What does a Circuit-Level Gateway primarily monitor?

A Circuit-Level Gateway primarily monitors the TCP handshake between packets. This type of firewall operates at the transport layer and is responsible for establishing a connection between two endpoints by using the TCP protocol. It allows traffic based on the established connection and monitors the state of that connection, which includes the TCP handshake process consisting of the SYN, SYN-ACK, and ACK packets.

By focusing on the handshake process, the Circuit-Level Gateway ensures that the connection initiated is legitimate and that packets exchanged during that session are part of an established communication, effectively controlling access and maintaining security over connections. This ability to track state is what differentiates Circuit-Level Gateways from other types of firewalls that may look at deeper application layer details or overall network logs.