TCP flow control purpose: In reliable transport protocols such as TCP, what is the primary purpose of flow control between sender and receiver?

Introduction / Context:
Transport protocols implement mechanisms to avoid overwhelming receivers. TCP uses a sliding window scheme so the receiver can advertise how much data it can accept before acknowledgment, preventing buffer overflow and improving end-to-end reliability.

Given Data / Assumptions:

• TCP provides reliable, ordered byte-stream delivery.
• Receivers have finite buffer capacity and processing speed.
• Flow control is separate from error control and congestion control.

Concept / Approach:

Flow control is receiver-driven: the receiver advertises a window size (rwnd) in ACKs indicating how much unacknowledged data the sender may have in flight. The sender must respect this limit, adjusting its send rate accordingly.

Step-by-Step Solution:

Verification / Alternative check:

Analyze a trace: the TCP header window field fluctuates with buffer availability; sender pacing changes accordingly.

Why Other Options Are Wrong:

Retransmission on missing ACKs (A) is error control, not flow control.

Reassembly order (B) is provided by sequence numbers and buffering.

Segment size regulation (D) is influenced by MSS/MTU, not flow control per se.

Common Pitfalls:

Confusing congestion control (network capacity) with flow control (receiver capacity); assuming fixed window sizes when modern TCP can use window scaling.

Final Answer:

To provide a means for the receiver to govern the amount of data sent by the sender.