## 2 Literature Review on IoT and Lightweight Security Protocols
### Standardized Protocols for IoT
A similar emphasis on application layer protocols is given in [20], which investigated test results obtained in 2017-2020 for IoT applications that use MQTT, CoAP, XMPP, AMQP, DDS, the Representational State Transfer (REST) interface using HTTP, and the WebSocket protocols. However, they focused on agricultural IoT applications, such as precision farming. It is very relevant to this review because it illustrates how IoT communication protocols and their security are used in different applications. According to [20], \"effective throughput and reliability are of major importance for smart farming since unreliable data can lead to inaccurate decisions and inappropriate automated procedures.\" Thus, when the CIA triad is considered, it shows that data integrity and availability (which are related to the reliability of the system) are the highest-priority factors for smart farming applications. The authors analyzed the application layer protocols by using performance indicators such as latency, energy and bandwidth requirements, throughput, reliability, security, and developers' preferences. They concluded that MQTT outperforms the others and is, at the same time, the most deployed application protocol for the IoT farming systems that they reviewed. MQTT runs over TCP protocols, which are very reliable, and re-transmits messages that are lost or corrupted. For security, MQTT would have to use TLS, which is heavy for constrained devices and is probably not adopted in most of the applications reviewed. One application protocol that is considered the most secure is XMPP, which includes a built-in TLS protocol, and also has a dedicated authentication function called the Simple Authentication Security Layer (SASL) that verifies a device's credentials. However, according to the study [16], XMPP is recommended for gateway nodes, not smart devices. In summary, some trends can be observed in these studies, with concern the types of communication technologies used at the device level. Thus, Figure 4 shows the distribution of protocols by layer. These observations are from the eight studies previously reviewed and represent commonly adopted technologies in IoT:  
* For the physical and data link protocols that are described in Table 1, it can be concluded that IEEE 802.15.4 and LoRaWAN are the most adopted protocols. For instance, IEEE 802.15.4 has been described in five of the eight papers, i.e., around 24% of the papers. IEEE 802.15.4 is a good example of short-range IoT and LoRaWAN for long-range applications. Moreover, the IEEE 802.15.4 security architecture provides a foundation for other protocols, such as Zigbee, BLE, and LoRaWAN. * At the network layer, the consensus is 6 LoWPAN as the main protocol used in the adaptation layer. The routing protocol RPL is mentioned in a few studies, but not as often as 6 LoWPAN. * At the application layer, there is a close tie between CoAP and MQTT. Although it is shown in Figure 4 that the percentage of CoAP is slightly higher, several studies, such as [27], which described a more specific application domain, mentioned that MQTT is definitely one of the most popular application layer protocols for IoT. Figure 4: In the eight studies, the number of times each protocol is described is recorded.