We introduce a generic service model aiming at ensuring Quality of Service management. This modelling approach has been proposed by the Next Generation Network and Service Management project. A fairly high integration level of the tools has been reached using object-oriented paradigm and Model Driven Interoperability approach. We provide an example of Quality of Service management through a Service Level Specification of the Virtual Private Network service. We consider important points to reflect the complexity introduced by the Service Level Agreement and Quality of Service management: reaction model, cooperation model and co-ordination model.