In Transport mode encryption of an IPsec server, only the payload of the data packet is encrypted. This mode is designed to encrypt the message within an IP packet, while the header remains unencrypted. Transport mode is used for end-to-end communication between a client and a server, where the server can interpret the headers to route the packet to the correct application or process.
References: The information is consistent with the IPsec standards and documentation, which specify that in Transport mode, the data within the original IP packet is protected, but not the IP header123. This ensures that the packet retains its original IP header, allowing it to be routed properly through the network.