TCP in Depth: Flow Control, Congestion Control and Loss

What TCP promises

Data arrives once and in order, or the connection fails.

Flow control versus congestion control

	Flow control	Congestion control
Protects	The receiver's buffer	The network between you
Limit	Receive window (rwnd), sent in each ACK	Congestion window (cwnd), computed by the sender
Signal	The receiver says how much room it has	Loss or delay

Congestion control: slow start and AIMD

A new connection probes the network.

Read more · 97 words

In slow start the window doubles every round trip from 10 segments. Past the threshold (ssthresh) it adds one per round trip, and on loss it cuts the window. This add-one, halve-on-loss rule (AIMD) lets many flows share a link fairly.

Try each loss event. is roughly one window per round trip, so 64 segments at 50 ms is about 15 Mbit/s.

Linux defaults to CUBIC; BBR models bandwidth and delay instead of reacting to loss.

Loss event

Loss at round: 8Receiver window: 128 seg

Initial ssthresh: 64 seg

Segments in flight per round trip

Congestion window
Receiver window

Show data table

Segments in flight per round trip
Round trip	Congestion window (segments)	Receiver window (segments)
0	10	128
1	20	128
2	40	128
3	64	128
4	65	128
5	66	128
6	67	128
7	68	128
8	69	128
9	34	128
10	35	128
11	36	128
12	37	128
13	38	128
14	39	128
15	40	128
16	41	128
17	42	128
18	43	128
19	44	128
20	45	128
21	46	128
22	47	128
23	48	128
24	49	128
25	50	128
26	51	128
27	52	128
28	53	128
29	54	128

Three duplicate ACKs at round 8: the window halves and then grows by one per round trip. Average in flight 46 segments, about 10.8 Mbit/s at 50 ms and 1460-byte segments.

// The congestion window (cwnd), in segments, for each round trip.
// Slow start doubles it until ssthresh, then congestion avoidance adds 1 per round trip.
// A loss cuts it: 3 duplicate ACKs halve it and carry on; a timeout drops it to 1 and starts over.
export function cwndSeries({ rounds = 30, initial = 10, ssthresh = 64, loss = null } = {}) {
  const out = [];
  let cwnd = initial;
  let threshold = ssthresh;
  for (let r = 0; r < rounds; r++) {
    out.push(cwnd);
    if (loss && r === loss.round) {
      threshold = Math.max(2, Math.floor(cwnd / 2));
      cwnd = loss.kind === 'timeout' ? 1 : threshold;
    } else if (cwnd < threshold) {
      cwnd = Math.min(cwnd * 2, threshold);
    } else {
      cwnd += 1;
    }
  }
  return out;
}

// Throughput is roughly one window per round trip.
export const throughputMbps = (segments, rttMs = 50, mss = 1460) => (segments * mss * 8) / (rttMs / 1000) / 1e6;

Quick checkIn slow start, how does the window grow each round trip?

Loss and retransmission

After three duplicate the sender retransmits at once (fast retransmit).

Head-of-line blocking

If segment 2 is lost, segments 3 to 5 may already have arrived, but the application sees none of them until segment 2 is resent.

Closing a connection and TIME-WAIT

Closing takes a FIN and an in each direction.

Other knobs worth knowing

Follow-up questions

What is the difference between flow control and congestion control?

protects the receiver with the window it advertises. protects the network with a window the sender computes from loss. The sender obeys the smaller.

Explain slow start and what happens on packet loss.

The window doubles each round trip until ssthresh, then grows by one. Three duplicate halve it; a timeout resets it to one segment.

What is TIME-WAIT and why can it cause trouble?

The side that closes first holds the connection about 60 seconds. Many short connections exhaust ports; reuse connections.

Check yourself

Question 1 of 7

Score 2 out of 3 or better to mark this page complete.

1. A receiver's application reads slowly and its buffer fills. Which mechanism tells the sender to slow down?Congestion controlFlow control, by shrinking the advertised receive windowSlow start

TCP in Depth: Flow Control, Congestion Control and Loss

Trade-offs

Bring it up before they ask

What TCP promises

Flow control versus congestion control

Congestion control: slow start and AIMD

TCP congestion window

Loss and retransmission

Head-of-line blocking

Closing a connection and TIME-WAIT

Other knobs worth knowing

Follow-up questions

Check yourself

CS fundamentals

Round trip	Congestion window (segments)	Receiver window (segments)
0	10	128
1	20	128
2	40	128
3	64	128
4	65	128
5	66	128
6	67	128
7	68	128
8	69	128
9	34	128
10	35	128
11	36	128
12	37	128
13	38	128
14	39	128
15	40	128
16	41	128
17	42	128
18	43	128
19	44	128
20	45	128
21	46	128
22	47	128
23	48	128
24	49	128
25	50	128
26	51	128
27	52	128
28	53	128
29	54	128

Round trip	Congestion window (segments)	Receiver window (segments)
0	10	128
1	20	128
2	40	128
3	64	128
4	65	128
5	66	128
6	67	128
7	68	128
8	69	128
9	34	128
10	35	128
11	36	128
12	37	128
13	38	128
14	39	128
15	40	128
16	41	128
17	42	128
18	43	128
19	44	128
20	45	128
21	46	128
22	47	128
23	48	128
24	49	128
25	50	128
26	51	128
27	52	128
28	53	128
29	54	128

Round trip	Congestion window (segments)	Receiver window (segments)
0	10	128
1	20	128
2	40	128
3	64	128
4	65	128
5	66	128
6	67	128
7	68	128
8	69	128
9	34	128
10	35	128
11	36	128
12	37	128
13	38	128
14	39	128
15	40	128
16	41	128
17	42	128
18	43	128
19	44	128
20	45	128
21	46	128
22	47	128
23	48	128
24	49	128
25	50	128
26	51	128
27	52	128
28	53	128
29	54	128