## 6 Results
### DTN Routing  
Routing is one of the main aspects that influences the performance of DTNs, and so, many authors have worked to tackle this topic. A total of 49 papers focus on routing. Most of the discussions revolved around route determination, the type of contact, whether the protocol sent only one copy or many, and the elements to design a routing protocol. In DTN, end-to-end connectivity is a key factor as connectivity might be available only for short periods or not available at all. Each node that receives a message might store it in its buffer, carries it, and forwards it whenever it finds a neighbor that could potentially be part of the route to the destination. When the nodes are highly sparse or highly mobile, the routing path can be unavailable to meet the request time. Contact type. One aspect largely studied concerning routing is the type of contacts. There are different kinds of contacts: persistent (always there), on-demand (when needed), scheduled, and opportunistic (found unexpectedly) [16]. Persistent contacts are always available, opportunistic contacts are random, scheduled contacts are established for a particular time and duration, and predicted contacts depending on the history and specific information can be guessed. Contact utilization is limited by the speed of the discovery mechanism. A lower contact utilization can influence end-to-end throughput and latencies. A long connection time, frequent encounters, and a high message delivery ratio are preferred node qualities to improve routing performance. The link quality indicates the effectiveness of the connection between contacted nodes [27]. DTN is an opportunistic network, characterized by the lack of continuous connectivity and thus the need for store-and-forward mechanisms to ensure the delivery of data. However, due to the lack of end-to-end connectivity, routing in DTN suffers data loss and a low delivery ratio [28]. The most used routing protocols in DNTs are first contact, direct delivery, epidemic, spray and wait, and prophet [4]. Prophet outperforms epidemic content dissemination to achieve higher packet delivery rates and consumes less energy [29]. Mobile users' social characteristics can be used for routing decisions under the premise that they are less volatile. In this sense, social aware routing mechanisms group mobile users into communities and use information, such as the contact duration, node centrality, and data delivery preferences [30]. Number of copies. Another aspect related to DTN routing protocols is the number of copies they issue. It can be single-copy and multi-copy schemes. The first is resource efficient, but it has long delivery delays. Epidemic routing is a multi-copy scheme [16]. Madni categorizes DTN routing protocols into three classes: flooding-based, which makes replicas of packets according to the number of encountered nodes; quota-based establishes a fixed number of packet replicas; forwarding-based makes only a single copy of the packet throughout the network [5]. Stochastic routing floods the message to the node and does not know the network [23]. Route. The determination of the route might be deterministic by a predefined mobility pattern or non-deterministic modeled by a probability function. Deterministic routing can predict the future movement of nodes and connections based on some characteristics [23]. Intermittent connectivity makes the waiting time vary from seconds to days, which affects contact the schedule and thus performance. Encounter-based routing protocols use the history of encounters between nodes to forward messages. They are divided into flooding and prediction based. Flooding does not take into consideration the encounter history, e.g., Epidemic and Spray and Wait. Prediction-based takes into consideration encounter information, using time metrics (interval, duration, inter-meeting time, or inter-contact time, etc. ), with examples as follows: EBR (Encounter Based Routing) and Prophet.