In the context of IPSec, what is the main purpose of the Transport Mode?

The primary purpose of the Transport Mode in IPSec is to encrypt only the payload data of an IP packet while leaving the header intact. This approach makes it suitable for scenarios where end-to-end communication security is required between specific applications or devices, rather than a full tunnel that would encapsulate an entire IP packet. By focusing solely on the payload, Transport Mode provides confidentiality for the data being transmitted without altering the routing information found in the original packet headers.

This is crucial for applications that need to maintain the capability to route packets through the network without needing to decrypt and re-encrypt the packet headers. It effectively secures the session data while keeping the necessary elements for proper data direction and handling intact, meaning that the communication's endpoint integrity and authentication aren’t lost, but are handled at a different layer, sometimes reusing existing infrastructures or services.