TCP congestion control in Computer Networks - Step-by-Step Execution

Concept Flow - TCP congestion control

Start Connection

↓

Initialize cwnd = 1 MSS

↓

Send packets

↓

Receive ACKs?

No→Timeout or Loss Detected

↓

Reduce cwnd (slow start or congestion avoidance)

↓

Increase cwnd

↓

Check cwnd vs ssthresh

↓

Slow Start

↓

Repeat Sending Packets

TCP congestion control starts with a small window, increases it when ACKs arrive, and reduces it when packet loss or timeout occurs, switching between slow start and congestion avoidance phases.

Execution Sample

Computer Networks

cwnd = 1  # start with 1 MSS
while sending data:
    send packets up to cwnd
    if ACK received:
        if cwnd < ssthresh:
            cwnd *= 2  # slow start
        else:
            cwnd += 1  # congestion avoidance

This code shows how TCP increases its congestion window size based on ACKs to control network congestion.

Analysis Table

Step	cwnd (MSS)	ssthresh (MSS)	Event	Action	Resulting cwnd
1	1	8	Start connection	Initialize cwnd and ssthresh	cwnd=1, ssthresh=8
2	1	8	ACK received	cwnd < ssthresh, slow start: cwnd *= 2	cwnd=2
3	2	8	ACK received	cwnd < ssthresh, slow start: cwnd *= 2	cwnd=4
4	4	8	ACK received	cwnd < ssthresh, slow start: cwnd *= 2	cwnd=8
5	8	8	ACK received	cwnd >= ssthresh, congestion avoidance: cwnd += 1	cwnd=9
6	9	8	ACK received	cwnd >= ssthresh, congestion avoidance: cwnd += 1	cwnd=10
7	10	8	Packet loss detected	Timeout: ssthresh = cwnd/2, cwnd=1 (slow start restart)	ssthresh=5, cwnd=1
8	1	5	ACK received	cwnd < ssthresh, slow start: cwnd *= 2	cwnd=2
9	2	5	ACK received	cwnd < ssthresh, slow start: cwnd *= 2	cwnd=4
10	4	5	ACK received	cwnd < ssthresh, slow start: cwnd *= 2	cwnd=8
11	8	5	ACK received	cwnd >= ssthresh, congestion avoidance: cwnd += 1	cwnd=9
12	9	5	No more data or stop	Transmission ends	Final cwnd=9

💡 Transmission ends after step 12 or when no more data to send.

State Tracker

Variable	Start	After 1	After 2	After 3	After 4	After 5	After 6	After 7	After 8	After 9	After 10	After 11	Final
cwnd	1	2	4	8	8	9	10	1	2	4	8	9	9
ssthresh	8	8	8	8	8	8	8	5	5	5	5	5	5

Key Insights - 3 Insights

Why does cwnd double during slow start but only increase by 1 during congestion avoidance?

During slow start (cwnd < ssthresh), TCP increases cwnd exponentially (doubling) to quickly find the network capacity (see steps 2-4). After reaching ssthresh, it increases linearly by 1 MSS per RTT to avoid congestion (steps 5-6).

What happens to cwnd and ssthresh when packet loss is detected?

When packet loss occurs (step 7), TCP assumes congestion and reduces ssthresh to half of current cwnd, then resets cwnd to 1 MSS to restart slow start, controlling congestion.

Why does cwnd sometimes stay the same after an ACK?

In this simplified trace, cwnd always changes on ACKs, but in real TCP, if ACKs are delayed or duplicate, cwnd may not increase immediately. Here, each ACK triggers an increase as shown in the table.

Visual Quiz - 3 Questions

Look at the execution table, what is the value of cwnd after step 4?

A 8

B 4

C 2

D 1

💡 Hint

Check the 'Resulting cwnd' column at step 4 in the execution_table.

At which step does TCP detect packet loss and reduce cwnd?

A Step 6

B Step 7

C Step 9

D Step 11

💡 Hint

Look for the 'Packet loss detected' event in the execution_table.

If ssthresh was initially 4 instead of 8, what would cwnd be after the second ACK?

A 4

B 2

C 1

D 8

💡 Hint

Refer to variable_tracker and how cwnd doubles during slow start until it reaches ssthresh.

Concept Snapshot

TCP congestion control manages data flow to avoid network overload.
Starts with cwnd=1 MSS, increases cwnd exponentially in slow start until ssthresh.
After ssthresh, cwnd grows linearly (congestion avoidance).
On packet loss, ssthresh halves and cwnd resets to 1 MSS.
This cycle repeats to balance speed and network safety.

Full Transcript

TCP congestion control is a method to prevent too much data from overwhelming the network. It begins by sending a small amount of data, called the congestion window or cwnd, starting at 1 maximum segment size (MSS). When acknowledgments (ACKs) come back, TCP increases cwnd quickly by doubling it during the slow start phase until it reaches a threshold called ssthresh. After that, it grows cwnd more slowly by adding 1 MSS per round trip time, called congestion avoidance. If TCP detects packet loss or a timeout, it assumes the network is congested, so it cuts ssthresh in half and resets cwnd to 1 MSS to start slow start again. This process repeats, balancing sending speed with network capacity to avoid congestion and packet loss.