This paper describes a proposal to improve the negotiation of value-added services. The approach aims at supporting user-network interactions in an automatic fashion. Stemming from the concept of metadata, we designed a specific model for on-line delivery of digital multimedia resources with provisioning of guaranteed performance. We exploited such features in an architecture that provides network operators and service providers with the capability to perform a service creation and delivery process on top of a QoS-enabled infrastructure, with automated configuration of the underlying communication network. In this architecture neither the users nor the service operators need to have information about the QoS requirements of service content since the QoS related network operations are performed transparently and can be dynamically managed during the trading of the service.