In IPsec, what mode protects all data and its associated headers during transportation?

Tunnel mode is the correct choice because it encapsulates and protects the entire original IP packet, which includes both the header and the payload. This mode is particularly useful for securing communication between two networks over an untrusted medium, such as the internet.

In tunnel mode, the original IP header is encapsulated within a new IP packet that is encrypted and sent to the destination. This provides a high level of security since both the data and the headers are encrypted, thus preventing unauthorized parties from analyzing the contents of the packets as they travel across the network.

On the other hand, transport mode only encrypts the payload of the IP packet, leaving the original IP header intact and visible. This mode is suitable for end-to-end communication between two hosts but does not offer the same level of protection for the packet's routing information. As a result, it does not meet the requirement for securing all data and headers during transmission as effectively as tunnel mode.

Therefore, tunnel mode is the preferred choice when the goal is to secure all data along with its associated headers.