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

The IPsec Authentication Header (AH) protocol provides data integrity and authentication for IP packets. In transport mode, the AH is inserted into the IP packet between the IP header and the upper-layer protocol. 1. Transport Mode Structure: In transport mode, IPsec protects the payload of the IP packet. The original IP header is preserved but modified to reflect the presence of the AH. The sequence of components from left to right (start of the packet to the end) is as follows:
• Original IP Header (A): This remains at the front of the packet.
• Authentication Header (C): The AH is inserted immediately after the IP header.
• Rest of Original Packet (D): This contains the transport layer header (TCP/UDP) and the application data.
• Padding (B): If necessary, padding is added to the end of the payload to meet block size requirements for the authentication algorithm. 2. Mapping to Options: Based on the standard construction of an AH transport-mode packet, the order is: IPsec Header (Original IP Header) $\to$ Authentication Header $\to$ Rest of original packet $\to$ Padding. 3. Conclusion: This corresponds to the sequence A, C, D, B, which is represented by Option (2).