A system is provided for peer-to-peer messaging. The system includes a first client machine which may transmit a sequence of messages related to a subject. The transmission of each message can be in the form of multicast delivery or reliable unicast delivery. At least one second client machine communicates with the first client machine. Each such second client machine may receive at least a portion of the sequence of messages transmitted in the multicast form of delivery from the first client machine, and determines if there is an interest in the subject of the sequence of messages. The second client machine may determine if any messages in the sequence have not been received if there is an interest, and can transmit a request for re-transmission to the first client machine. The request identifies any messages of the sequence that were not received so that such messages can be re-transmitted by the first client machine to the at least one second client machine in the form of reliable unicast delivery.