Type 3 – Application Proxies. Unlike the other two examples of firewalls, a proxy is visible to the applications. The codec acts like a client and treats the proxy as a server; on the other side of the network the proxy appears as the client and the remote location appears as the server. Because of the nature of this client-server relationship, there inevitably is some address translation that must take place. Because of this close relationship with the actual application, it is possible that a proxy would use portions of the H.323 stack to maintain both sides of the network connections. Since this relationship requires that the endpoint calls the proxy (not the callee), the codec must make use of other H.323 fields to relay the callee’s information so that the proxy knows where to forward the call. This being the case, the endpoint must be proxy aware in some capacity. In the video conferencing world, gatekeepers are used to relieve the endpoints from being proxy aware.
