The TCP (checksum) control sum, intended for package integrity control, uses a pseudo-header that consists of the source IP of origin, destination, reserved file name (identified by 0000,0000), protocol (x06) and the length of the header Tcp.
| IP source | 192.168.0.1 |
| IP destination | 192.168.0.2 |
| Reserved/TCP Control | 0 / 6 |
| Padding/Lenght | 0 / 10 |
| TCP source port | 20 |
| TCP port destination | 10 |
| Sequence number | 11 |
| ACK Acknowledgement Number | 0 |
| Offset / Reserved / Flag | 5/0/2 (Flag SYN) |
| Window (c) | 8192 |
| Checksum (c) | 0 |
| Urgent pointer | 0 |
| Data | "Ah" (41 68 unicode converts to hexadecimal) |
| Source IP (32-bit) | 11000000.10101000. |
| 00000000.00000001 | |
| IP destination (32 bits) | 11000000.10101000. |
| 00000000.00000010 | |
| Reserved/TCP Control (16 bts) | 00000000.00000110 |
| Padding/Lenght – 0/10 (16 bts) | 00000000.00001010 |
| TCP source port – 20 (16 bit) | 00000000.00010100 |
| TCP port destination – $10 | 00000000.00001010 |
| Sequence number – 11 (32 bits) | 00000000.00000000 |
| 00000000.00001011 | |
| ACK Acknowledgement Number | 00000000.00000000 |
| Offset / Reserved / Flag | |
| 5 / 0 / 2 (Flag SYN) | 01010000.00000010 |
| Window – 8192 | 01010000.00000010 |
| Checksum – 0 | 00100000.00000000 |
| Urgent Pointer – 0 | 00000000.00000000 |
| Data – "Ah" | |
| (41 68 unicode | |
| converts to hexadecimal) | 01000001.01101000 |
| SOMME DE CONTROLE | |
| Step 1 (addition) | 10.10000010.00001110 |
The binary addition is based on the rules:
- Rule 1: 0 – 0 – 0;
- Rule 2: 1 – 0 – 1;
- Rule 3: 1 – 1 – 0 with carry of 1;
- Rule 4: 1 – 1 – 1 – 1 with carry of 1.
To add up several binary numbers you have to proceed in pairs and carry out postponements.
CONTROLE SOMME (step 1) 00000000.00000010.10000010.00001110
The calculated sum is then on 32 bits the first 16 bits are added with the last 1
6 bits.
| CONTROLE SOMME (step 2) | 00000000.00000010 |
| 10000010.00001110 | |
| 10000010.00010000 |
The next step is to calculate the supplement to 1 of the binary number found previously.
The complement to one of a binary number is the value obtained by reversing all the bits of that number (by swapping the 0 by 1 and vice versa).
CONTROLE SOMME – 01111101.1110111 1 I.e.
7D EF in hexadecimal
The conversion to hexadecimal base 16 is done by breaking the binary number into 4-bit packets:
0111/1101/1110/1111
7/D/E/F