In a multicasting system content is multicast from a sender to a plurality
of receivers over a data network. Each receiver independently determines
whether it is missing elements or packets of the content. Receivers having
missing content each initiate a random timer. The receiver having the
shortest random interval unicasts a negative acknowledgement to the
sender, which immediately multicasts the negative acknowledgement to the
other receivers. All other receivers having the same missing packet
thereupon suppress their own negative acknowledgements as to that packet.
A repair transmission is then multicast by the sender to all receivers.
The random intervals have upper and lower bounds according to the round
trip transmission time and the size of the largest missing data element.
