A technique is disclosed that can prevent any meaningless multicast stream transmission in a multicast. According to this technique, a transmitting source communication apparatus (source) (11) uses a particular algorithm to derive a test data for some of data included in a multicast stream being transmitted, and sends, to a multicast router (12), the test data, information for determining data related to the derivation of the test data and information for determining the algorithm. When checking whether there exist any listeners of multicast packet, the multicast router delivers the information for determining the data and algorithm to communication apparatuses (listeners) (13) at the receiving ends in charge of the multicast router. Each of the communication apparatuses at the receiving ends sends, back to the multicast router, a response data generated by using the designated algorithm for the designated data. If determining that the response data is coincident with the test data, then the multicast router determines that there exists a listener of the present multicast packet.
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