cn/yangxc/dcc_teach/fcs-calc.pdf

Any ideas?

Answer 1

The PDF mentions NTCIP and AB 3418.

Cite: NTCIP and AB 3418 protocol developers typically use the inverse polynomial method to calculate the Frame Check Sequence (FCS).

If you try to create an FCS for an ethernet frame with a method defined for NTCIP it may not work for one of the following reasons:

either: the FCS algorithm for ethernet is different. Then that's the reason for the discrepancy.
or: the FCS algorithm defined in the PDF (for NTCIP) is the same as for ethernet (I did not check). In that case, your implementation is buggy.

For a detailed information how to calculate the FCS please check the following file:

http://www.xilinx.com/support/documentation/application_notes/xapp209.pdf

You can also read the Wireshark source code:

packet-eth.c:add_ethernet_trailer()

        guint32 fcs = crc32_802_tvb(tvb, tvb_length(tvb) - 4);

crc32-tvb.c:

crc32_802_tvb(tvbuff_t *tvb, guint len)
{
    guint32 c_crc;
c_crc = crc32_ccitt_tvb(tvb, len);
/* Byte reverse. */
c_crc = GUINT32_SWAP_LE_BE(c_crc);
return ( c_crc );

}

Regards
Kurt