Communicating Through a Firewall

H.323 is a complex, dynamic protocol that consists of several interrelated subprotocols. The ports and addresses used with H.323 require detailed inspection as call setup progresses. As the dynamic ports are negotiated, the firewall must maintain a table of current ports associated with the H.323 protocol. As calls are torn down, the firewall must remove those ports from the table. The process of adding and removing ports from the table is called stateful inspection of packets. In addition to checking static ports and recognizing protocols that negotiate dynamic ports as in H.323, the firewall looks into the packets of that protocol to track the flows.