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Efficient delivery of real-time multicast traffic imposes on the underlying network infrastructure the burden of supporting quality of service. This can be quite a complex issue in a differentiated services IP network, especially if multicast users are allowed to join and leave the multicast tree dynamically. In fact, since DiffServ lacks explicit reservation states, a replicating node cannot test whether a corresponding reservation exists on an output link, and upon a dynamic join of a QoS multicast user, the DiffServ network lacks control functions to verify whether resources are available along the new path. We present a solution to support dynamic multicast with QoS over a DiffServ network. Our solution combines two ideas. First, resource availability along a new QoS path is verified via a probe-based approach. Second, QoS is maintained by marking replicated packets with a special DSCP (differentiated service code point) value before forwarding them on the QoS path.