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ETHERNET IP frame checksum calculation

1. The following is a specman method that calculates the header checksum.The IP header structure is shown below:
   
   
2. The calculation is done using one's complement.
3. The code is checked in a bigger project. A VHDL DUT is used, which knows how to analyses ETHERNET frames. The project can be seen at: VHDL IP stack main page.
4. The code is first checked on a sample frame, which was downloaded from the WEB. This frame was also checked with the DUT, using a small VHDL test-bench.
5. The numerical values, which were used in the WEB example are:
   
   0x4500+0x0054+0xaafb+0x4000+0x0000+0xfc01+0x8b85+0xe902+0x8b85+0xd96e=0x505ca
   0x5+0x05ca=0x05cf -> 0xfa30 checksum.
   Note: the checksum field is zero in the calculatation.
6. The e-code follows:

calc_check_sum(data : list of bit) : uint(bits:16) is {
  var chk_sum : uint(bits:32);
  var tmp : uint(bits:16);
  var tmps: uint(bits:16);
  for i from 0 to data.size()-16 step 16 do {
    unpack(packing.low, data[i+15:i], tmp);
    --shuffle nibbles to the right sum order
    tmps[ 3: 0]=tmp[ 7: 4];
    tmps[ 7: 4]=tmp[ 3: 0];
    tmps[11: 8]=tmp[15:12];
    tmps[15:12]=tmp[11: 8];
    chk_sum += tmps.as_a(uint(bits:32));
    --hanlde carry
    if(chk_sum[19:16] != 0) then {
      chk_sum = chk_sum[19:16]+chk_sum[15:0];
    };
  };--for
  --do one's complement
  for i from 0 to 15 do {
    result[i:i]=~chk_sum[i:i];
  };
};--calc_check_sum

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