1. A data processing system comprising: a host computer system supporting a software entity and a receive queue for the software entity; a network interface device having a controller unit configured to provide a data interface for receiving data packets from a network and a data bus interface that connects to the host computer system, the network interface device being connected to the host computer system by means of the data bus interface; and an accelerator module, addressable as a network endpoint, arranged between the controller unit and the network and having one or more interfaces that connect to the network and a second medium access controller coupled to the data interface of the controller unit, the accelerator module being configured to: on behalf of the software entity, process incoming data packets received from the network in one or more streams associated with a first set of one or more network endpoints wherein incoming data packets that are not associated with the first set of one or more network endpoints are forwarded onto the controller unit; encapsulate data resulting from said processing in network data packets directed to the software entity; and deliver the network data packets to the data interface of the controller unit wherein the network data packets are written to the receive queue of the software entity upon delivery.