A disruption-tolerant network (DTN) is a network designed so that temporary or intermittent communications problems, limitations and anomalies have the least possible adverse impact. There are several aspects to the effective design of a DTN, including:
- The use of fault-tolerant methods and technologies.
- The quality of graceful degradation under adverse conditions or extreme traffic loads.
- The ability to prevent or quickly recover from electronic attacks.
- Ability to function with minimal latency even when routes are ill-defined or unreliable.
Fault-tolerant systems are designed so that if a component fails or a network route becomes unusable, a backup component, procedure or route can immediately take its place without loss of service. At the software level, an interface allows the administrator to continuously monitor network traffic at multiple points and locate problems immediately. In hardware, fault tolerance is achieved by component and subsystem redundancy.
Graceful degradation has always been important in large networks. One of the original motivations for the development of the Internet by the Advanced Research Projects Agency (ARPA) of the U.S. government was the desire for a large-scale communications network that could resist massive physical as well as electronic attacks including global nuclear war. In graceful degradation, a network or system continues working to some extent even when a large portion of it has been destroyed or rendered inoperative.
Electronic attacks on networks can take the form of viruses, worms, Trojans, spyware and other destructive programs or code. Other common schemes include denial of service attacks and malicious transmission of bulk e-mail or spam with the intent of overwhelming network servers. In some instances, malicious hackers commit acts of identity theft against individual subscribers or groups of subscribers in an attempt to discourage network use. In a DTN, such attacks may not be entirely preventable but their effects are minimized and problems are quickly resolved when they occur. Servers can be provided with antivirus software and individual computers in the system can be protected by programs that detect and remove spyware.
As networks evolve and their usage levels vary, routes can change, sometimes within seconds. This can cause temporary propagation delays and unacceptable latency. In some cases, data transmission is blocked altogether. Internet users may notice this as periods during which some Web sites take a long time to download or do not appear at all. In a DTN, the frequency of events of this sort is kept to a minimum.