Which layer do circuit level gateway firewalls operate on?

Circuit level gateway firewalls operate primarily on the session layer of the OSI model. This layer is responsible for establishing, managing, and terminating connections between devices. Circuit-level gateways monitor the TCP handshake and track the state of the connection to determine if a session is legitimate but do not inspect the actual content of the packets. They allow packets to flow freely once a connection is established, providing a level of security based on the legitimacy of the communication session rather than analyzing the content itself.

This is distinct from other layers; for instance, application layer firewalls inspect the data included in the application payload, while network layer firewalls manage IP traffic and address-based rules without concerning themselves with the sessions. Transport layer firewalls would generally focus on TCP/UDP traffic without the session management aspects. Therefore, the correct answer emphasizes the role of the session layer in managing established connections for circuit-level gateway firewalls.