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Delay-tolerant networks are partitioned wireless ad hoc networks with intermittent connectivity. Within Delay Tolerant Networks, a store and forward policy is often applied. Therefore, data are typically stored for extended periods of time before they are transmitted to their next hop. The goal, when routing decisions are made in DTNs, is typically to maximize the probability of the data reaching the final destination while minimizing a communication cost.
Communication between nodes in DTNs could be either scheduled or opportunistic, depending on the knowledge we have about the initial status and the time evolution of the network.
Connectivity between nodes in DTNs while the network evolves could be unstable. Asymmetric data rates, high error rates, and long delays are key features of DTNs. Thus, these networks are not ideal to support real time services. Nevertheless, some kind of QoS should be ensured at some level, because critical data are often transferred over DTNs. Hence, there is a research area to design routing algorithm aim of select a path for better performance.
The need for a delay-tolerant networking architecture stems from the fact that there are situations in which the existing Internet protocols do not work well, or may even fail completely. Protocols, such as the ubiquitous TCP, make certain assumptions about the characteristics of the underlying network: that an end-to-end path exists between the communicating nodes, that the maximum round-trip time between nodes is not excessive and that the data loss rate is low. When these assumptions hold true, the protocols perform well. When they do not, however, protocol performance can be severely degraded. Networks, that violate one or more of these assumptions, are called challenged networks .
In todayâ€™s Internet, network access is usually based on high-bandwidth wire line links (with perhaps a single wireless hop from an end-node to a wireless access point), and the aforementioned assumptions are usually met. But if we consider, for example, interplanetary communication it is clear that the network characteristics are completely different. DTNs support interoperability of region networks by accommodating long delays between and within regional networks, and by translating between regional network communication characteristics .
A network conforming to the DTN architecture, often simply called a DTN, is an overlay network that can comprise multiple regional networks, each of which may use different lower-layer technologies. Thus, the DTN overlay provides a common ground for applications that might not otherwise be able to communicate.
1.3 The Delay-Tolerant Network Architecture
The DTN architecture proposes the introduction of another layer just above the transport layer, called as the bundle layer in order to ferry data across the DTN . Figure 1.1  is a depiction of a traditional Ethernet-based network being incorporated into a DTN so that it can communicate with a challenged network via a DTN gateway. The challenged network may use transport and network protocols different from TCP/IP. The DTN gateway communicates with the Internet using TCP/IP and with the challenged network using the specific protocols.
Figure 1.1 Interworking of network protocols in a Delay-Tolerant Network
Instead of packet-switching, DTNs use the concept of store-and-forward message (or bundle) switching . This is because, at any given instant, there may not be any route to the next hop. In this case, the node must buffer the message in persistent storage, until a contact becomes available. Once the next hop stores the bundle in persistent storage, it is said to have taken custody of the bundle, and the upstream node can delete its own copy of the bundle. Instead of waiting for the next hop to become available, the DTN gateways may themselves be mobile. This extension of the message switching concept is referred to as store-carry-forward routing .
1.4 The concepts of DTN
The concepts of DTN are described below:
Bundles are also called messages. The bundle layer stores and forwards entire bundles between nodes. A single bundle-layer protocol is used across all networks that make up a DTN. The bundle layer ties together specific lower layers so that application programs can communicate across multiple regions.
1.4.2 Custodial transfer
A sending node can request custodial transfer of a bundle, meaning that any node on the path can take Custody of the bundle. If a node chooses to take custody of a bundle, it takes over all responsibilities regarding the bundle, such as retransmission, and related resources can be released from the previous custodian.
1.4.3 Regions and nodes
A large DTN network can consist of nodes from several different network topologies, each with a different addressing scheme. The use of different addressing schemes is usually a reason why nodes from different networks are unable to communicate.
DTN solves this problem by defining a region part in the endpoint ID, the DTN addressing scheme. DTN regions are defined in Delay-Tolerant Network Architecture  and a region can be described as a group of nodes in a network, using the same protocol set for communication.
Since nodes in different regions often use different protocol sets, consequently using different addressing schemes, only the region part of the endpoint ID is meaningful until the bundle has arrived somewhere within its destination region, where the other, administrative part of the endpoint ID can be interpreted for correct delivery.
In DTN, routing is primarily done based on the region part of the endpoint ID and then according to local rules used by each network topology. It is not defined in detail how the routing should be done in practice; instead this is left for the implementation, since it is not necessarily done consistently and can depend on network characteristics.
Since the resources of a DTN network can be limited, steps need to be taken to ensure that only the intended data is accepted into the network. To accomplish this, two additional headers are used; payload security and authentication header.
