An engineer is configuring IPsec VPN and needs an authentication protocol that is reliable and...

 IKEv1 is the authentication protocol that is reliable and supports ACK and sequence for IPsec VPN. IKEv1 is a key management protocol that is used in conjunction with IPsec to establish secure and authenticated connections between IPsec peers. IKEv1 uses UDP port 500 and consists of two phases: phase 1 and phase 2. In phase 1, the peers authenticate each other and negotiate a shared secret key that is used to encrypt the subsequent messages. In phase 2, the peers negotiate the security parameters for the IPsec tunnel, such as the encryption and authentication algorithms, the lifetime, and the mode (transport or tunnel). IKEv1 uses ACK and sequence numbers to ensure the reliability and integrity of the messages exchanged between the peers. ACK is an acknowledgment message that confirms the receipt of a previous message. Sequence number is a unique identifier that is assigned to each message to prevent replay attacks and to detect missing or out-of-order messages. IKEv1 also supports various authentication methods, such as pre-shared keys, digital certificates, and extended authentication (XAUTH). References : Internet Key Exchange for IPsec VPNs Configuration Guide, Security for VPNs with IPsec Configuration Guide, IPSec Architecture