2.Circuit-level gateways Firewall

These firewalls work at the session layer of the OSI model, or TCP/IP layer of the TCP/IP.  They monitor TCP handshaking between packets to determine whether a requested session is legitimate. Traffic is filtered based on the specified session rules, such as when a session is initiated by the recognized computer. Information passed to remote computer through a circuit level gateway appears to have originated from the gateway. This is useful for hiding information about protected networks. Circuit level gateways are relatively inexpensive and have the advantage of hiding information about the private network they protect. On the other hand, they do not filter individual packets.Unknown traffic is allowed up to level 4 of network stack. These are hardware firewalls and apply security     mechanisms when a TCP or UDP connection is established.