The payload security header is used to verify the payload integrity, and can also be used to authenticate the original sender on an end-to-end basis. Verification is done by calculating a hash of the payload and comparing it to the hash supplied in the header, whereas the authenticity is verified by checking a supplied signature of the hash.
The authentication header is on the other hand used to verify the entire bundle on a hop-by-hop basis, using the same types of hashes and signatures as in the payload verification. The use of a hop-by-hop verification has several benefits, such as early discard of non-authenticated or damaged bundles and simplified key exchange.
1.5 Routing in Delay-Tolerant Networks
DTNs incorporate a wide range of intermittently-connected networks. Contacts in a DTN may, in general, be opportunistic or scheduled. Opportunistic contacts occur, for example, in the case of terrestrial mobile networks. On the other hand, in a network where a low-earth orbit (LEO) satellite acts as a bundle forwarder, contact instants follow a pre-determined schedule. Routing methods proposed in the literature can be broadly classified as deterministic or stochastic based on whether the network topology is deterministic and predictable with time or whether it is randomly evolving . The former corresponds to networks where contacts are opportunistic and the latter to those where contacts are scheduled. We survey these methods in the following sections.
1.5.1 Deterministic Routing
Deterministic routing protocols are applied in the situation where future connectivity opportunities in the network can be predicted. It assumes varying degrees of knowledge of the network topology and constructs a notion of its evolution with time. With this knowledge, transmissions are scheduled so that some objective function can be optimized .
This strategy uses network topology information to select the best path, and the message is then forwarded from node to node along this path. A path can be found using location-based routing, assigning metrics to nodes or by assigning metrics to links. In reality, however, it may not be possible to have such comprehensive knowledge of the system.
1.5.2 Stochastic Routing
Stochastic Routing protocols do not assume any knowledge of the evolution of network topology with time. One of the earliest works in this class of protocols was the epidemic routing protocol proposed in . This protocol is motivated toward routing in military and sensor networks. Routing is done with the objective of maximizing message delivery rates, and minimizing message delivery latency while also minimizing the total amount of system resources consumed.
Types of Routing Strategies:
Forwarding  Based: This strategy uses network topology information to select the best path, and the message is then forwarded from node to node along this path. A path can be found using location-based routing, assigning metrics to nodes or by assigning metrics to links.
Example of Forwarding Based Routing Strategies: location based routing, gradient routing, link metrics etc.
Flooding Based: In this strategy node deliver multiple copies of each message to a set of nodes, called relays. The relays store the messages until they connect with the destination, at which point the message is delivered.
Example of Flooding Based Routing Strategies: direct contact, two hop relay, tree based flooding, epidemic routing etc.
1.6 Mobility Models
Mobility models represent the movement of mobile users, and how their location, velocity and acceleration change over time. It is imperative to use a mobility model that accurately represents the mobile nodes (MNs) that will eventually utilize the given protocol. Mobility models characterize  user movement patterns, i.e. the different behaviors of subscribers.
Figure 1.2 Classifications of Mobility Models
There are two types of mobility models used in the simulation of networks: traces and RWP models .
Traces are those mobility patterns that are observed in real life systems. Traces provide accurate information, especially when they involve a large number of participants and an appropriately long observation period.
New network environments (e.g. ad hoc networks) are not easily modeled if traces have not yet been created. In this type of situation it is necessary to use RWP models. RWP models attempt to realistically represent the behaviors of mobile nodes without the use of traces.
For mobility modeling, the behavior or activity of a userâ€™s movement can be described using both analytical and simulation models. The input to analytical mobility models are simplifying assumptions regarding the movement behaviors of users. Such models can provide performance parameters for simple cases through mathematical calculations. In contrast, simulation models consider more detailed and realistic mobility scenarios. Such models can derive valuable solutions for more complex cases.
Here the different RWP entity mobility models for ad hoc networks are discussed mostly used for the experiments :
Random Walk Mobility Model: A simple mobility model based on random directions and speeds.
Random Waypoint Mobility Model: A model that includes pause times between changes in destination and speed.
Random Direction Mobility Model: A model that forces MNs to travel to the edge of the simulation area before changing direction and speed.
A Boundless Simulation Area Mobility Model: A model that converts a 2D rectangular simulation area into a torus-shaped simulation area.
Gauss-Markov Mobility Model: A model that uses one tuning parameter to vary the degree of randomness in the mobility pattern.
A Probabilistic Version of the Random Walk Mobility Model: A model that utilizes a set of probabilities to determine the next position of an MN.
City Section Mobility Model: A simulation area that represents streets within a city.