What is the relationship between congestion control and flow control in TCP?

• A. They are the same concept
• B. Flow control limits the sender’s rate
• C. Congestion control limits the receiver’s capacity
• D. The effective window is the minimum of rwnd and cwnd

Answer: (D)

In TCP, there are two separate controls shaping how much data can be in flight: congestion control and flow control. Congestion control adjusts the sender’s window (cwnd) in response to network conditions to avoid packing too much data into a congested path. Flow control uses the receiver’s advertised window (rwnd) to prevent overwhelming the receiver’s buffer. The actual amount the sender may transmit is the effective window, which is the smaller of cwnd and rwnd. If the network can handle more data (large cwnd) but the receiver’s buffer is small (small rwnd), rwnd limits the flow; if the receiver can accept data but the network is congested (small cwnd), cwnd limits the flow. This is why the precise relationship is that the effective window equals the minimum of rwnd and cwnd. The other statements either misstate the concepts or describe only part of the picture.