Abstract: A data distribution system comprises a plurality of processing devices (8) connected by a packet-based transport network (2), and a session manager (12). The session manager (12) is configured to manage a distribution session by (i) associating two or more of the processing devices (8) with a multicast group on the transport network, (ii) storing and/or accessing a mapping between a session identifier for the distribution session and the multicast group address, and (iii) configuring the transport network (2) and/or the processing devices (8) so that data output to the multicast group by any one of the processing devices in the group is received by all the other processing devices in the group. Each of the processing devices (8) can receive data from a respective node (16) located on an external network, and send the received data, or data derived therefrom, over the transport network (2) to a multicast group.