FastTCP works by detecting congestion a different way. Reno detects congestion by watching for lost data. FastTCP monitors the real-time transfer speed of each packet of information, watching for delays. This way, adjustments can be made smoothly. Andrew says that the difference is particularly important for wireless networks. Since transfer over a wireless network is often less reliable, packets of information are lost fairly often. FastTCP, he says, detects that there’s no need to slow down the information flow: it simply resends the lost packets.
“In order to be smooth, which is what FastTCP does, you have to be more gentle and assess queuing delay,” says Aleksandar Kuzmanovic, an associate professor in the department of electrical engineering and computer science at Northwestern University who researches TCP. By keeping the transfer from reaching the point of packet loss, he says, it’s possible to avoid sending bursts of data, which can create a jittery effect for the recipient.
The problem, Kuzmanovic says, is that FastTCP isn’t the only attempt to improve on Reno. Other protocols have been designed that could speed file transfer while still using packet loss to monitor congestion. If those protocols send over the same line as FastTCP, they could hog the bandwidth. A protocol that senses delays could be at a disadvantage, Kuzmanovic says, since it might politely back away from congestion where more-aggressive protocols push to the point of packet loss.
Low says that his research group has been studying the issue of fairness–the question of how polite and aggressive protocols should interact. In theory, he says, it is always possible to create a fair situation, even between different types of protocols. Low’s group has been working on an algorithm that could be used to manage how bandwidth is shared among FastTCP flows and other protocols. Although there have not yet been any problems with fairness in practice, Low says, the algorithm could be made part of Aria if the need arose.
On July 30, FastSoft will announce its next commercial product in the Aria line: the Aria 2000, which the company says can accelerate throughput to up to one gigabit per second. Pricing for Aria appliances starts at $10,000 and goes up to more than $200,000, depending on model and configuration.