NetworkGenie/Test_Cases/Test_Case3/readme

Hello,

This folder contains details for Test_Case3.

1. Version = 4
2. IHL = 5, which indicates 20 bytes.
3. Type Of Service (ToS) = 0,
4. Total Length = 40 bytes, which is 28 in hex.
5. Identification = 3,
6. Flags = The flags are set to zero,
7. Fragment Offset = 0 for now,
8. Time To Live (TTL) = 40s, which is 28 in hex.
9. Protocol = 6, which indicates TCP protocol is used.
10. Header Checksum = initially zero and then changed to the sum of 16 bit values of header + data.
11. Source Address = 172.168.5.3, which is aca8 0503 in hex.
12. Destination Address = 172.168.5.4, which is aca8 0504 in hex.
13. Source Port = 5555, which is 15b3 in hex.
14. Destination Port = 6666, which is 1a0a in hex.
15. Sequence Number = 0,
16. Acknowledgement Number = 0,
17. Data Offset = 5, as it indicates the number of bytes after which data starts. The number 5 represents that data starts after 20 bytes. The data offset value is scaled by 4. 
18. RSV = The reserved bits are set to zero for now.
19. Flgas: set according to the packet type.
20. Window Size = 0
21. 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. 
22. Urgent Pointer = 0 for now.

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] = 0x03, 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] = 0xac, Source IP,
header[13] = 0xa8, Source IP,
header[14] = 0x05, Source IP,
header[15] = 0x03, Source IP,
header[16] = 0xac, Destination IP,
header[17] = 0xa8, Destination IP,
header[18] = 0x05, Destination IP,
header[19] = 0x04, Destination IP,

The value of IP_Header_Checksum comes out to be: 2f76.