3.1.1.4 Acknowledgments

An acknowledgment (ACK) is sent from the receiver to the sender, informing the sender about the receipt of a Source Packet. An acknowledgment MUST be generated for every Source Packet received. However, because acknowledgments are cumulative, the number of Source Packets for which a receiver generates an acknowledgment is implementation-specific.<3> Only Source Packets MUST be acknowledged by the receiver; FEC Packets MUST NOT be acknowledged by the receiver.

Each acknowledgment contains an ACK vector (section 2.2.2.7.1).