When a firewall acts as an application-level gateway (ALG), what does it require in order...

An ALG is designed toinspect and modify the payloadof application-layer protocols (like SIP, FTP, etc.) to manage dynamic port allocations and session information.

“Application Layer Gateways (ALGs) inspect the payload of certain protocols to dynamically manage sessions that use dynamic port assignments. By modifying payloads, the ALG ensures that NAT and security policies are correctly applied.”

(Source: ALG Support)