[RESOLVED] ICMPv6 Checksum

Hey there,
I am trying to figure out how to calculate the [ICMPv6 checksum](https://tools.ietf.org/html/rfc4443#section-2.3).

Let's assume I have an IPv6 packet like this:

Source IP: fe 80 00 00 00 00 00 00 be ae c5 ff fe df 31 d1

Destination IP: ff 02 00 00 00 00 00 00 00 00 00 00 00 00 00 01

Payload length: 00 18

Next header: 3a

ICMPv6: 80 00 00 c8 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00

What I am doing wrong when performing this calculation:
sum of:
fe80
0
0
0
beae
c5ff
fedf
31d1
ff02
0
0
0
0
0
0
1
18
3a
8000
now moving the bytes and adding them to the end

Code:

---

sum += sum >> 16;

---

result now is 3337
inverting the value

Code:

---

sum = ~sum;

---

result is c800
wireshark says it should be ccc8

I can't find the mistake here, can anyone please take a look at it?
Thanks and have a nice day!

Here is the wireshark screenshot:
http://i.imgur.com/Y7HaDZS.jpg

What is the type of sum? Consider

Code:

---

#include <iostream>
using namespace std;

int main()
{
        short int chk = 0x3337;

        short int inv = ~chk;

        cout << hex << "0x" << chk << " 0x" << inv << endl;
}

---

which displays

Code:

---

0x3337 0xccc8

---

From the data in post #1,

Code:

---

#include <iostream>
using namespace std;

int main()
{
        const unsigned short int bytes[] = {
                0xfe80, 0x0000, 0x0000, 0x0000, 0xbeae, 0xc5ff, 0xfedf, 0x31d1,
                0xff02, 0x0000, 0x0000, 0x0000, 0x0000, 0x0000, 0x0000, 0x0001,
                0x0018,
                0x003a,
                0x8000};

        unsigned int sum = 0;

        for (auto i : bytes)
                sum += i;

        while (sum & 0xffff0000)
                sum = (sum >> 16) + (sum & 0xffff);

        sum = ~sum;

        cout << hex << "0x" << (unsigned short) sum << endl;
}

---

displays

Code:

---

0xccc8

---

Quote:

---

Originally Posted by 2kaud

From the data in post #1,

Code:

---

#include <iostream>
using namespace std;

int main()
{
        const unsigned short int bytes[] = {
                0xfe80, 0x0000, 0x0000, 0x0000, 0xbeae, 0xc5ff, 0xfedf, 0x31d1,
                0xff02, 0x0000, 0x0000, 0x0000, 0x0000, 0x0000, 0x0000, 0x0001,
                0x0018,
                0x003a,
                0x8000};
....

---

---

Thank you so much for this check. The problem was in the data type but this has really helped me!

Quote:

---

Originally Posted by 2kaud

What is the type of sum? Consider

Code:

---

#include <iostream>
using namespace std;

int main()
{
        short int chk = 0x3337;

        short int inv = ~chk;

        cout << hex << "0x" << chk << " 0x" << inv << endl;
}

---

which displays

Code:

---

0x3337 0xccc8

---

---

Many thanks. Now I feel really dumb. It was the data type after all...