A method for transmitting data by means of a data processing system, the system supporting an operating system and at least one application and having access to a memory and a network interface device capable of supporting a communication link over a network with another network interface device, the method comprising the steps of: forming by means of the application data to be transmitted; requesting by means of the application a user-mode operating system functionality of the data processing system to direct the data to be transmitted, the request including an indication of a protocol by which the data is to be transmitted; responsive to that request, the user-mode operating system functionality selecting a protocol processing entity in accordance with the indication of a protocol by which the data is to be transmitted; forming within the context of the application by means of the protocol processing entity the data into packets for transmission in accordance with the protocol by which the data is to be transmitted and writing by means of the protocol processing entity the packets to be transmitted to an area of the memory; initiating by means of communication between the protocol processing entity and the network interface device a transmission operation of at least some of the packets over the network.