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

• A. Application Layer (Layer 7)
• B. Presentation Layer (Layer 6)
• C. Transport Layer (Layer 4)
• D. Network Layer (Layer 3)

Answer: (C)

The Transport Layer is responsible for end-to-end reliability and flow control between applications. It sits between the network (which moves packets) and the application processes, coordinating how data is sent from one process to another across a network.

Reliability is provided by mechanisms like acknowledgments, sequence numbers, and retransmission of lost data. These features are implemented by protocols at this layer (for example, TCP in common networks), which ensure data arrives correctly and in order. Flow control, often implemented with a sliding window, lets the receiver tell the sender to slow down if it can’t handle more data, preventing overload and loss.

Other layers don’t handle this end-to-end reliability and flow control. The Application Layer focuses on user-facing services and interfaces; the Presentation Layer deals with data formatting and encryption; the Network Layer handles routing and addressing but generally provides best-effort delivery without guarantees about reliability or ordering.