An engineer is configuring IPsec VPN and wants to choose an authentication protocol that is reliable and supports ACK and sequence. Which protocol accomplishes this goal?

IKEv1 is a key exchange and security association management protocol used during IPsec tunnel setup - it does not operate on data packets and provides no per-packet authentication, sequencing, or anti-replay function.

AES-192 is a symmetric block cipher used within ESP for confidentiality - it is an encryption algorithm, not a protocol, and provides no authentication, ACK, or sequence numbering capability.

ESP is defined in RFC 4303 and operates at the IPsec data plane, providing authentication, integrity checking, optional encryption, and anti-replay protection through a mandatory sequence number field in every packet. The combination of the sequence number field and the Integrity Check Value (ICV) directly addresses the requirement for a reliable, sequenced authentication mechanism for IPsec traffic.

AES-256 is a symmetric block cipher used for data encryption within ESP - it is an algorithm, not a protocol, and has no built-in authentication or packet sequencing mechanism.