Answer:
Stateful packet inspection (SPI) is a firewall type that scans network traffic based on TCP or UDP transmission. It monitors the state of active connections and checks incoming packets against authorized communication patterns. SPI filters packets based on protocols, port numbers, and other rules defined in the firewall's configuration. It inspects packets at the application layer to detect attacks that try to exploit vulnerabilities in specific services or applications. SPI is considered more secure and efficient than traditional packet filtering firewalls because it can filter traffic at a more granular level and can maintain a session state to facilitate more accurate filtering decisions.