Which layer ensures that data units are sent without losses or duplication?

The transport layer is responsible for ensuring that data units are sent without losses or duplication. This layer manages the delivery of messages between computers and establishes a reliable communication channel. It ensures that data packets are delivered in order, without errors, and that any lost packets are retransmitted. Protocols such as Transmission Control Protocol (TCP) operate at this layer to provide end-to-end error detection and recovery mechanisms. This is crucial for applications that rely on accurate data delivery, such as web browsers and file transfer applications.

In contrast, the application, network, and session layers serve different functions. The application layer is focused on providing network services directly to user applications, while the network layer is responsible for data routing and addressing but does not ensure reliable delivery. The session layer manages sessions between applications but does not handle the error recovery or data integrity that the transport layer does.