When does a circuit-level gateway operate during a session?

A circuit-level gateway operates during the establishment of a TCP or UDP connection. This type of gateway functions at the session layer of the OSI model and is responsible for monitoring the establishment and termination of connections. By doing so, it can verify and manage the sessions between communicating parties, ensuring that only legitimate connections are allowed to proceed.

When a connection is being established, the circuit-level gateway can enforce security policies and manage the required parameters for the session. This capability allows it to effectively control the flow of traffic and ensure that unauthorized access is prevented.

Other phases, such as before the connection is established or after it is terminated, do not involve the active monitoring and verification that the gateway provides during the session establishment phase. The focus of a circuit-level gateway is on the connection lifecycle, which makes this option the most appropriate representation of its operational characteristics.