ethernet checksum nightmare

A

axr0284

Guest
Hi,
I have been fighting with this for some time now and I cannot figure
it out. I have read the 802.3 information and I have read a lot of
forums and they all have some kind of general answer.

I am trying to create a very simple MAC module that will send data to a

PHY for transmission. I am currently using the aesic crc VHDL function
to perform my CRC checks.


Let's say I want to send the following packet (I know this is not a
correct ethernet packet. Bear with me):
00 00 12 33 FF FF (in bytes) The bytes will be sent with the left most
byte first.


0000 0000 0000 0000 0001 0010 0011 0011 1111 1111 (in bits)


I would first switch the bits in each byte individually and then feed
it to the CRC module which has been initialized with all 1s.


Therefore


0000 0000 0000 0000 1000 0100 1100 1100 1111 1111


is fed into the CRC module starting with the byte on the left most
side.


Hopefully this is correct so far.


After the last byte is fed through, I will grab the output of the CRC
module, Invert it and shift it's bits.


The output from the CRC is


7E 11 64 34 ( in bytes)


0111 1110 0001 0001 0110 0100 0011 0100 ( in bits)


After inverting


1000 0001 1110 1110 1001 1011 1100 1011 ( in bits)


After switching


1101 0011 1101 1001 0111 0111 1000 0001 ( in bits)


D3 D9 77 81 ( in bytes)


This means that my packet that I will send to the PHY will be start
with the left most byte


00 00 12 33 FF FF D3 D9 77 81 ( in bytes)


This value seems to agree with some CRC32 software I found on the net.


**************************************************


NOW lets say i am receiving this data from the PHY.


00 00 12 33 FF FF D3 D9 77 81 ( in bytes)


I do the same as for sending, I switch the bits in each byte (including

the FCS) and feed it to the CRC module which has been initialized to
all 1. After sending the right most byte, I should get the magic number

C7 04 DD 7B out which says that it is working but i don't. Below is the

exact sequence of CRC output:


Previous CRC Value | Input | Next CRC Value | Next FCS Value
FFFFFF | 00 | 4E08BFB4 |
D202EF8D
4E08BFB4 | 00 | 00B7647D | 41D912FF
00B7647D | 12 | A5EAE0CF | 0CF8A85A
A5EAE0CF | 33 | 648CF998 | E660CED9
648CF998 | FF | 82AF68FF | 00E90ABE
82AF68FF | FF | 7E116434 | D3D97781
7E116434 | D3 | BB9FD215 | 57B40622
BB9FD215 | D9 | 07F1A3E0 | F83A701F
07F1A3E0 | 77 | 16C33676 | 91933c97
16C33676 | 81 | F86C1D9B | 2647C9E0


The VHDL code I used is below:
process(clk)
begin
if(clk'event and clk = '1') then
crc_32_out <= nextCRC32_D8(input0(0) & input0(1) & input0(2) &
input0(3) & input0(4) & input0(5) & input0(6) & input0(7), input1);


for i in 31 downto 0 loop
crc_32_final(i) <= NOT crc_32_out(31-i);
end loop;
end if;


end process;


My biggest issue is not getting the magic number after running the
packet + CRC through the CRC module. I am at a lost here. I know this
is a lot of information but i hope
somebody can help me out. I guess this will also help anybody that
researches this topic after me. Thanks a lot,
Amish
 
axr0284 wrote:

My biggest issue is not getting the magic number after running the
packet + CRC through the CRC module. I am at a lost here. I know this
is a lot of information but i hope
Click to expand...
There are many ways to get this wrong,
and most have been discussed here:
http://groups.google.com/groups/search?q=treseler+crc+32

-- Mike Treseler
 
I read through it and I still could not figure it out correctly but I
think i got it right this time:

So if I send 00 00 12 33 FF FF to the PHY from the FPGA,
this is what is sent on the WIRE from the PHY

00 00 84 CC FF FF

lsb first for each byte

So I feed the original data "00 00 12 33 FF FF" as is through the
EASICS module with initialization of all 1, I get the <RESIDUE>
AA C5 98 B0

Now it clearly states in the 802.3 specs that "The bit sequence is
complemented and the result is the CRC"

So I need to complement the <RESIDUE> to get the <CRC> : 55 3A 67 4F

Then I need to send it MSB first since the 802.3 spec says

"The 32 bits of the CRC value are placed in the frame check sequence
field so that the x31 term is the leftmost bit of the first octet,
and the x0 term is the right most bit of the last octet.
(The bits of the CRC are thus """transmitted""" in the order x31,
x30..., x1, x0.)"

So I need to switch the bits in each byte before appending to the
Packet
Therefore the <CRC> becomes the <FCS>
AA 5C E6 F2

So the full packet is
00 00 12 33 FF FF AA 5C E5 F2

but what is sent on the wire is
00 00 84 CC FF 55 3A 67 4F
since it is lsb first

On receiving , I guess i need to flip the CRC back to 55 3A 67 4F
before feeding it into the EASICS module to get the magic number.

I hope I got it right this time. This took a while.
I think i will just recompute the packet and compare the incoming FCS
with that. It might be easier. Thanks
Amish
 
"axr0284" <axr0284@yahoo.com> writes:

I read through it and I still could not figure it out correctly but I
think i got it right this time:

So if I send 00 00 12 33 FF FF to the PHY from the FPGA,
this is what is sent on the WIRE from the PHY

00 00 84 CC FF FF

lsb first for each byte

So I feed the original data "00 00 12 33 FF FF" as is through the
EASICS module with initialization of all 1, I get the <RESIDUE
AA C5 98 B0

Now it clearly states in the 802.3 specs that "The bit sequence is
complemented and the result is the CRC"

So I need to complement the <RESIDUE> to get the <CRC> : 55 3A 67 4F

Then I need to send it MSB first since the 802.3 spec says

"The 32 bits of the CRC value are placed in the frame check sequence
field so that the x31 term is the leftmost bit of the first octet,
and the x0 term is the right most bit of the last octet.
(The bits of the CRC are thus """transmitted""" in the order x31,
x30..., x1, x0.)"

So I need to switch the bits in each byte before appending to the
Packet
Therefore the <CRC> becomes the <FCS
AA 5C E6 F2

So the full packet is
00 00 12 33 FF FF AA 5C E5 F2

but what is sent on the wire is
00 00 84 CC FF 55 3A 67 4F
since it is lsb first

On receiving , I guess i need to flip the CRC back to 55 3A 67 4F
before feeding it into the EASICS module to get the magic number.
Click to expand...
No don't do that. You need exactly one flipping to get the magic
number (well, any odd number of bit inversions will do...)


Kai
--
Kai Harrekilde-Petersen <khp(at)harrekilde(dot)dk>
 
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