Because there are security-critical information (such as group secret key) being exchanged over the multicast protocol, we are building our security multicast framework on top of a reliable multicast service. It is worth noting that the multicast messages related to maintaining secure service may piggy back on top of the regular multicast messages. Since most reliable multicast services are initiated by receiver, receiver has a choice not to recover the lost message if the receiver determines that the multicast message is not critical.
Providing a reliable multicast is still under heavy research. Proposed schemes include SRM described in [FJM95] and STROM [XMZY97].
Alternatively, we may choose to replace the assumption of a reliable multicast service with unreliable multicast service, and have a reliable unicast as a channel to convey security-critical information that is lost in the multicast protocol. However, we speculate that when the multicast group grows large in size, the chance of missing packets increase up to a point that it becomes more economical to use existing reliable multicast than opening individual reliable unicast sessions.