Circuit-level gateway firewall technology operates at the session layer of the OSI model. This type of firewall validates TCP or UDP sessions before allowing traffic through, without inspecting the contents of the data packets. It acts as a handshaking device between trusted clients or servers and untrusted hosts, ensuring that the session packets adhere to established rules for a connection. The session layer, which is Layer 5 in the OSI model, is responsible for setting up, managing, and terminating the connections between applications1234.

References: The information about the operation of circuit-level gateways at the session layer is supported by networking resources and documentation1234.