A data processing system comprising: a host computing device supporting an operating system and a network protocol stack, the network protocol stack being operable to support one or more transport streams by performing transport stream protocol processing of data packets received over the streams; a network interface device arranged to couple the host computing device to a network and operable to receive data packets over a transport stream supported by the network protocol stack; and a message engine configured to perform upper layer protocol processing; wherein the network interface device is configured to, on receiving a data packet over one of a predetermined set of transport streams, pass the payload data of the data packet to the message engine and the message engine is configured to, in response to receiving the payload data, identify and process any upper layer messages in the payload data in accordance with the upper layer protocol.