TCP Congestion Control: Slow Start & AIMD

Link Conditions

Round-trip time (RTT) 120 ms

Packet loss probability 2%

Initial ssthresh 32 pkt

Loss Recovery Mode

Congestion State

SLOW START

cwnd (window)

1.0 pkt

ssthresh

32 pkt

RTT elapsed

Throughput

0 pkt/s

How it works

TCP's congestion window (cwnd) controls how many unacknowledged packets may be in flight at once. Growth follows RFC 5681, evaluated once per round trip:

Slow start (cwnd < ssthresh):
  cwnd ← cwnd × 2        (exponential)

Congestion avoidance (cwnd ≥ ssthresh):
  cwnd ← cwnd + 1        (linear, AIMD "additive increase")

On loss — fast recovery (3 dup ACKs):
  ssthresh ← max(cwnd / 2, 2)
  cwnd ← ssthresh         ("multiplicative decrease")

On loss — timeout (no ACKs at all):
  ssthresh ← max(cwnd / 2, 2)
  cwnd ← 1                 (restart from slow start)

RTT slider — sets how often the window is re-evaluated; a shorter RTT reaches a given window size sooner in wall-clock time, exactly as on a low-latency link.
Loss probability — the chance any given round trip loses a packet, standing in for real network congestion/queue drops.
ssthresh slider — the initial slow-start threshold; once cwnd reaches it, growth switches from doubling to +1 per RTT.
Fast Recovery vs Timeout — fast recovery (triple duplicate ACK) halves the window and resumes at congestion-avoidance speed; a full timeout is far more punishing and restarts slow start from cwnd = 1.
The 3D scene shows one packet sphere per unit of cwnd in flight between client and server each RTT; a dropped packet flashes red and falls away instead of reaching the server.

Real-world relevance: this exact additive-increase/multiplicative-decrease (AIMD) logic is what makes TCP flows converge to a fair share of bandwidth and back off under congestion — it is the reason the internet doesn't collapse under load, and it underlies every TCP implementation (Reno, and the slow-start/AIMD skeleton inside CUBIC and BBR too).

In-flight packet Dropped packet Client / Server

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