Circuit and application gateways
It will provide UDP and TCP connection security, which actually means that it can reassemble examine or block any packets in a TCP or UDP connection. So this will actually work between a transport layer and application layer like a session layer. Unlike application gateway, what this circuit gateway does is it will monitor TCP data, packet handshaking and also the session fulfillment of Firewall rules and policies. This circuit gateway can also act as virtual private network over the Internet by doing encryption from Firewall to Firewall