IMAGINE an internet connection so fast it will let you download a whole movie in just 5 seconds, or access TV-quality video servers in real time. That's the promise from a team at the California Institute of Technology in Pasadena, who have developed a system called Fast TCP. A key feature of Fast TCP is that it could run on the same internet infrastructure we have today. The first practical test of Fast TCP took place in November at a supercomputing conference. Researchers from Caltech, Stanford and CERN near Geneva in Switzerland, sent data 10,000 kilometres from Sunnyvale, California, to CERN at an average rate of 925 megabits per second. Ordinary TCP managed just 266 megabits per second on the same routes. Steven Low, who led the Caltech team, likens the way the internet works now to driving a car while looking only 10 metres ahead. You slowly increase the car's speed until an obstacle comes into view, but then you have to hit the brakes. "This is OK for driving slowly in a parking lot," says Low. But on the open road you need to be able to look further ahead: "That's what we are doing with Fast TCP." Today, all traffic on the internet uses a system called the Transmission Control Protocol (TCP) developed in the 1970s by network engineers Vinton Cerf at Stanford University and Bob Kahn at the Pentagon's Defense Advanced Research Projects Agency. TCP breaks down large files into small packets of about 1500 bytes, each carrying the address of the sender and the recipient. The sending computer transmits a packet, waits for a signal from the recipient that acknowledges its safe arrival, and then sends the next packet. If no receipt comes back, the sender transmits the same packet at half the speed of the previous one, and repeats the process, getting slower each time, until it succeeds. This means that even minor glitches on the line can make a connection very sluggish. Because Fast TCP uses the same packet sizes as regular TCP, the hardware that carries messages around the net will still work. The difference is in software and hardware on the sending computer, which continually measures the time it takes for sent packets to arrive, and how long acknowledgements take to come back. This reveals the delays on the line, giving early warnings of likely packet losses. The Fast TCP software uses this to predict the highest data rate the connection can support without losing data. Since the packets are the same size as those used in TCP, none of the equipment along the internet itself will have to be modified, and no new hardware will be needed on computers receiving the data. By ganging 10 Fast TCP systems together, the researchers have achieved transmission speeds of over 8.6 gigabits per second, which is more than 6000 times the capacity of ordinary broadband links. The improved "Internet 2" infrastructure, currently being developed for scientific data transmission between 200 universities around the world, will use conventional TCP to run at around 350 megabits per second, but will run even faster with Caltech's technology. And the bandwidth-hungry entertainment industry is also looking at Fast TCP. Caltech is already in talks with Microsoft and Disney about using it for video on demand.