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TCP Flow Control and Windowing

Flow control prevents a fast sender from overwhelming a slow receiver: the Window field in every ACK tells how much buffer space remains. Congestion control (slow start, congestion avoidance, fast retransmit) is separate — it reacts to network loss, not just receiver buffers.

Sliding window idea
📤
Send
Bytes in flight ≤ min(cwnd, rwnd)
📬
ACK
Advances left edge of window
📏
rwnd
Receiver window in header
🐢
Zero window
Probe with window updates
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