What is a Stateful Packet Inspection (SPI)?

Stateful Packet Inspection (SPI) is essential for modern network security as it allows a firewall to track the state of active connections. This means that the firewall can monitor the state and context of network traffic, remembering characteristics such as the connection's source, destination, and protocol. By maintaining state information, the firewall can make more informed decisions about which packets to allow or block based on the established connections, rather than merely filtering packets based on predefined rules.

This method enhances security by ensuring that only packets belonging to established connections are permitted through the firewall, thereby preventing unauthorized access and potential threats from entering the network. In contrast, the other options represent different security mechanisms or features, but none address the specific tracking nature of SPI, which is key to understanding how it operates in a firewall context.