Why are UPD servers inconvenient to be used as concurrent servers?

UPD (User Datagram Protocol) servers are inconvenient as concurrent servers primarily because they are connectionless, meaning they don't establish a dedicated connection between the client and the server. This results in challenges with packet ordering and reliability, as packets can arrive out of sequence or be lost without acknowledgment. Additionally, managing concurrent requests becomes complex, as the server must handle multiple incoming packets from different clients simultaneously without a straightforward way to track session states. Consequently, ensuring data integrity and effective communication can become cumbersome in a concurrent environment.