Data Grids address computational and data intensive applications that combine very large datasets and a wide geographical distribution of users and resources [1, 2]. In addition to computing resource scheduling, Data Grids address the problems of storage and data management, network-intensive data transfers and data access optimization, while maintaining high reliability and availability of the data (see References [2, 3] and references therein). The Open Grid Services Architecture (OGSA) [1, 4]