Research papers in Datagram Transport Layer Security (DTLS) for the Constrained Application Protocol (CoAP) focus on strengthening end-to-end security in resource-constrained IoT environments where lightweight communication is critical. Since CoAP is designed to run over UDP for efficiency, it lacks built-in security features, making it vulnerable to eavesdropping, replay, message forgery, and man-in-the-middle attacks. DTLS, being the adaptation of TLS for datagram protocols, is commonly employed to provide confidentiality, integrity, authentication, and protection against replay attacks in CoAP-based communication. However, its direct application in constrained devices introduces challenges such as high handshake overhead, increased energy consumption, latency, and memory footprint, which can degrade performance in Low-Power and Lossy Networks (LLNs). To address this, researchers have proposed lightweight DTLS implementations, session resumption techniques, pre-shared key (PSK) and raw public key (RPK) modes, header compression, and optimized cryptographic algorithms suitable for IoT devices. Several studies also explore combining DTLS with object security approaches like OSCORE (Object Security for Constrained RESTful Environments) to achieve end-to-end security with reduced overhead. Other works leverage hardware-assisted cryptography, context-aware session management, and group-based DTLS for secure multicast communication in CoAP. Furthermore, adaptive DTLS mechanisms integrated with energy-aware scheduling, cross-layer design, and Software-Defined Networking (SDN) have been proposed to balance security with efficiency. Recent research trends also incorporate blockchain-based trust management and lightweight certificate-less authentication schemes to overcome key management complexities in DTLS for CoAP. Despite significant progress, open challenges remain in achieving scalable group security, mitigating denial-of-service attacks, and designing ultra-lightweight DTLS variants that preserve security strength while fitting the stringent constraints of IoT devices. Overall, the literature demonstrates that DTLS remains a cornerstone for securing CoAP, but ongoing innovations are necessary to make it fully compatible with the diverse and resource-limited IoT landscape.