Question

How do you interpret this phrase?

Checksum
This is the value to make zero on the addition of the lower 8 bits from the header to the checksum.

With this protocol description:

Protocol
Consist of header (1 byte) + data length (1 byte) +
command data (13 bytes) + check sum (1 bytes) + connection ID (1
byte).

(I literally copied this protocol description, so I’dont know why there is 1 bytes (in plural). But I can tell you it is only one byte)

Here are some sample TCP packets of this protocol:

HE:DL:------------Command Data -------------:CS:ID
02:0d:be:ef:03:06:00:19:d3:02:00:00:60:00:00:ed:01
02:0d:be:ef:03:06:00:cd:d2:02:00:00:20:00:00:7a:01
02:0d:be:ef:03:06:00:10:f6:02:00:ba:30:00:00:49:01
02:0d:be:ef:03:06:00:c8:d8:02:00:20:30:00:00:49:01
// Requested Packets
02:0d:be:ef:03:06:00:13:d3:01:00:02:30:01:00:21:01
02:0d:be:ef:03:06:00:c2:ff:02:00:90:10:00:00:d8:01

Where

• HE is the HEader (which is fixed to 0x02)
• DL is the DataLength (which is always 0x0d, because the packets are all the same length)
• CS is the CheckSum (which is my question about)
• ID is the connection ID (seems to be always 01 in my tests)

I can’t figure out how the checksum is computed. I hope I gave enough info.

Thanks in advance.

Answer 1

I think there’s just an error in the description. It looks like they are summing all the bytes between the header and the checksum. And the checksum is just a number that clears the lower 8 bits. So, for the first example, the sum of all bytes between the header and the checksum is 0x0313. Or

0x0313 0000 0011 0001 0011
0x00ED 0000 0000 1110 1101

When it’s lined up like that, you can clearly see you’ll be zeroing out the lower 8 bits and returning:

0x0400 0000 0100 0000 0000

You didn’t specify a language, but you could also quickly calculate your own checksum by doing (0 XOR calculatedSum) + 1.