## 2 Literature Review on IoT and Lightweight Security Protocols  
This section provides a detailed review of previous studies on IoT and lightweight security protocols. First, a brief background on the IoT network architecture has been provided to make understanding the details of each work easier. Subsequently, this section reviews related work in the field of IoT and security. In particular, the review presented in this paper focuses on communication protocols and lightweight security, which spans three different categories of related work, namely:  
* [noitemsep]
* standardized communications protocols for IoT;
* lightweight solutions for IoT;
* the challenges, trends, and requirements in implementing a secure IoT system. The first category focuses on surveys about standardized communication protocols for IoT, from the application to data link and physical layers of the network stack. The second category covers research studies that describe or propose lightweight encryption and authentication solutions for IoT. The last category is concerned with analyzing the research papers that discuss the challenges and the requirements in implementing IoT in different fields. For these reasons, the communication protocols and lightweight algorithms that will be addressed later in this study are positioned against these three categories of related work. More specifically, this paper concentrates on providing a review of previous studies on IoT and lightweight security protocols and it directly relates to the first two categories, noting that the research works developed for these two categories also serve as input to the third category of related works. For each category, Tables 1-3 indicate the reference, the publication year, and the topics discussed in each study. Additionally, Table 4 focuses on lightweight encryption algorithms--more specifically, symmetric block ciphers--that have been proposed for IoT devices. \begin{table}
\begin{tabular}{c c c} \hline \hline
**Reference** & **Year** & **Topic** \\ \hline [PERSON] et al. (2014) & 2020 & Discusses the evolution of IoT, the taxonomy, and proposed architectures, probes the various efforts for the standardization of IoT, and illustrates some IoT applications. \\ \hline [PERSON] et al. (2015) & 2017 & Discusses application layer protocols (CoAP, MQTT), service discovery protocols (mDNS, DNS-SD, uBonjour), infrastructure protocols (IEEE 802.15.4, 6 LoWPAN, LoRaWAN), and smart home systems with wireless sensors and robot assistants that use IEEE 802.15.4, 6 LoWPAN, CoAP. \\ \hline [PERSON] et al. (2016) & 2016 & Standardized IoT communication protocols are explained in detail, with analysis of their security in terms of integrity, confidentiality, authentication, and protection against replay attacks. \\ \hline [PERSON] et al. (2017) & 2017 & Communication protocols at each OSI (Open Systems Interconnection) layer. Briefly describes a few communication protocols used for IoT and whether they support security at the respective layer. \\ \hline [PERSON] et al. (2017) & 2017 & Review of forty authentication protocols in four environments, M2M, IoV, IoE, and IoS, that are evaluated under thirty-five attacks. Countermeasures and formal security verification techniques are also addressed. \\ \hline [PERSON] et al. (2018) & 2019 & Secure key generation and distribution (or key bootstrapping) in the context of IoT applications and constrained devices. With a focus on low-power and short-distance technologies, it reviews how public-key algorithms and authentication protocols are being adapted for IoT. \\ \hline [PERSON] et al. (2019) & 2019 & Application layer protocols for IoT in the context of specific challenges in fog and cloud computing integration. \\ \hline [PERSON] et al. (2020) & 2020 & Discusses IoT application protocols—MQTT, CoAP, XMPP, AMQP, DDS protocol, including HTTP-based applications, and the WebSocket protocol—within the context of IoT systems in agriculture, which must be reliable. \\ \hline \hline \end{tabular}
\end{table} 
Table 1: Standardized protocols for IoT.