Which layer is responsible for transporting data between processes (server and client programs)?

• A. Application layer
• B. Network layer
• C. Physical layer
• D. Transport layer

Answer: (D)

End-to-end communication between processes on different machines is handled by the transport layer. It uses port numbers to multiplex multiple conversations on a single host and ensures data is delivered to the correct process on the remote end. The transport layer can provide reliable, connection-oriented delivery (as with TCP) or lightweight, best-effort delivery (as with UDP), handling segmentation, reassembly, error detection, and flow control. The network layer is responsible for routing packets between hosts, the application layer defines the actual data and protocols used by programs, and the physical layer deals with the hardware signaling. In a server–client interaction, the transport layer ensures the data from a client process reaches the intended server process via the appropriate port, making it the layer that transports data between processes.