The central problem
Let feedback shape the sending rate.
TCP keeps data in flight inside a congestion window. Acknowledgements allow the sender to increase its sending opportunity, while loss or growing delay warns that the path may be under pressure.
This is different from detecting congestion in the network. The network exposes signals; TCP interprets them and changes one flow's behavior.
Adapt after a signal
Loss changes the next sending window.
When TCP detects likely loss, it retransmits missing data and reduces how much new data it sends into the path. As acknowledgements arrive again, the sender can cautiously increase its congestion window.
The exact response depends on the TCP implementation and the evidence it receives, but the operating principle is stable: probe for available capacity, then back off when the path shows stress.
Build the model
Four TCP congestion signals
Congestion window
Limits how much unacknowledged data the sender may place in the path.
Acknowledgements
Confirm delivered data and let the sender advance its sending window.
Loss and delay
Suggest that queues, retransmissions, or a pressured path need attention.
Rate adaptation
Changes the sending pace instead of continuing at the same rate.
Mini investigation
Inspect TCP feedback
Select the observed behavior and identify the most direct next check.
The sender may be limited by its congestion window rather than by the link's advertised capacity.
Compare the congestion window, bytes in flight, and acknowledgement progress.
Knowledge check
Test your TCP congestion model
Answer five questions.