Co-operative multimedia systems are characterized by different properties (interactivity, group communication support and QoS sensitiveness). In addition to constraints that can be dealt with in multimedia systems (particularly synchronisation and communication constraints), integrity constraints can be defined and verified, and also temporal constraints can be described in the definition of multimedia flows. In this paper, we present a model to deal with multimedia constraint representation. We propose a model that allows the description of the constraints and the characterization of multimedia applications in the context of co-operative environments. Our model defines a framework with different functionality levels to tackle the complexity of distributed multimedia systems. We identify the entities and describe their functions. In addition, we define a set of services associated with each level. We describe these services, and we illustrate, by an example, message exchanges between entities.