## 4 Conclusions  
This research presents an SLR for mitigating frequent attacks at the network layer in wireless sensor networks using the RPL protocol. It is essential to mention that the IoT security analysis must also be seen from the WSN perspective because the problems that are present in WSN are also inherited to IoT. As a general trend, this work emphasized how the provision of security related to IoT is gaining more attention in recent years, even though there is a decrease in literary production between 2017 and 2019, as shown in Figure 3a. In 2020 the publications of the year 2019 were already matched, and it is expected that, by the end of 2020, this production will increase in number. According to the bibliographic review, nine attacks have been found: rank, blackhole, selective forwarding, wormhole, DODAG version number, DAO inconsistency, DIO suppression, Sybil, and sinkhole. Each of the 53 proposals analyzed in this review has an associated mitigation strategy, these strategies have been categorized into four groups: based on authentication or cryptography, based on network monitoring, based on secure parent node selection and other. According to the results, the primary mitigation strategy used by the authors is based on parent selection or trust model, with 36% (See Figure 2). Figure 4 shows that some authors have been constant in the investigation of the subject, and this is concluded because these authors have more than one proposal included in this review. An important conclusion is that there is a tendency to mitigate a single attack within a unique paper. This is evidenced in Figure 8, which shows that 35 proposals were classified with a thematic code. Only one research was classified with four thematic codes. This figure shows that the number of publications is inversely proportional to the number of mitigated attacks. The implications made in this review were as follows: (1) RPL security is a topic highly associated with the industry since the highest percentage of publications are in proceedings of international conferences (see Table 8). (2) The investigation takes strength in the field of security in RPL. Since its inception, its authors stated that this protocol did not manage security at the network layer. Therefore this has been a starting point for the investigation and implementation of mechanisms to mitigate attacks in this protocol (see Figure 3). (3) 10 authors have more than one publication in this SLR. This situation shows a trend of the persistence of researchers in the area. On the other hand, according to Table 8, the collaboration index is 2.71, which shows temporary work among the authors. (4) New Zealand, India, Iran and Algeria are evidenced as strong countries at the level of technological research, according to Figure 5.  
There is a tendency to improve the metrics of the RPL protocol. One of these metrics is energy consumption, given the nature of WSN, and it is necessary to maximize the lifetime to the sensors. In the protocol proposal [12], the authors affirm that they do not have security mechanisms implemented for the routing layer, leaving the line of investigation open to those who wish to improve the said protocol. In the first instance in the research, it was evidenced not to take into account the energy consumption. However, as a future trend, it is a metric that is intended to improve even by implementing the security mechanisms. The biggest obstacle to developing security mechanisms for IoT is the nature of the network. The reason is that things or sensors are small devices with little processing capacity, which makes it challenging to execute robust algorithms that strengthen the security of the system. RPL does not have a defined implementation for its security operations, as the standard only advises on how to improve the security of the protocol. It is vital to continue working on strengthening the security aspects of RPL since even a small security flaw can be exploited and misused by third parties.