What is Application level Gateway?

An application layer gateway (ALG) is a feature on ScreenOS gateways that enables the gateway to parse application layer payloads and take decisions on them. Although there are other ScreenOS features, such as deep inspection, in which the gateway inspects traffic at the application layer, ALGs are typically employed to support applications that use the application layer payload to communicate the dynamic Transmission Control Protocol (TCP) or User Datagram Protocol (UDP) ports on which the applications open data connections. Such applications include the File Transfer Protocol (FTP) and various IP telephony protocols. The dynamic TCP, UDP, or other ports that are opened by the ScreenOS gateway to permit these data or secondary channels are referred to as pinholes, and are active strictly for the duration of activity on the data channel.