What is the name assigned to the protocol data unit (PDU) at the transport layer?

The protocol data unit (PDU) at the transport layer is referred to as a segment. This terminology is specific to the transport layer of the OSI (Open Systems Interconnection) model, which is responsible for providing end-to-end communication services for applications. The transport layer ensures that data is transferred reliably and in the correct order.

Segments are created when data from the application layer is broken down into manageable parts to be transmitted over the network. This segmentation allows for error detection and correction, ensuring that the receiving end can accurately reassemble the original data stream. Additionally, the transport layer adds its own header information to each segment, which includes critical information such as source and destination port numbers, helping direct the segment to the correct application process.

In contrast, the other terms refer to different layers in the OSI model. A packet is the PDU at the network layer, and a frame is the PDU at the data link layer. The term data is more of a generic descriptor rather than a specific name for a PDU at any layer. Each layer of the OSI model has its specific responsibilities and corresponding PDU terminology, making "segment" the correct choice for the transport layer.