in XMODEM-CRC. This was a very simple change to the original protocol; if requested, the receiver would attempt to trigger the transfer by sending an initial instead of a . If the remote sender supported the CRC option, it would begin sending packets as normal, but with a 16-bit CRC in the footer rather than the 1-byte checksum. YAM supported this option with no changes. 1k 1024-byte packets had been introduced in XMODEM-1k. This version did not change the trigger character from the receiver, so there was no way for the sender to know whether the receiver supported larger packets. Instead, XMODEM-1k was presented as a separate protocol on both ends of the connection. When such a connection was started, the sender could choose to send either 1024 bytes in a packet or 128, indicating the larger with an character in the header rather than the normal . Normally only the last few packets would use the smaller packets, to avoid sending large amounts of padding. 1k also assumed CRC for all connections. YAM supported 1k with no changes. 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 the packet, as packet zero is illegal. The sender would then see the 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 , 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