The invention relates to the monitoring of the flow of a data stream travelling between a client and a server system. The invention is intended particularly for such communications protocols carrying representation data above some connection-oriented protocol layer. The objective of the present invention is to bring about a flow monitoring mechanism enhancing system security. This is achieved by analyzing a data stream travelling from the server to the client in order to identify at least one response descriptor in the data stream. The identified response descriptors are stored in a set of available states for said client. Then the data stream travelling from the client to the server is analyzed in order to identify at least one request descriptor. The request descriptors identified are compared with the set of available states for said client, and in response to the comparing step, a monitoring result is generated.