and compares it to the received checksum or CRC to determine whether it was received properly. If it was, the receiver sends back an ACK (acknowledgement) message, prompting the sender to send the next packet. If there was any problem, it instead sends a NAK (not-acknowledged) message, and the sender resends the damaged packet.

This process introduces overhead into the transfer. For one, the additional checksum or CRC uses up time in the channel that could otherwise be used