# An Improved Hybrid Routing Protocol Combining MANET and DTN
## 1 Introduction  
In mobile data networks, data delivery between mobile nodes can be provided using infrastructure, such as a base station (BS) in cellular networks [1]. If communication infrastructure does not exist, a mobile ad hoc network (MANET) can be used for data delivery, where dynamic routing path are maintained between nodes [2]. Cellular networks have been developed very actively, from the first generation (1G) analog networks to current fifth generation (5G) networks, launched last year. Although a cellular network is sufficient to meet the various requirements, such as high data rate, low latency, and high mobility, if such communication infrastructure is not available for some reason, such as failure of infrastructure due to natural disaster or if node-to-node communication is more effective for some scenarios, then self-organizing networks can be a good candidate for enabling data communication between mobile nodes [3, 4]. In a self-organizing network, such as MANET, mobile nodes form a network topology in a distributed way without a central entity, and communication between nodes is achieved through neighboring mobile nodes, where the topology varies due to the mobility of mobile nodes. Therefore, appropriate routing protocol to maintain the routing path between a source node and a destination node without the help of any centralized infrastructure should be essential to enable an efficient communication between mobile nodes. In a peer-to-peer (P2P) network, which is a logical overlay network, resources are shared between peers and actual routing is achieved using underlying physical networks. A distributed hash table (DHT) is used to search for the resources requested by peers in the underlying networks. Works on MANET have been carried out significantly during the last decade and has been extended to vehicular ad hoc networks (VANET) or flying ad hoc networks (FANET) [5, 6, 7]. In MANET, routing protocol is managed proactively or reactively. In proactive routing protocol, routing pathbetween nodes is actively maintained periodically. On the other hand, routing path is maintained on demand in reactive routing protocol [8; 9; 10]. MANET basically assumes that there exist at least one routing path between a source node S and a destination node D at any time even in mobile environment. However, if this assumption fails, in case where if the density of node is sparse, connection between nodes is broken and communication is not possible any more. Delay tolerant networks (DTN) can provide communication between nodes using store-carry-forward mechanism, even when connectivity between nodes is not guaranteed and it was proposed for communication in disaster or extreme environments [11; 12]. To tradeoff the pros and cons of MANET and DTN protocols, hybrid protocols combining MANET and DTN have been proposed recently [13; 14; 15; 16; 17; 18; 19; 20; 21]. In these works, it is generally assumed that a whole network is fragmented due to disaster or sparse density of nodes, and MANET is used for communication between nodes within the same fragmented network (intra-fragment communication) and DTN is used for communication between nodes belonging to different fragmented networks (inter-fragment communication), as shown in Figure 1.  
In this paper, we propose an improved hybrid routing protocol. If the routing path to the destination node from the source node is not successfully established within a predefined time period, a source node selects virtual source nodes, which was originally proposed in Reference [13], in fragmented networks based on the delivery predictability to the destination node defined in PRoPHET protocol [22]. After that, the source node operates based on DTN protocol for a predefined time period and forwards message to other node based on delivery predictability to the destination node. After the time period, the source node switches operates based on PRoPHET protocol.