The present invention essentially comprises a Packet Receiver and a Stream Manager for a computer network. When a stream of packets passes through the present invention, they are received by the Packet Receiver. The Packet Receiver identifies the session to which the packet stream belongs, and passes the packet to the Stream Manager. The Stream Manager identifies the application generating the packet stream by scanning an Application ID Hash Table, which is a table that contains a mapping of destination ports to corresponding applications. Thereafter, it uses a State Machine Execution Engine to execute application decode instructions on the packet stream. The application decode instructions are stored in a table called Expression Action Table, and are generated based on a decode script created by the user for each application. The application decode instructions are chosen by the State Machine Execution Engine based on the current state of an Application Decode State Machine, which is a state machine that keeps track of the application decode process. The results of the State Machine Execution Engine are passed to the Stream Manager, and are stored in tables called Session Cache Table and Application Instance Table. Once the Stream Manager has decoded all the transactions and parameters of the application, the decoded information is sent with the packets in the packet stream.