Routing in delay-tolerant networking concerns itself with the ability to transport, or route, data from a source to a destination, which 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 AODVC. E. Perkins and E. M. Royer. Ad hoc on-demand distance vector routing. In The Second IEEE Workshop on Mobile Computing Systems and Applications, February 1999. and DSRD. B. Johnson and D. A. Maltz. Mobile Computing, chapter Dynamic source routing in ad hoc wireless networks, pages 153–181. Kluwer Academic Publishers, February 1996. 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.John Burgess, Brian Gallagher, David Jensen, and Brian Neil Levine. MaxProp: Routing for vehicle-based disruption-tolerant networks. In Proc. IEEE INFOCOM, April 2006.Philo Juang, Hidekazu Oki, Yong Wang, Margaret Martonosi, Li Shiuan Peh, and Daniel Rubenstein. Energy-efficient computing for wildlife tracking: design tradeoffs and early experiences with zebranet. SIGOPS Oper. Syst. Rev., 36(5):96–107, 2002.Augustin Chaintreau, Pan Hui, Jon Crowcroft, Christophe Diot, Richard Gass, and James Scott. Impact of human mobility on opportunistic forwarding algorithms. IEEE Transactions on Mobile Computing, 6(6):606–620, 2007. A common technique used to maximize the probability of a message being successfully transferred is to replicate many copies of the message in hopes that one will succeed in reaching its destination.Amin Vahdat and David Becker. Epidemic routing for partially connected ad hoc networks. Technical Report CS-2000-06, Department of Computer Science, Duke University, April 2000.
Routing considerations
There are many characteristics DTN protocols, including routing, must take into consideration. A first consideration is if information about future contacts is readily available. For example, in interplanetary communications, many times a planet or moon is the cause of contact disruption, and large distance is the cause of communication delay. However, due to the laws of physics, it is possible to predict the future in terms of the times contacts will be available, and how long they will last. These types of contacts are known as scheduled or predictable contacts.Sushant Jain, Kevin Fall, and Rabin Patra. Routing in a delay-tolerant network. In Proc. ACM SIGCOMM, 2004. On the contrary, in disaster recovery networks the future location of communicating entities, such as emergency responders, may not be known. These types of contacts are known as intermittent or opportunistic contacts.
A second consideration is if mobility can be exploited and, if so, which nodes are mobile. There are three major cases, classifying the level of mobility in the network. First, it is possible that there are no mobile entities. In this case, contacts appear and disappear based solely on the quality of the communication channel between them. For instance, in interplanetary networks, large objects in space, such as planets, can block communicating nodes for a set period of time. Second, it is possible that some, but not all, nodes in the network are mobile. These nodes, sometimes referred to as Data Mules,Jea D., Somasundara A. A, and Srivastava M. B. Multiple Controlled Mobile Elements (Data Mules) for Data Collection in Sensor Networks. In Proc. IEEE/ACM International Conference on Distributed Computing in Sensor Systems (DCOSS), June 2005.Rahul C. Shah, Sumit Roy, Sushant Jain, and Waylon Brunette. Data MULEs: Modeling a Three-tier Architecture for Sparse Sensor Networks. In Proc. IEEE SNPA Workshop, May 2003. are exploited for their mobility. Since they are the primary source of transitive communication between two non-neighboring nodes in the network, an important routing question is how to properly distribute data among these nodes. Third, it is possible that the vast majority, if not all, nodes in the network are mobile. In this case, a routing protocol will most likely have more options available during contact opportunities, and may not have to utilize each one.Aruna Balasubramanian, Brian Neil Levine, and Arun Venkataramani. DTN routing as a resource allocation problem. In Proc. ACM SIGCOMM, August 2007.Thrasyvoulos Spyropoulos, Konstantinos Psounis, and Cauligi S. Raghavendra. Spray and wait: An efficient routing scheme for intermittently connected mobile networks. In WDTN ’05: Proceeding of the 2005 ACM SIGCOMM workshop on Delay-tolerant networking, 2005.Thrasyvoulos Spyropoulos, Konstantinos Psounis, and Cauligi S. Raghavendra. Spray and focus: Efficient mobility-assisted routing for heterogeneous and correlated mobility. In Fifth Annual IEEE International Conference on Pervasive Computing and Communications Workshops, 2007. An example of this type of network is a disaster recovery network where all nodes (generally people and vehicles) are mobile.Samuel C. Nelson, Albert F. Harris, and Robin Kravets. Event-driven, role-based mobility in disaster recovery networks. In CHANTS 07: Proceedings of the second workshop on Challenged Networks, 2007. A second example is a vehicular network where mobile cars, trucks, and buses act as communicating entities.
