Which OSI layer handles end-to-end communication and reliability?

• A. Physical Layer
• B. Data Link Layer
• C. Network Layer
• D. Transport Layer

Answer: (D)

End-to-end communication and reliability are provided by the layer that delivers data between applications across the network. This layer takes data from a sending program, breaks it into manageable pieces, and ensures those pieces arrive at the destination in the proper order and without errors when using reliable protocols. It also handles flow control so the receiver isn’t overwhelmed and uses acknowledgments and retransmission to recover from lost or corrupted data. Additionally, it supports multiplexing with port numbers so multiple applications can communicate simultaneously over the same network path. The lower layers handle transferring data between neighbors and across networks, while this layer ensures the overall data stream between endpoints stays intact. Within this layer, reliable delivery is achieved by protocols like TCP, while UDP offers a faster but unreliable option. This combination makes it the layer best matching end-to-end communication and reliability.