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TCP congestion control (slow start and loss recovery)

Computer Networks Lab · free online experiment, runs in your browser with no install or sign-up.

Open this experiment in the network simulator

Objective

Send 2 MB over a link that loses 1 % of the frames and watch the congestion window: slow start, then halving on loss.

Procedure

  1. Run and read the Summary: goodput, retransmissions, timeouts, maximum cwnd.
  2. Open What is happening: lines tagged TCP tell when the window was cut.
  3. Set the link loss to 0 %: the transfer is limited only by the 10 Mbit/s link.

Theory

TCP Reno grows cwnd exponentially in slow start and linearly afterwards. Three duplicate ACKs trigger fast retransmit and halve cwnd; a timeout drops it to 1. Random loss therefore keeps the average window — and the throughput — far below the link rate.

Network blocks used

2 × Host / PC

Open this experiment in the network simulator

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