Packets in a stateful inspection firewall are queued and then analyzed at all OSI layers, providing a more complete inspection of the data. By examining the state and context of the incoming data packets, it helps to track the protocols that are considered “connectionless”, such as UDP-based applications and Remote Procedure Calls (RPC). Source: KRUTZ, Ronald L. & VINES, Russel D., The CISSP Prep Guide: Mastering the Ten Domains of Computer Security, John Wiley & Sons, 2001, Chapter 3: Telecommunications and Network Security (page 91).