Resource-sharing is one of the Grid intrinsic properties enabling efficient usage of services by clients from many Virtual Organizations. In such an environment, the provisioning of Quality of Service requires mechanisms for the control of the concurrent access to resources and services, as well as protocols for the establishment of Service Level Agreements between users and service providers. Nevertheless, while the availability of guarantees is important to allow a more flexible usage of the distributed infrastructure, most of the Grids today only offer best-effort services and do not support Service Level Agreement signalling. In this paper, we present the architecture and the prototype implementation of a novel resource-independent set of workload and resource management services supporting discovery, Service Level Agreement signalling and monitoring, and adopting the functional division into agreement and service provider layers currently under definition by the Grid Resource Allocation and Agreement Protocol Working Group of the Global Grid Forum.