Stateful inspection works by adding greater awareness of which layer?

1. A stateless filter judges each packet alone. 2. Tracking a connection means understanding the layer that makes them. 3. Stateful inspection is accomplished by incorporating greater awareness of the transport layer. 4. So the answer is the transport layer. _Source: NIST SP 800-41 Rev. 1, 'Guidelines on Firewalls and Firewall Policy'_