Which OSI layer is responsible for end-to-end reliability and flow control?

• A. Physical
• B. Data Link
• C. Transport
• D. Session

Answer: (C)

End-to-end reliability and flow control are provided by the Transport layer. This layer manages communication between processes across a network, ensuring data is delivered accurately and in the correct order, and it prevents the sender from overwhelming the receiver. Protocols at this layer, like TCP, implement segmentation, sequencing, acknowledgments, and retransmission of lost data, along with flow control through a sliding window to match sender speed to the receiver’s capacity. The Data Link layer handles reliability only over a single link between two devices, dealing with frame errors on that link. The Session layer focuses on coordinating and managing conversations between applications, not on guaranteeing delivery or flow control. The Physical layer is concerned with transmitting raw bits over a medium, with no reliability guarantees.