A security engineer configured a remote access VPN. The remote access VPN allows end users to connect to the network by using an agent that is installed on the endpoint, which establishes an encrypted tunnel. Which of the following protocols did the engineer most likely implement?

IPSec (Internet Protocol Security) is the most likely protocol implemented. IPSec operates at the network layer and provides secure communication over an untrusted network by authenticating and encrypting each IP packet. It commonly uses an agent (VPN client) on the endpoint to establish an encrypted tunnel, fitting the description perfectly. GRE (Generic Routing Encapsulation) is a tunneling protocol but does not inherently provide encryption or security, so it would typically be used in conjunction with IPSec, not as a standalone solution for a secure remote access VPN. SD-WAN (Software-Defined Wide Area Network) is an architectural approach for managing network connections, not a specific tunneling or encryption protocol. EAP (Extensible Authentication Protocol) is an authentication framework used within other protocols (like WPA2-Enterprise or VPNs) but doesn't establish the encrypted tunnel itself.