Delay-Tolerant Networking (DTN) is an approach to computer network architecture that seeks to address the technical issues in heterogeneous networks that may lack continuous network connectivity. Examples of such networks are those operating in mobile or extreme terrestrial environments, or planned networks in space.

Recently, the term Disruption-Tolerant Networking has gained currency in the United States due to support from DARPA, who have funded many DTN projects. Disruption may occur because of the limits of wireless radio range, sparsity of mobile nodes, energy resources, attack, and noise.

History

In the 1970s, spurred by the micronization of computing, researchers began developing technology for routing between non-fixed locations of computers. While the field of ad-hoc routing was inactive throughout the 1980s, the wide-spread use of wireless protocols reinvigorated the field in the 1990s as mobile ad-hoc routing and vehicular ad-hoc networking became areas of increasing interest.

Concurrently with (but separate from) the MANET activities, DARPA had funded NASA, MITRE and others to develop a proposal for the the Interplanetary Internet (IPN). Internet pioneer Vint Cerf and others developed the initial IPN architecture, relating to the necessity of networking technologies that can cope with the significant delays and packet corruption of deep-space communications. In 2002, Kevin Fall started to adapt some of the ideas in the IPN design to terrestrial networks and coined the term Delay Tolerant Networking and the DTN acronym. A paper published in 2003 SIGCOMM conference gives the motivation for DTNs. The mid-2000s brought about increased interest in DTNs, including a growing number of academic conferences on delay and disruption tolerant networking, and growing interest in combining work from sensor networks and MANETs with the work on DTN. This field saw many optimizations on classic ad-hoc and delay-tolerant networking algorithms and began to examine factors such as security, reliability, verifiability, and other areas of research that are well understood in traditional computer networking.

Routing

The ability to transport, or route, data from a source to a destination is a fundamental ability all communication networks must have. Delay and disruption-tolerant networks (DTNs), are characterized by their lack of connectivity, resulting in a lack of instantaneous end-to-end paths. In these challenging environments, popular ad hoc routing protocols such as AODV and DSR fail to establish routes. This is due to these protocols trying to first establish a complete route and then, after the route has been established, forward the actual data. However, when instantaneous end-to-end paths are difficult or impossible to establish, routing protocols must take to a "store and forward" approach, where data is incrementally moved and stored throughout the network in hopes that it will eventually reach its destination. A common technique used to maximize the probability of a message is successfully transferred is to replicate many copies of the message in hopes that one will succeed in reaching its destination.

Other concerns

Bundle Protocols

In efforts to provide a shared framework for algorithm and application development in DTNs, RFC 4838 and RFC 5050 were published in 2007 to define a common abstraction to software running on disrupted networks. Commonly known as the Bundle Protocol, this protocol defines a series of contiguous data blocks as a bundle—where each bundle contains enough semantic information to allow the application to make progress where an individual block may not. Bundles are routed in a store and forward manner between participating nodes over varied network transport technologies (including both IP and non-IP based transports). The transport layers carrying the bundles across their local networks are called bundle convergence layers. The bundle architecture therefore operates as an overlay network, providing a new naming architecture based on Endpoint Identifiers (EIDs) and coarse-grained class of service offerings.

Protocols using bundling must leverage application-level preferences for sending bundles across a network. Due to the store and forward nature of delay-tolerant protocols, routing solutions for delay tolerant networks can benefit from exposure to application-layer information. For example, network scheduling can be influenced if application data must be received in its entirety, quickly, or without variation in packet delay. Bundle protocols collect application data into bundles that can be sent across heterogeneous network configurations with high-level service guarantees. The service guarantees are generally set by the application level, and the RFC 5050 Bundle Protocol specification includes 'bulk', 'normal', and 'expedited' markings.

Security

Addressing security issues has been a major focus of the bundle protocol.

Security concerns for delay-tolerant networks vary depending on the environment and application, though authentication and privacy are often critical. These security guarantees are difficult to establish in a network without persistent connectivity because the network hinders complicated cryptographic protocols, hinders key exchange, and each device must identify other intermittently-visible devices. Solutions have typically been modified from mobile ad hoc network and distributed security research, such as the use of distributed certificate authorities and PKI schemes. Original solutions from the delay tolerant research community include the use of identity-based encryption, which allows nodes to receive information encrypted with their public identifier.

Research efforts

Various research efforts are currently investigating the issues involved with DTN:

• The The Delay Tolerant Networking Research Group
• The Technology and Infrastructure for Developing Regions project at UC Berkeley
• The KioskNet research project at the University of Waterloo.
• The DieselNet research project at the University of Massachusetts, Amherst.
• The ResiliNets Research Initiative at the University of Kansas and Lancaster University.
• The Haggle EU research project.
• The Saratoga project at the University of Surrey.
• The N4C EU/FP7 research project.
• The WNaN DARPA project.
• The NASA JPL's Deep Impact Networking (DINET) Experiment on board the Deep Impact/EPOXI spacecraft.

Footnotes