NetworkGenie/Test_Cases/Test_Case2/readme

This folder contains detail for Test_Case2.

1. Verison = 4, to indicate IPv4 version is used.
2. IHL = 5, which indicates 20 bytes.
3. Type of Service = 0,
4. Total Length = 40, which is 28 in hex. The total windows size contains the total lenght of IP header plus data.
5. Identification = 0002,
6. Flags = All flags are set to zero,
7. Fragment Offset = For now, there is no fragment offset so, this value is set to 0.
8. TTL = 40s, which is 28 in hex.
9. Protocol = 6, which indicates TCP.
10. Header Checksum = initially zero and then changed to the 1's complement of the sum of 16 bit values of IP header + data.
11. Source IP        = 192.168.10.20, which is c0a8 0a14 in hex
12. Destination IP   = 192.168.10.10, which is c0a8 0a0a in hex
13. Source Port      = 2341, which is 0925 in hex,
14. Destination Port = 1200, which is 04B0 in hex.
15. Sequence Number = 0 for now,
16. Acknowledgement Number = 0,
17. Flags: 
For the SYN packet, the SYN Flag in the header has a value of 1.
For the SYN+ACK packet, the SYN and ACK flags in the header have a value of 1.
For the ACK packet, the ACK flag in the header has a value of 1.
For the FIN packet, the FIN flag in the header has a value of 1.
For the FIN+ACK packet, the FIN and ACK flags in the header have a value of 1.
18. TCP Checksum = initially zero, then:
(i) Concatenate TCP Pseudo header (Protocol, Source IP, Destination IP, Total Length) with TCP header and TCP data. This creates a big integer.
(ii) Convert this integer to binary and compute bit wise 1's complement. 
19. Urgent Pointer = 0, no urgent data in the header.

The value of header comes out to be:
header = {}
header[0] = 0x45, Version, IHL
header[1] = 0x00, TOS, 
header[2] = 0x00, Total Length,
header[3] = 0x28, Total Length, 
header[4] = 0x00, Identification,
header[5] = 0x02, Identification,
header[6] = 0x00, Flags + Fragment Offset,
header[7] = 0x00, Flags + Fragment Offset,
header[8] = 0x28, TTL
header[9] = 0x06, Protocol
header[10] = 0x00, Header Checksum
header[11] = 0x00, Header Checksum
header[12] = 0xc0, Source IP,
header[13] = 0xa8, Source IP,
header[14] = 0x0a, Source IP,
header[15] = 0x14, Source IP,
header[16] = 0xc0, Destination IP,
header[17] = 0xa8, Destination IP,
header[18] = 0x0a, Destination IP,
header[19] = 0x0a, Destination IP,

The value of IP_Header_Checksum comes out to be: fd5f.

References:
1. https://www.thegeekstuff.com/2012/05/ip-header-checksum/
2. https://www.tutorialspoint.com/calculation-of-tcp-checksum