Recently, a framework for the development of multiservice IP networks based on a mixture of DiffServ-based QoS management and MPLS-based traffic engineering has been defined. This approach is based on the aggregation of individual flows into classes and on the provisioning of QoS to the class instead of a single flow. With these developments, it is important to model and characterize the traffic flows on a per class basis in order to plan and manage these networks to meet performance objectives (QoS) as required by various traffic classes. This paper addresses the problem of characterizing class-based traffic flows in a multiservice IP network. It provides simple models that can be used to take measurement-based data and translate it into a form that is suitable for implementing dimensioning and planning functions in these networks.