There is a strong application demand for reliable multicast. Widespread use of the Internet makes the economy of multicast transport attractive. The current Internet multicast model offers best-effort many-to-many delivery service but no guarantees. One-to-many and many-to-many services will become more important in the future. Reliable multicast transports add delivery guarantees, not necessarily like those of reliable unicast TCP, to the group-delivery model of multicast. To meet this growing demand for reliable multicast, there is a large number of protocol proposals. Because of the complexity of the technical issues, and the abundance of proposed solutions, we wish to put into place a classification of the various aspects involving reliable multicasting.  XTP, one of the well-known transport protocols, provides a toolkit of mechanisms over which the user may build the protocol he so desires. It has been observed that the multipoint-to-multipoint mechanisms as described in the appendix to the XTP specification are not very efficient. Hence, we also wish to provide a few suggestions on how to improve the existing mechanisms.