Which OSI layer is responsible for end-to-end data transport and reliability?

• A. Layer 1
• B. Layer 2
• C. Layer 3
• D. Layer 4

Answer: (D)

End-to-end data transport and reliability are handled by the Transport layer. This layer takes data from higher layers, breaks it into pieces, and ensures those pieces reach the destination in order and without corruption. It uses port numbers to distinguish multiple conversations between the same hosts and, in reliable implementations like TCP, provides acknowledgments and retransmission of lost segments, along with flow control to prevent overwhelming the receiver. The data link layer focuses on delivery and error checking over a single link, while the network layer handles routing and addressing across multiple links. Higher layers manage sessions, presentation, and applications, but the guarantee of dependable end-to-end delivery comes from the Transport layer.