But things do not always go okay. Sometimes packets are altered en route, which the receiver, R, can detect by recomputing the checksum and comparing it to what was sent in the packet. If the packet is damaged, R must request a retransmission of that packet, which it does by sending a NAK (negative acknowledgment) packet with the sequence number of the altered packet. The sender, S, then retransmits that packet and continues with the other packets, as shown in Fig. 23.8.2.