RMCast is a reliable source-ordered multicast protocol implementation for message-oriented multi-sender group communication built on top of IPv4 multicast. It uses sequence numbers for re-ordering, duplicate suppression and loss detection. Negative acknowledgments (NAK) with retransmissions are used to recover from losses. One new and interesting idea implemented in this protocol is history transmission (dubbed negative retransmission). In a nutshell, each sender, along with normal payload, transmits a list of other sender's IDs along with sequence numbers of last messages received from those senders by this member. This, in some sense, builds a pyramid of information: each subsequent message carries some information for a number of previous messages (from other senders). This helps other receivers detect losses. The protocol does not track group membership. Messages are retained for retransmission for a predefined amount of time. The "last message or lost message" dilemma is solved by periodic history transmissions in cases when there is no useful traffic (idle network). -- Boris Kolpackov