## 7 Discussion  
As shown in the results, most of the publications occurred between 2010 to 2019. However, in 2023, publications are still coming out, which means that the topic is of interest, moving from architecture and routing to applying DTN to new scenarios and technologies, such as IoT, 5G, and satellite technologies to support terrestrial communication. The most frequent topics in 2014, when the highest number of papers were found, include security, sensors, vehicular, and cellular networks. Given that this protocol sits between the application layer and the transport layer, it makes it possible to work with any underlaying protocol, especially in the data link layer. It was shown in the results that most papers focus on aspects related to performance and routing. This makes sense as the purpose of DTN is precisely to ensure that packets are not dropped and can reach the destination in the presence of communication infrastructure limitations of different kinds, which has to do with routing. Also, we could see that most papers dealing with issues, such as security or applications, did include performance as part of their discussion or parameters to evaluate them. Even though there has been a great amount of research work related to DTN routing, it continues to be one of the main interests, especially because of the conditions that make it different from traditional TCP/IP routing, where packets are dropped after a specific time. And, so, many different routing schemes were found in the literature, varying from how to choose a route depending on the type of contact, its social characteristics, or history. Others focus on the elements to design a routing protocol for these special conditions. However, still, there is not a strong word on the right combination of elements for a routing algorithm that gives an acceptable performance for different scenarios in the context of DTN. Therefore, here, there is an opportunity for further research. An aspect that affects routing and performance in this type of network is the dynamicity of the topology. As nodes move around, disappearing from time to time due to energy constraints or mobility, routes change. This requires more computing resources for nodes to perform routing algorithms and calculate routes, which again comes to energy. Many papers discussed aspects, such as energy consumption and computing capabilities as elements that affect performance and that are directly linked to the routing capabilities of nodes, yet no overall solution has been found. Even though a variety of routing algorithms are in the literature, it looks like each performs better in specific scenarios and not in all the possible situations. That is the case of algorithms like Intercontact, which is ideal for scenarios where nodes are mobile, but they have a recurrent path, and so they are recommended for emergency scenarios. Here, the focus of the algorithm is reducing energy consumption by reducing computation work to find routes. Routing algorithms based on society like CNS use the contact history to predict routes; as the network increases, so does the overhead. Likewise, Epidemic-based algorithms that flood the network with copies of messages have a lower performance as the network grows in nodes. There is the case of Hybrid MANET-DTN devoted to IoT, drones, and smart grid scenarios, which combines MANET with store-and-forward, and even there, different approaches perform differently. In this case, there are considerable computing and storage resources needed to make the calculations depending on which one must be applied in different times. Regarding the design of DTN routing protocols, it seems that the biggest issues include the energy constrains of nodes, as well as storage limitations. In both cases, the performance is affected, as packets will take longer to reach their destination. Works intended to design light routing protocols that might consume less energy and computing resources were not found.