Question

The attributes in IPsec for Authentication Header protocol in transport mode from left to right are :
A. IPsec header
B. Padding
C. Authentication header
D. Rest of original packet
Choose the correct answer from the options given below :

• A. D, C, A, B
• B. A, C, D, B
• C. D, C, B, A
• D. A, C, B, D

Hint:

In Transport Mode, think of IPsec as a "sandwich filling." The original IP Header stays on top (left), the AH is added in the middle, and the original data follows. Padding always goes at the very end to round off the size.

Answer 1

The Correct Option is B

Concept:
• IPsec (Internet Protocol Security) provides security services at the IP layer.
• Authentication Header (AH) provides data integrity and authentication.
• Transport Mode is used for end-to-end communication between two hosts.

Step 1: Analyze the AH Transport Mode packet structure
In transport mode, the IPsec header is inserted immediately after the original IP header and before the upper-layer protocol header (TCP/UDP). The sequence from the start of the packet (left) to the end (right) is:
• Original IP Header: This is often referred to as the initial "IPsec header" in packet diagrams because it is the outer header of the secured packet.
• Authentication Header (AH): Inserted to protect the rest of the packet.
• Upper Layer Protocol Data: The "Rest of the original packet" (Payload).
• Padding: Added at the end to satisfy block size requirements or alignment.

Step 2: Map the attributes to the sequence
Matching with the given letters: A (IP header) $\rightarrow$ C (AH) $\rightarrow$ D (Payload) $\rightarrow$ B (Padding).

Step 3: Conclusion
The correct sequence is A, C, D, B.