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Circuit Level Gateways

These operate at the level of the TCP connection (or circuit). In effect, the application gateway transfers a copy of application-level data between the internal network and the outside Internet instead of the IP packets themselves.
 
Such a structure might be used where an internal host wishes to, for example, fetch a WWW page from a server in the external Internet. The client web browser establishes a connection to a server which is running on the gateway host. The gateway server in turn establishes a second connection to the actual WWW server on the external Internet, and passes the incoming HTML document back to the internal browser process. Such a server (running on the gateway, or bastion, host) is called a proxy.
 
One of the advantages of a proxy server is the caching of external web pages, thereby potentially reducing traffic on the connection to the outside Internet.
 


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