**3.3.1 Session configuration**

One key strength of MetaMPICH is that it can be configured in a very flexible manner. For that purpose, MetaMPICH relies on a dedicated configuration file that is parsed before each session startup. This configuration file contains information about the communication topologies as well as user-related information about the requested MPI session. The information must be coded in a special description language customized to coupled clusters in Grid environments. Such a configuration file is structured into three parts: a header part with basic information about the session, a part describing the different clusters and a part specifying the overall topology. The header part gives, for example, information about the number of clusters and the number of nodes per cluster and thus the total number of nodes. The second part describes each participating cluster in terms of access information, environment variables, node and router lists as well as information about type and structure of the cluster-internal network. In the third part, the individual links between router nodes in case of a router-based architecture are described in terms of protocols and addresses. The same applies to clusters that are connected in an all-to-all manner: Here, a transport protocol must be specified<sup>9</sup> and additional netmasks may be stated, too. Moreover, MetaMPICH even supports mixed configurations, where some clusters are connected via an all-to-all network, whereas others are simultaneously connected via router nodes. Figure 6 shows an example for such a mixed session configuration.
