My UDP checksum calculation gives wrong results every time

Question

I am trying to manually calculate checksums for various UDP packets, but I am always getting the wrong results compared to what is shown in Wireshark. Below is an example of how I do it:

Source Address: 192.168.0.103 (0xC0A8, 0x0067) Destination Address: 192.168.0.1 (0xC0A8, 0x0001) Source Port: 57090 (0xDF02) Destination Port: 8000 (0x1F40) Packet length: 19 (0x0013) Data: hello world (0x6865, 0x6C6C, 0x6F20, 0x776F, 0x726C, 0x6400) Expected checksum: 0xEDFD (from wireshark)

I understand that the UDP checksum is calculated with the following variables:

Source IP + Destination IP + 17 (0x0011 - protocol code) + 10 (0x000A - pseudoheader length) + Source Port + Destination Port + UDP Packet Length + Data

All of them in groups of 16 bits.

So my calculations are (in the same order):

0xC0A8 + 0x0067 + 0xC0A8 + 0x0001 + 0x0011 + 0x000A + 0xDF02 + 0x1F40 + 0x0013 + 0x6865 + 0x6C6C + 0x6F20 + 0x776F + 0x726C + 0x6400

The result of the above sum is:

0x511F4

Now, since the result is higher than 0xFFFF I do the following:

0x11F4 + 0x0005 = 0x11F9

After flipping the bits I get the result:

0xEE06 <- which, as you can see, it is different from the expected one

So my question is what am I doing wrong? I am always getting almost the result of what is shown in wireshark.

Below is a screenshot of the packet inside Wireshark, as a reference https://www.evernote.com/l/AWl0H1AGoxpGX4_zjgDlVBcytJM-HP_PvQE

There is an RFC on this. Have you read it?

user207421
– user207421

2017-08-28 01:26:34 +00:00
Commented Aug 28, 2017 at 1:26 — user207421
– user207421, Commented Aug 28, 2017 at 1:26
Yes, I read RFC768.

Ibrahim
– Ibrahim

2017-08-28 04:40:40 +00:00
Commented Aug 28, 2017 at 4:40 — Ibrahim
– Ibrahim, Commented Aug 28, 2017 at 4:40

Ibrahim · Accepted Answer · 2017-08-28 18:52:02Z

I found what I was doing wrong, in case somebody else has a similar issue.

The way I was computing the pseudo header was like this:

Source IP + Destination IP + 17 (0x0011 - protocol code) + 10 (0x000A - pseudoheader length)

The problem was at the length byte. It should be the UDP packet length, not the pseudoheader length.

The final formula is this:

Source IP + Destination IP + 17 (0x0011 - protocol code) + UDP Packet Length + Source Port + Destination Port + UDP Packet Length + Data

Notice the UDP Packet length appears twice. First for the pseudoheader and second for the actual UDP header.

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