Abstract: The frequency of a local clock of a local data processor in communication with an asynchronous switched packet network is synchronized to the frequency of a reference clock of a source data processor also coupled to the network. Timing packets each including a field containing the destination address of the local processor and a field containing reference clock data indicating the time at which the packet is launched onto the network are sent to the local data processor from the source data processor across the network. The frequency of the local clock is controlled in dependence on the reference clock data and the times of arrival of the packets.