Editing YMODEM (section)

====Zero packet====
In order to support automated transfers of [[FidoNet]] mail, MODEM7 introduced the ability to send the filename as plain text before sending the first block of data. This was not reliable, and TeLink improved this by placing the filename, and optionally other data like the creation date and file length, in a complete 128-byte packet. XMODEM started transfers with packet number one, so TeLink sent this packet as number zero. This "zero packet" or "block zero" became common in other FidoNet systems like SEAlink and others.

YAM supported the zero packet format, but it was ignored by many 3rd party implementations of YMODEM. When one implementation attempted to send the zero packet to a non-aware version, the receiver would naturally {{mono|NAK}} the packet, as packet zero is illegal. The sender would then see the {{mono|NAK}} as a transmission error and try to send the packet again, attempting this ten times before failing.

For reasons that are not entirely clear, many implementations of YMODEM did not implement this feature. Because they were unaware of it, they sent a {{mono|NAK}}, triggering a series of resend attempts before failing out. This meant that if the user chose to use a compliant YMODEM with a non-compliant version, the transfers would fail. Nevertheless, such non-compliant versions were common.

As a result, it was common to see both YMODEM and YMODEM Batch listed as two separate protocols. Further confusion was created by the similarity between XMODEM-1k and these non-compliant YMODEMs, which were similar to the point that they were often incorrectly listed as the same.