The (multicast acknowledgment) MACK layer is an acknowledgment-based loss-less error correction layer. Whenever a message is sent, every receiver has to acknowledge it with an ACK message. When ACKS from all members have been received, the message was successfully sent to all members. When an ACK from a member is missing, MACK retransmit the message only to that member. When no ACK for a message has been received after n retries, that member is suspected: a SUSPECT event is passed up the stack. The MACK layer is best used on a noisy communication line, where packet drop is frequent. On a good communication substrate, layer MNAK should be used instead.