What is the PDU of the Application Layer?

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

Answer: (B)

The thing being carried at the Application Layer is the actual user data—the message produced by the application, such as an HTTP request, email body, or file content. That data is handed down to the Transport Layer, which adds its own header and creates a segment (for TCP) or a datagram (for UDP). As it moves downward, each layer adds headers, turning the data into a packet at the Network Layer, a frame at the Data Link Layer, and finally bits at the Physical Layer. So the PDU at the Application Layer is data—the payload from the application. The other terms describe PDUs created by lower layers (segments by Transport, frames by Data Link, packets by Network).