Disruption-tolerant nets set for large-scale test

Scientists at BBN Technologies have begun readying a large-scale field test of a mobile network designed to keep working despite transmission failures, glitches and long delays.

The test is the third phase of a Department of Defense project to create disruption-tolerant networks, or DTNs. It builds on a field prototype of 20 nodes that was successfully demonstrated last November at the Army's Fort A.P. Hill in Virginia. The large-scale trial, due in late 2009, is intended to show that big DTNs are not only possible, but also commercially viable and able to be built with off-the-shelf, standard components.

To fund Phase 3, the Defense Department's Defense Advanced Research Projects Agency (DARPA) just awarded almost $9 million to BBN. Key priorities involve work on DTN scalability and robustness to support thousands of nodes, and designing and implementing new algorithms for several key tasks. The BBN team also will be working with the U.S. Marines to introduce DTN into the Marines' CONDOR mobile network program, which is designed to link maneuvering units with command centers beyond line-of-sight (about 20 to 30 miles).

Click to see: Aerial photo of DARPA's Fort A.P. Hill demonstration

Though driven by military networking requirements, DTNs potentially have a much wider applicability. They can sustain communications without the stability, connectivity and predictability required by today's IP networks, including the Internet. If these networks lose a connection or suffer delays, packet deliveries plummet because the existing routing protocols assume an end-to-end path that becomes stable fairly quickly. But those assumptions break down in the face of repeated disconnections and long delays, which can be caused by equipment failures, weather, terrain or jamming.

One civilian prototype is the DieselNet project at the University of Massachusetts-Amherst. DieselNet consists of off-the-shelf single-board computers, GPS receivers and radios mounted in 40 UMass buses. As two buses near each other, their DTN nodes query each other to find out what other nodes each sees most frequently. If one of those other nodes is related to the final network destination of a message, that message is handed off to the passing node in the seconds they're close enough for the Wi-Fi connection. At some point, the message is handed to a node attached to the wired Internet.

Central to DTN's effectiveness is the technology's tenacity.

"IP networks have as a philosophy the idea [that] ‘if there's a problem, give up. The user will resend.' DTN doesn't give up. It's constantly trying to move the information forward," says Christopher Small, senior scientist with the Networking Research Group at BBN's Cambridge, Mass., headquarters. "DTN will work around breaks, and route the information any way it can."

That tenacity is due in large part to a new BBN-written routing protocol, called Bundle, which makes use of queuing and other techniques, including one called late binding. With late binding, a source node in a DTN can send a message even though the final destination IP address can't be known due to disruptions of name servers or routers. It's sort of like mailing an envelope that has blank spaces in the address.