## 2 Literature Review on IoT and Lightweight Security Protocols
### Standardized Protocols for IoT  
Table 1 presents a summary of eight studies that discuss standardized protocols for IoT. Next, the main contributions of each paper are summarized. [PERSON] et al. [14] focused on IoT security and emphasized the recent research on authentication. They explained in detail the architecture of IoT and its layers. They reviewed a three-layered architecture that includes, from the bottom, the _perception_, _network_, and _application_ layers. The perception layer includes the sensors and devices and how they are  
Figure 3: General network architecture diagram of LPWA networks. connected, while the network layers offer services such as routing, data encapsulation, and service discovery. The application layer includes protocols to support requests/responses, publish/subscribe exchanges, and RESTful interfaces (where REST stands for representation state transfer and is commonly used for machines and computers to retrieve data on a Web server). The application layer also uses transport layer protocols such as a transmission control protocol (TCP) and user datagram protocol (UDP). They can provide security with a transport layer security (TLS) protocol. In addition, [PERSON] et al. [14] described different IoT applications and classified them either as collaborative aware (e.g., smart home, smart city, industrial internet) or information aggregation (smart energy, healthcare, wearable devices). As an IoT use case, they described a smart home system with wireless sensor nodes and robot assistants that use communication standards such as IEEE 802.15.4, 6 LoWPAN, and a constrained application protocol (CoAP). However, security is addressed concisely in their paper. Similarly referencing a layered architecture, [PERSON] et al. [15] reviewed standardized protocols for IoT, according to the layers that they operate:  
* Data link layer protocols such as IEEE 802.15.4 and long-range WAN (LoRaWAN). * Network layer protocols such as 6 LoWPAN, which allows devices to connect to IPv6 routers with compressed IP headers. * Application layer protocols: they compare CoAP, which runs over UDP, and message queuing telemetry transport (MQTT), which runs over TCP. * Domain name service (DNS) protocols translate between a domain name and an IP address since smart devices can join and leave the network at any time. Multicast domain name service (mDNS) supports multiple devices in the same DNS query, DNS service discovery (DNS-SD) discovers devices that provide a certain service, and UBonjour combines mDNS and DNS-SD. [PERSON] et al. [16] analyzed security requirements such as confidentiality, origin authentication, integrity, and protection against replay attacks, and how these are supported by standardized protocols. They focused on communication protocols for IoT at different layers of the protocol stack. The physical and data link communication protocols surveyed include IEEE 802.15.4, Bluetooth Low Energy, Zigbee, Wi-Fi, Near Field Communications (NFC), LoRaWAN, and Z-Wave. At the network layer, they reviewed 6 LowPAN and IP Security (IPSec). They also reviewed application layer protocols such as CoAP, MQTT and Advanced Message Queueing Protocol (AMQP), eXtensible Messaging and Presence Protocol (XMPP), and how they can be integrated with transport layer security protocols, such as TLS or Datagram TLS (DTLS), in the case of CoAP, which runs on UDP. They also addressed a full IoT protocol stack called Thread, which is owned by a consortium of industries including Google and ARM. The authors conclude that some protocols, such as IEEE 802.15.4 and LoRaWAN at the lower layers and CoAP and MQTT at the application layer, are better for resource-constrained devices. However, they proposed two points as future works that require the analysis of these standard protocols considering other security requirements, such as resilience and understanding how the security features of each protocol \"transfer to practical applications\" [16], which is addressed in this review.