A normal XMODEM implementation would simply discard the packet, the assumption being that the packet number had been corrupted. But this led to a potential time delay if the packet were discarded, as the sender could not tell whether the receiver had responded with a <NAK> because it did not understand the zero packet or because there was a transmission error. As TeLink was normally used only by FidoNet software, which demanded it as part of the FidoNet standards, this did not present a real-world problem as both ends would always support this standard.