s, in conjunction with RTP. UDP and Firewalls Most organizations that deploy Internet audio and video technologies to conduct business and communicate with their global counterparts demand that these communications be secured through corporate firewalls. Firewalls are usually placed between internal LANs and WANs and external networks such as the Internet (they usually reside directly behind the router). Although it may provide reporting functions, a firewall's primary purpose is access control. Firewalls require configuration; the forfeiture in allowing a service (e.g., UDP) through the firewall is that one creates another door through which a hacker may intrude. Setting up firewall access rules on routers (what can pass through, what will be denied) is a time- consuming, error-prone process. However, it is generally accepted that static filters are not adequate for security, and firewalls are being designed to be simpler to configure. Still, network administrators generally adopt a "whatever is not expressly permitted is forbidden" approach to incoming Internet traffic. Most Internet (and intranet) applications are delivered as connectionless streams over UDP sockets. Since firewalls approach security in connection-oriented terms, they do not permit packets that are associated with connectionless streams to penetrate the corporate network. Different types of firewalls handle security differently, of course. There are four general categories: packet-level firewalls, circuit-level firewalls, proxy servers, and dynamic or intelligent packet filters (a well-known term for what this type of firewall performs is stateful inspection). Today most categories are combined into hybrid products that offer multiple security levels. At the lowest security level are packet- filtering routers; they are not truly firewalls, but they do examine each (or at least some) incoming IP packets. The router checks the packet's source, destination addresses, or services and, based on what it learns, decides whether to allow the packet on