What is the PDU at Layer 4 (Transport)?

The Layer 4 (Transport) PDU is the segment — data plus a TCP/UDP header carrying source/destination ports (and, for TCP, sequence numbers).

* The Layer 4 segment: TCP/UDP header plus the application data. *

The Transport layer takes data from the Application layer and creates segments.

What happens at Layer 4:

1. Data is segmented into smaller pieces
2. TCP/UDP header is added
3. Contains source and destination ports
4. TCP adds sequence numbers for ordering

Mnemonic for all PDUs (top → bottom): "Do Some People Fear Birthdays" — Data, Segment, Packet, Frame, Bits.

Go deeper:

• Transport layer — the layer that produces segments and uses ports for multiplexing.
• Transmission Control Protocol — TCP's segment header, ports, and sequence numbers.