title: How can a Data Transfer Protocol using NAKs only be reliable?
body: <p>I have been studying a book on Computer Networking (this is not a homework question)</p>
  
  <p>One of the questions compares ACK and NAK based data transfer protocols, with the emphasis being that for a NAK based protocol, packet loss can for packet x can be detected by the received when the (x + 1)th packet is received.</p>
  
  <p>However, my issue is that what happens if the NAK sent by the receiver is lost before it gets to the sender? The sender will not be aware of errors and will not retransmit. Furthermore, what if the packet is the last one in the sequence? (there is no subsequent packet to test with) </p>
  
  <p>I do not see how a NAK-only protocol can be reliable (delivers each packet in the correct order)</p>