Positive and negative acknowledgment messages are commonly used to signal receipt or non-receipt of a message. This approach does not scale well to a multicast system, because each destination must send an ACK for each packet received. This largely defeats the advantage of using Multicast packets, because it decreases both the efficiency and the performance of the protocol. Even though these acknowledgments are small, they are all sent simultaneously and can cause network congestion. In addition, processing the ACKs returning from each destination increases the load on the sender and decreases the performance of the protocol.
Negative acknowledgments shift the burden of error detection from the source to the destinations.
Packets are stamped with sequential numbers. Receivers at target destinations detect missing packets by any gaps in the sequence numbers. The target receiver can request retransmission of the missing packets corresponding to the gaps. Because only negative acknowledgments of non-receipt are transmitted back to the sender, the sender receives no assurance that a packet has reached its destination. If the protocol is to be considered truly reliable, senders must keep a copy of each packet sent, for an indefinite period.