Which firewall type requires significant amounts of memory to track TCP connections?

Stateful Packet Inspection (SPI) Firewalls are designed to monitor the state of active connections and make decisions based on the context of those connections. Unlike stateless firewalls that inspect each packet in isolation without maintaining information about the state of the traffic, stateful firewalls keep track of the state of all ongoing connections. This capability requires a significant amount of memory, as the firewall must store information about each connection's state, including details such as source and destination IP addresses, port numbers, and the connection state (e.g., established, closing, or closed). This tracking allows stateful firewalls to make informed decisions about whether to allow or block traffic based on the state of the connection, enhancing security and efficiency in managing data packets.

The other types of firewalls mentioned may vary in their functionality and resource requirements, but they do not require the same level of memory and state management as stateful packet inspection firewalls. For instance, appliance firewalls can include both stateful and stateless capabilities, next-generation firewalls integrate advanced features like intrusion prevention and application awareness, and host-based firewalls are software-based and generally rely on local system resources rather than extensive memory for connection tracking.