## 6 Results
### DTN Routing
From our perspective, routing is definitely another crucial element to consider in the performance of DTN; however, there is no common agreement. The papers reviewed all focus in one or two algorithms for given scenarios. This variety of algorithms, as well as the diversity of contexts, makes it difficult to have a final statement in terms of which one is the best. It remains an opportunity to find a routing protocol that can be the standard. _RQ3: What are the trends of terrestrial applications of DTN?_  
DTN terrestrial applications are the focus of this SLR, and therefore, one research question was set for it. A total of 34 papers were found in this category. This includes implementations at the software or protocol level or applications in different case scenarios, such as communication in remote areas without network infrastructure, low-cost alternatives to satellite-assisted communication for sea applications, sensor applications for environmental purposes, and smart cities. Rural remote areas and environment. Within the category of implementations for remote areas, there is Daknet, a low-cost DTN infrastructure for communication in rural areas in India [37]. KioskNet is another implementation of DTN for TCP/IP for rural areas that uses the cellular phone network [38]. Sami is an implementation to provide communication to a remote population in Sweden [39]. ZebraNet and LUSTER are sensor networks based on DTN for wildlife and environment monitoring [40]. Smart cities. In the category of smart cities, there are different implementations. CarTel is a mobile sensor platform for traffic analysis [41]. DieselNet is a DTN network using WiFi nodes attached to buses to transfer data between them [42]. BikeNet is a mobile sensor that supports DTN and real-time sensing [43]. TrainNet is a DTN vehicular network where nodes and storage devices are attached to trains to transfer data between them [44]. DAPHNE, is a Delay Tolerant Application Proxy for e-health Network Environments [45]. An implementation of DTN in a smart city scenario running a Spray and Hop Distance(SNHD) routing protocol shows that more tests are needed to evaluate the performance in IoT scenarios [46]. DistressNet is a platform for emergency responders' communications that implements DTN through networking equipment, including smartphones, routers, and sensors, which are carried by vehicles and mobile personnel acting as mules. The system also implements a cloud computing interface that works as Fog routers through Mikrotik RB433 UAH devices, which allows users to share files. The platform also provides users with a social networking app to share their status or to message other users. All information is kept in the fog; therefore, regular synchronization with third-party services is required [31]. Protocol or software implementations. In the category of software or protocol implementations, several works were found. One of the issues of DTN is streaming, as applications require in-order delivery or full reliable services. Tetrys, an on-the-fly coding scheme, is a strategy for long-delay networks where missing packets are not possible with traditional retransmission mechanisms. It is based on an elastic encoding window updated dynamically using feedback information from the receiver. It generates repair bundles for every k-source bundle [47]. Zhang proposes a Software Defined Network (SDN) to separate the control function and the transmission function in mass sensor networks in satellite DTNs. Transmission requirements are translated into separate controls for satellite IoT. Zhang also states that satellite IoT, which combines satellite and IoT, is key for technologies, such as satellite manufacturing, satellite navigation, driverless, artificial intelligence, big data, wide-area narrowband low-power wireless communication, and 5G [26]. Srivastava proposes the use of throwboxes, cheap wireless storage devices used as dumping spots in fickle connections.