Stateful inspection works at Network Layer .

Many Thanks,

Chinmoy Roy

Stateful firewalls use session-based rules to control traffic.

The disadvantage of a stateful firewall is that it can be vulnerable to Denial of Service (DoS) attacks if a lot of new connections are opened very fast.

cisco

Stateful Inspection.

A stateful inspection firewall uses a technique known as stateful packet filtering to keep track of communication channels. This is different when compared to basic firewalls. Once the packet and connection has been sent, a normal firewall will not remember the communication channel, whereas the stateful inspection firewall will. This also proves useful to protect connectionless communication protocols.

The method you are referring to is called SPI - Stateful Packet Inspection. A firewall employing SPI is commonly called a stateful firewall.