What characteristic defines a circuit-level gateway?

A circuit-level gateway is defined by its operation at the session layer of the OSI model. This gateway establishes a virtual circuit between the client and server for the duration of the session. It enables the two parties to communicate without directly passing data packets; instead, it manages the session and the state of the connection.

This operation allows for the inspection of the session without checking the individual packets along the way, unlike packet-filtering firewalls, which evaluate each packet independently. Circuit-level gateways are particularly useful for maintaining the integrity and flow of the communication between two endpoints while ensuring that only authorized sessions can be established.

The focus on the session layer distinguishes circuit-level gateways from other types of firewalls and security devices that might operate at different layers, such as application or network layers.