1. A data processing system comprising: an operating system that stores a first network routing table comprising one or more entries each indicating at least part of a route over a network to a respective destination, the operating system being arranged to transmit data using the first network routing table; a transmission support function internal to the operating system and arranged to maintain a copy at the operating system of the first network routing table by updating the copy network routing table to include changes to the first network routing table; and a network data transmission function external to the operating system and at user level arranged to transmit data in accordance with the copy network routing table without accessing the first network routing table, the network data transmission function being arranged to read the copy network routing table by means of memory mapping.