Abstract: A parallel computer system comprises a number of processing elements, at least one communication element, an internal network interconnecting the communications and processing elements, at least one external network connected to the communications element, and a number of services resident in the elements. Each of the elements includes a protocol stack comprising transport, network, logical link and MAC (media access control) layers. Routing messages between services and the external network is performed in the MAC layer, rather than by a relay service at application level as in conventional systems. This improves the efficiency of the system, by obviating the need to pass messages up the stack to the routing function and then to pass them back down the stack again.