Editing XMODEM (section)

===Protocol spoofing===
Over reliable (error-free) connections, it is possible to eliminate latency by "pre-acknowledging" the packets, a technique known more generally as "[[protocol spoofing]]". This is normally accomplished in the link hardware, notably Telebit modems. The modems, when the option was turned on, would notice the XMODEM header and immediately sent an <code>ACK</code>. This would cause the sending XMODEM program to immediately send the next packet, making the transfer continuous, like an infinite-sized window. The modems also suppress the <code>ACK</code> being sent by the XMODEM software at the far end, thereby freeing up the low-speed return channel.

The system can also be implemented in the protocol itself, and variations of XMODEM offered this feature. In these cases, the receiver would send the <code>ACK</code> as soon as the packet started, in the same fashion as the Telebit modems. Since this feature is only an alteration of the receiver-side behavior, it does not require any changes in the protocol on the sender's side. [[YMODEM]] formalized this system.

This concept should be contrasted with the one used in SEAlink, which changes the behavior on both sides of the link. In SEAlink, the receiver stops sending the <code>ACK</code> entirely, and the sender changes its behavior to not expect them.