What service would BEST meet the requirement for securely transferring files while authenticating both the IP header and the payload?

The service that best meets the requirement for securely transferring files while authentically verifying both the IP header and the payload is the Authentication Header (AH). This protocol is part of the Internet Protocol Security (IPsec) suite and is specifically designed to provide authentication and integrity for IP packets.

AH ensures that the sender of the data is authenticated, providing a means to confirm that the data originated from a legitimate source and has not been altered during transmission. It adds an integrity check and can be used to authenticate both the IP header and the entire payload of the packet. This level of security makes it a robust option for secure file transfers that need both authentication and data integrity.

The other options mentioned do not fulfill these specific requirements as effectively as AH. TLS primarily secures applications rather than individual packets and does not handle all aspects of an IP header. PFS (Perfect Forward Secrecy) refers to a method of ensuring that session keys are not compromised even if the server key is compromised in the future but does not directly contribute to the authentication of IP headers. ESP (Encapsulating Security Payload), while it does provide encryption and integrity, does not authenticate the original IP header, which is crucial in this context. Thus, AH is the most suitable choice