Application-level gateway is a feature of ScreenOS gateways that allows the gateway for parsing the application-layer payloads. Even though we have other ScreenOS features like deep inspection, in which gateway checks traffic at the application layer.
We use application-level gateways for supporting the applications, which use the application layer payload for interacting with the dynamic Transmission Control Protocol(TCP) or the User Datagram Protocol(UDP) on which applications open data connections. The dynamic UDP, TCP, or the other ports which we open through the ScreenOS gateway for allowing the secondary or data channels.
