A software networking arrangement for operation in a data processing system, the data processing system: having access to a network interface device; and comprising an operating system capable of processing a flow of traffic data received from an application supported by the data processing system, the operating system being operable in: a first mode, in which the operating system performs protocol processing of a flow of traffic data and then passes the protocol processed data to the network interface device for transmission; and a second mode, in which the operating system does not perform protocol processing of a flow of traffic data, the operating system being arranged to, on entering the second mode, transmit a state handover message to the network interface device indicating that the network interface device is to perform protocol processing of the flow of traffic data; the software networking arrangement comprising: a protocol processing entity in user space; and a driver for the network interface device, the driver being configured to indicate to the operating system that said driver represents a network interface device supporting a TCP offload engine; the driver being configured to receive said state handover message from the operating system and in response thereto, to configure the protocol processing entity to: perform in user mode TCP protocol processing of a flow of traffic data received from an application supported by the data processing system within the context of that application, and then pass the protocol processed data to the network interface device for transmission. having access to a network interface device; and comprising an operating system capable of processing a flow of traffic data received from an application supported by the data processing system, the operating system being operable in: a first mode, in which the operating system performs protocol processing of a flow of traffic data and then passes the protocol processed data to the network interface device for transmission; and a second mode, in which the operating system does not perform protocol processing of a flow of traffic data, the operating system being arranged to, on entering the second mode, transmit a state handover message to the network interface device indicating that the network interface a first mode, in which the operating system performs protocol processing of a flow of traffic data and then passes the protocol processed data to the network interface device for transmission; and a second mode, in which the operating system does not perform protocol processing of a flow of traffic data, the operating system being arranged to, on entering the second mode, transmit a state handover message to the network interface device indicating that the network interface device is to perform protocol processing of the flow of traffic data; a protocol processing entity in user space; and a driver for the network interface device, the driver being configured to indicate to the operating system that said driver represents a network interface device supporting a TCP offload engine; the driver being configured to receive said state handover message from the operating system and in response thereto, to configure the protocol processing entity to: perform in user mode TCP protocol processing of a flow of traffic data received from an application supported by the data processing system within the context of that application, and then pass the protocol processed data to the network interface device for transmission. perform in user mode TCP protocol processing of a flow of traffic data received from an application supported by the data processing system within the context of that application, and then pass the protocol processed data to the network interface device for transmission.