A delay-tolerant network is a network designed to operate effectively over extreme distances such as those encountered in space communications or on an interplanetary scale. In such an environment, long latency -- sometimes measured in hours or days -- is inevitable. However, similar problems can also occur over more modest distances when interference is extreme or network resources are severely overburdened.
Delay-tolerant networking involves some of the same technologies as are used in a disruption-tolerant network but there are important distinctions. A delay-tolerant network requires hardware that can store large amounts of data. Such media must be able to survive extended power loss and system restarts. It must also be immediately accessible at any time. Ideal technologies for this purpose include hard drives and high-volume flash memory. The data stored on these media must be organized and prioritized by software that ensures accurate and reliable store-and-forward functionality.
In a delay-tolerant network, traffic can be classified in three ways, called expedited, normal and bulk in order of decreasing priority. Expedited packets are always transmitted, reassembled and verified before data of any other class from a given source to a given destination. Normal traffic is sent after all expedited packets have been successfully assembled at their intended destination. Bulk traffic is not dealt with until all packets of other classes from the same source and bound for the same destination have been successfully transmitted and reassembled.