A method for transmitting data by means of a data processing system having the Winsock Direct architecture, the system supporting a Microsoft Windows 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 Winsock socket library 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, said protocol being one of TCP/IP and UDP/IP; responsive to that request, the Winsock socket library selecting a transport library in accordance with the indication of said protocol, the selected transport library being a user-level Winsock Service Provider and the primary transport library for said protocol, there being a transport library of the operating system further down a hierarchy of transport libraries for the protocol; forming within the context of the application by means of the Winsock Service Provider the data into packets for transmission in accordance with said protocol and writing by means of the Winsock Service Provider the packets to be transmitted to a buffer allocated by the operating system; and initiating by means of communication between the Winsock Service Provider and the network interface device a transmission operation of at least some of the packets over the network; wherein said buffer is: accessible to the Winsock Service Provider by reference to a virtual address associated with said buffer by the operating system and programmed into the network interface device by the operating system. forming by means of the application data to be transmitted; requesting by means of the application a user-mode Winsock socket library 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, said protocol being one of TCP/IP and UDP/IP; responsive to that request, the Winsock socket library selecting a transport library in accordance with the indication of said protocol, the selected transport library being a user-level Winsock Service Provider and the primary transport library for said protocol, there being a transport library of the operating system further down a hierarchy of transport libraries for the protocol; forming within the context of the application by means of the Winsock Service Provider the data into packets for transmission in accordance with said protocol and writing by means of the Winsock Service Provider the packets to be transmitted to a buffer allocated by the operating system; and initiating by means of communication between the Winsock Service Provider and the network interface device a transmission operation of at least some of the packets over the network; accessible to the Winsock Service Provider by reference to a virtual address associated with said buffer by the operating system and programmed into the network interface device by the operating system.