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Monitoring the TCP handshake for session legitimacy is best accomplished using a circuit-level gateway. Circuit-level gateways operate at the Transport Layer of the OSI model and are designed to monitor and manage TCP connections. They can establish a connection between a client and a server, allowing them to exchange packets, while ensuring that only legitimate sessions are processed and maintained.

Circuit-level gateways track the state of the TCP handshake process, which includes the SYN, SYN-ACK, and ACK segments. By watching for these specific packets, a circuit-level gateway can ensure that the connection is authentic and that no unauthorized or malformed packets disrupt the communication. This capability makes it particularly effective for session legitimacy verification, as it can enforce security policies and potentially mitigate the risks of certain types of network attacks, such as SYN flooding.

While packet sniffers, network analyzers, and proxy servers can capture and analyze traffic, they do not inherently monitor the state of sessions in the same manner as circuit-level gateways. Packet sniffers capture data packets that traverse the network, network analyzers can help perform in-depth traffic analysis, and proxy servers act as intermediaries facilitating requests from clients to servers but do not primarily focus on the TCP handshake's legitimacy monitoring. Therefore, the choice of a circuit-level gateway is