Flow Control (Stop-and-Wait, Sliding Window)

Flow Control ensures that a fast sender does not overwhelm a slow receiver. The receiver must have time to process each frame before the next one arrives.

1. Stop-and-Wait Protocol

The simplest flow control mechanism. The sender sends one frame and then stops and waits for an acknowledgment (ACK) from the receiver before sending the next frame.

• Pros: Dead simple. No buffer overflow possible.
• Cons: Extremely inefficient. The sender is idle for the entire round-trip time (RTT). On a high-latency, high-bandwidth link, utilization can be less than 1%.

Utilization: U = T_frame / (T_frame + 2 × T_propagation)

2. Go-Back-N (GBN)

The sender can send up to $N$ frames (the window size) without waiting for an ACK. The receiver only accepts frames in order.

• If frame 3 is lost, the receiver discards frames 4, 5, 6 (even if they arrive correctly) and sends a NAK or duplicate ACK for frame 3.
• The sender must retransmit frame 3 AND all subsequent frames (4, 5, 6). Hence "Go Back N."
• Pros: Much better utilization than Stop-and-Wait.
• Cons: Wasteful retransmission of correctly received frames.

3. Selective Repeat (SR)

Like GBN, the sender can send up to $N$ frames ahead. But the receiver buffers out-of-order frames and only requests retransmission of the specific lost/corrupted frames.

• If frame 3 is lost, the receiver accepts and buffers frames 4, 5, 6. It sends a NAK only for frame 3.
• The sender retransmits ONLY frame 3. When it arrives, the receiver delivers frames 3, 4, 5, 6 in order.
• Pros: Maximum efficiency. Only lost frames are retransmitted.
• Cons: Requires buffer space at the receiver. More complex to implement.

4. Comparison