Why does the checksum add a constant number(2)?

[yinyeyy]

Hello!
I have read your code, it is excellent!
I have two questions about the UDP checksum calculation.

1. I don't understand why add a constant 2 when calculating the checksum in "udp_checksum_gen.v".
   You have write a annotation here like "add 2 for length calculation (two length fields in pseudo header)", but I didn't get it.

2. And why does the "checksum_next" have an initial value of "16'h0011 + 16'h0010"?

Can you give me more annotation please?

[alexforencich]

Both of these are to handle the UDP pseudo header, which is included in the UDP packet checksum. See https://en.wikipedia.org/wiki/User_Datagram_Protocol . The pseudo header contains a copy of the protocol field (which is 0x11 for UDP) and two copies of the UDP length (why two copies, I have no idea...). The length fields include the UDP header length (8 bytes). The checksum is computed on a byte-by-byte basis, so instead of computing the checksum and length separately and then adding them together, that 2 added per byte accounts for 1 byte added to both copies of the length field. And the 0x0010 is double the UDP header length, again to account for both copies of the length field.

[yinyeyy]

Thanks a lot!
I understand that. The pseudo header and UDP header all include UDP length. The checksum calculation of Length no longer take 1 extra cycle, but scattered and move to the payload calculation process.