Fast TCP uses the same packet sizes as regular TCP, and so will work with the current internet infrastructure. Instead of waiting for the acknowledgment of the receipt of each packet before sending the next, it instead uses software and hardware on the sending computer to continually measure the time it takes for sent packets to arrive, and how long acknowledgements take to come back. The Fast TCP software uses this warning of likely packet losses to predict the highest data rate the connection can support without losing data.
Caltech is already in talks with Microsoft and Disney about using Fast TCP for video on demand.