Which of the following is not a protocol used in IPSec?

In the context of IPSec, ESP (Encapsulating Security Payload) and AH (Authentication Header) are both integral protocols used to provide security services for IP packets. ESP is responsible for providing confidentiality, authentication, and integrity by encrypting the payload of the packet. AH similarly provides authentication and integrity but does not offer encryption.

IKE (Internet Key Exchange) is another component of IPSec, used for negotiating security associations and establishing shared keys for secure communication.

TCP (Transmission Control Protocol), on the other hand, is not a protocol used in IPSec. Instead, it is a transport layer protocol that manages data transmission over IP networks. While TCP operates at the transport layer to ensure reliable communication between hosts, it does not provide the security services or functionality associated with IPSec protocols.

Thus, the identification of TCP as not being a protocol used in IPSec accurately reflects the role and purpose of the other options related to the IPSec framework.