Which layer of the OSI model is responsible for error-checking and ensuring proper order of data units?

The transport layer of the OSI model is responsible for error-checking and ensuring that data units are delivered in the correct order. This layer provides end-to-end communication services for applications. It segments data into smaller packets, adds sequence numbers for ordering, and employs error detection mechanisms, such as checksums, to ensure that data is transmitted accurately.

By managing these aspects, the transport layer helps maintain the integrity of the communication process between two endpoints in a network. For example, if segments of data are received out of order, the transport layer can reorder them before passing them onto the upper layers. Additionally, if errors are detected, it can request retransmission of the affected segments, ensuring reliable communication.

In contrast, the application layer focuses on providing network services to end users, the network layer is responsible for routing data between devices across multiple networks, and the session layer establishes, manages, and terminates connections between applications. None of these layers incorporate the error-checking and sequencing capabilities that define the transport layer's functionality.