A third consideration is the availability of network resources. Many nodes, such as mobile phones, are limited in terms of storage space, transmission rate, and battery life. Others, such as buses on the road, may not be as limited. Routing protocols can utilize this information to best determine how messages should be transmitted and stored to not over-burden limited resources. As of April 2008, only recently has the scientific community started taking resource management into consideration, and this is still an active area of research.
Routing protocol classifications
While there are many characteristics of routing protocols, one of the most immediate ways to create a taxonomy is based on whether or not the protocol creates replicas of messages. Routing protocols that never replicate a message are considered forwarding-based, whereas protocols that do replicate messages are considered replication-based.
There are both advantages and disadvantages to each approach, and the appropriate approach to use is probably dependent on the scenario at hand. Forwarding-based approaches are generally much less wasteful of network resources, as only a single copy of a message exists in storage in the network at any given time.Dan Henriksson, Tarek F. Abdelzaher, and Raghu K. Ganti. A caching-based approach to routing in delay-tolerant networks. In Proceedings of 16th International Conference on Computer Communications and Networks, 2007. ICCCN 2007, 2007. Furthermore, when the destination receives the message, no other node can have a copy. This eliminates the need for the destination to provide feedback to the network (except for, perhaps, an acknowledgments sent to the sender), to indicate outstanding copies can be deleted. Unfortunately, forwarding-based approaches do not allow for sufficient message delivery rates in many DTNs. Replication-based protocols, on the other hand, allow for greater message delivery rates, since multiple copies exist in the network, and only one (or in some cases, as with erasure coding, a few) must reach the destination. However, the tradeoff here is that these protocols can waste valuable network resources. Furthermore, many flooding-based protocols are inherently not scalable. Some protocols, such as Spray and Wait,<ref name="spyropoulos2005" /> attempt to compromise by limiting the number of possible replicas of a given message.
The vast majority of DTN routing protocols are heuristic-based, and non-optimal. Optimality, in the general DTN case, is NP-hard. More specifically "online algorithms without complete future knowledge and with unlimited computational power, or computationally limited algorithms with complete future knowledge, can be arbitrarily far from optimal".
Replication-based routing
Replication-based protocols have recently obtained much attention in the scientific community, as they can allow for substantially better message delivery ratios than in forwarding-based protocols. These types of routing protocols allow for a message to be replicated; each of the replicas, as well as the original message itself, are generally referred to as message copies or message replicas. Possible issues with replication-based routing include: network congestion in clustered areas, being wasteful with network resources (including bandwidth, storage, and energy), and network scalability.
Since network resources may quickly become constrained, deciding which messages to transmit first and which messages to drop first play critical roles in many routing protocols.
Epidemic routing
Epidemic routing is flooding-based in nature, as nodes continuously replicate and transmit messages to newly discovered contacts that do not already possess a copy of the message. In the most simple case, epidemic routing is flooding; however, more sophisticated techniques can be used to limit the number of message transfers. Epidemic routing has its roots in ensuring distributed databases remain synchronized, and many of these techniques, such as rumor mongering, can be directly applied to routing.
PRoPHET routing protocol
Epidemic routing is particularly resource hungry because it deliberately makes no attempt to eliminate replications that would be unlikely to improve the delivery probability of messages. This strategy is effective if the opportunistic encounters between nodes are purely random, but in realistic situations, encounters are rarely totally random. Data Mules (mostly associated with a human) move in a society and accordingly tend to have greater probabilities of meeting certain Mules than others. The Probabilistic Routing Protocol using History of Encounters and Transitivity (PRoPHET) protocol uses an algorithm that attempts to exploit the non-randomness of real-world encounters by maintaining a set of probabilities for successful delivery to known destinations in the DTN (delivery predictabilities) and replicating messages during opportunistic encounters only if the Mule that does not have the message appears to have a better chance of delivering it. This strategy was first documented in a paper from 2003.A. Doria, and O. Scheln. Probabilistic routing in intermittently connected networks. In Proceedings of the Fourth ACM International Symposium on Mobile Ad Hoc Networking and Computing (MobiHoc 2003), 2003.
An adaptive algorithm is used to determine the delivery predictabilities in each Mule. The Mule M stores delivery predictabilities P(M,D) for each known destination D. If the Mule has not stored a predictability value for a destination P(M,D) is assumed to be zero. The delivery predictabilities used by each Mule are recalculated at each opportunistic encounter according to three rules: When the Mule M encounters another Mule E, the predictability for E is increased: P(M,E)new = P(M,E)old + (1 - P(M,E)old) * Lencounter where Lencounter is an initialisation constant. The predictabilities for all destinations D other than E are 'aged': P(M,D)new = P(M,D)old * γK where γ is the aging constant and K is the number of time units that has elapsed since the last aging.