PGM provides a reliable sequence of packets to multiple recipients simultaneously, making it suitable for applications like multi-receiver file-transfer.
Multicast is a network addressing method for the delivery of information to a group of destinations simultaneously using the most efficient strategy to deliver the messages over each link of the network only once, creating copies only when the links to the multiple destinations split (typically network switches and routers).
However, like the User Datagram Protocol, multicast does not guarantee the delivery of a message stream.
A reliable multicast protocol, like PGM, adds the ability for receivers to detect lost and/or out-of-order messages and take corrective action (similar in principle to TCP), resulting in a gap-free, in-order message stream.
A NAK is sent unicast back to the host via a defined network-layer hop-by-hop procedure whenever there is a detection of data loss of a specific sequence.