What is the PDU at the Transport Layer?

• A. bytes
• B. segments
• C. frames
• D. data

Answer: (B)

At the Transport Layer, the PDU is a segment. The transport layer takes the application’s data stream and divides it into segments, attaching a header that includes port numbers, sequencing information, and control flags. This segmentation enables reliable, ordered delivery (as with TCP) and supports multiplexing multiple connections over the network. In TCP, the receiver uses the segment sequence numbers to reassemble the original data in the correct order and to acknowledge received segments. Compare with the other ideas: frames belong to the Data Link Layer, and bytes or generic data don’t capture the layering term used for the unit at this layer. If UDP were used, the PDU is often called a datagram, but in the context of Transport Layer terminology, segments are the standard PDU reference for TCP.