It has been recognized that an effective support for multimedia applications must provide Quality of Service (QoS) guarantees. Current methods propose to provide such QoS guarantees through coordinated network resource reservations. In our approach, we extend this idea providing system-wide QoS guarantees that consider the data manipulation and transformations needed in the intermediate and end sites of the network. Given a user's QoS requirements, multisegment virtual channels are established with the necessary communication and computation resources reserved for the timely, synchronized, and reliable delivery of the different datatypes. Such data originate in several distributed data repositories, are transformed at intermediate service stations into suitable formats for transportation and presentation, and are delivered to a viewing unit. In this paper, we first review NETWORLD, an architecture that provides such QoS guarantees and an interface for the specification and negotiation of user-level QoS requirements. Our user interface supports both expert and non- expert modes. We then describe how to map user-level QoS requirements into low-level system parameters, leading into a contract between the application and the network. The mapping considers various characteristics of the architectures (such as the hardware and software available at each source, destination, or intermediate site) as well as cost constraints.