**3. Message-passing in the grid**

When running large parallel applications with demands for resources that exceed the capacity the local computing site offers, the deployment in a distributed Grid environment may help to satisfy these demands. Advances in wide-area networking technology have fostered this trend towards geographically distributed high-performance parallel computing in the recent years. However, as Grid resources are usually heterogeneous by nature, this is also true for the communication characteristics. Especially the inter-site communication often constitutes a bottleneck in terms of higher latencies and lower bandwidths than compared to the site-internal case. The reason for this is that the inter-site communication is typically handled via wide area transport protocols and respective networks, whereas the internal communication is conducted via fast local-area networks or even via dedicated high-performance interconnections. That in turn means that an efficient utilization of such a hierarchical and heterogeneous infrastructure demands a communication middleware providing support for all these different kinds of networks and transport protocols (Clauss et al., 2008).
