Abstract: A proxy which is part of a firewall program controls exchanges of information between two application entities. The proxy interrogates attempts to establish a communication session by requesting entities with a server entity in lower layers in accordance with defined authentication procedures. The proxy interfaces with networking software to direct a communication stack to monitor connection requests to any address on specific ports. The requestor's address, and the server's address are checked against an access control list. If either address is invalid, the proxy closes the connection. If both are valid, a new connection is setup such that both the requestor and server are transparently connected to the proxy with variable higher levels being connected in a relay mode. Protocol data units are interrogated for conformance to a protocol session, and optionally further decoded to add additional application specific filtering. In one embodiment, an OSI architecture comprises the levels.