Editing ZMODEM (section)

===Restart===
XMODEM, and most protocols based on it, managed packet order by prefixing the data with a packet number from 1 to 255. Windowed versions used this packet number to indicate which packets had been received properly, or specify one that had not. Since the packets were 128&nbsp;bytes long, this meant the maximum amount of data that could be transferred before the packet numbers rolled over was 32&nbsp;KB.

ZMODEM replaced the packet number with the actual location in the file, indicated by a 32-bit number. This allowed it to send {{mono|NAK}} messages that re-wound the transfer to the point of failure, regardless of how long the file might be. This same feature was also used to re-start transfers if they failed or were deliberately interrupted. In this case, the receiver would look to see how much data had been previously received and then send a {{mono|NAK}} with that location, automatically triggering the sender to start from that point.