[Ches94] defines another firewall component that other authors sometimes include under the category of application gateway. A circuit-level gateway relays TCP connections but does no extra processing or filtering of the protocol. For example, the TELNET application gateway example provided here would be an example of a circuit-level gateway, since once the connection between the source and destination is established, the firewall simply passes bytes between the systems. Another example of a circuit-level gateway would be for NNTP, in which the NNTP server would connect to the firewall, and then internal systems' NNTP clients would connect to the firewall. The firewall would, again, simply pass bytes.