## 2 DTN Routing Classification  
Many survey papers have addressed the challenges and opportunities in DTN. Some of them give a taxonomy of routing protocols, while others focus only on design issues. Although DTN has been studied for several years, we believe it is useful to recall the importance of surveying DTN routing solutions in order to better understand their applicability in IoT domains. In this section, we cover important surveys on DTN routing protocols together with our proposed classification illustrated in Figure 1.  
The classification in [8] is based on information about network topology such as historical data used by the routing protocol to decide where and when to forward the packets. Two approaches were proposed: Deterministic and Stochastic. In the Deterministic approach, the nodes assume that future network behavior is known apriori. In the Stochastic approach, the nodes assume that no knowledge about network behavior is known. Figure 1: Classification of DTN routing protocols. [PERSON] et al. [9] classified routing protocols based on the property used to find the destination into two families Flooding and Forwarding. The Flooding family uses replication and the Forwarding family uses knowledge about the network. [PERSON] et al. [10] gave more details about the Forwarding family. [PERSON] et al. [11] proposed a classification based on the type of knowledge used for routing into Flooding based approach: History based approach and Special devices based approach. In Flooding based approach, there is no knowledge about the network and the nodes flood the packets in the network. In History based approach, history of encounters between nodes is used for routing decisions. The nodes use additional devices to communicate such as stationary or mobile nodes in the Special devices based approach. The surveys presented above proposed two main classes based on knowledge about the network, Deterministic ( Forwarding-based or History-based ) and Stochastic ( Flooding-based or Replication-based ). [PERSON] et al. [12] talked about unicast and broadcast routing in DTN. [PERSON] et al. [13] introduced a taxonomy of routing in DTNs into Unicasting, Multicasting and Anycasting issues, which addresses the number of copies of the packets sent from a destination and whether a packet is sent to one or more destinations. The Unicasting category is detailed depending on the design characteristics of routing algorithms into Naive replication, Utility forwarding and Hybrid. Nevertheless, they focused on Opportunistic routing in which relay node selection is based on factors such as mobility patterns and probability of contacting the destination. [PERSON] et al. [14] presented a survey and comparison of Social-based routing protocols in DTN. They listed positive and negative social routing properties such as community and centrality, used to make routing decisions. The survey in [15] considers both Social-based and Opportunistic routing techniques. A survey on Delay Tolerant Underwater Wireless Sensor Network (UWSN) routing is presented in [16]. The authors classified DTN routing protocols in UWSNs, based on the definition of DTN, into Scheduled contact, Opportunistic contact and Predicted contact. In Scheduled contact, a base station is used as a scheduler and a relay node to ensure communication between nodes. Relay nodes are deployed to convey information between sensors in Opportunistic contact. In Predicted contact, communication depends on a prediction scheme. The work in [17] summarizes previous classifications to three principal categories: Network topology, Routing strategy and Message replication and semantics. Network topology includes Replication-based and Forwarding-based approaches. Routing strategy groups Social-based routing and Opportunistic routing. Replication and semantics covers Unicast, Multicast and Anycast routing. However, the classification does not cover the routing techniques that use auxiliary devices such as relay nodes and base stations.