TCP flow control (sliding window) in Computer Networks - Step-by-Step Execution

Concept Flow - TCP flow control (sliding window)

Sender has data to send

↓

Check window size

↓

Send data

↓

Wait for ACK

↓

Receive ACK

↓

Slide window forward

↓

More data to send?

No→End

↩Back to Check window size

The sender checks if it can send data within the allowed window size, sends data if possible, waits for acknowledgments, and slides the window forward to send more data.

Execution Sample

Computer Networks

Window size = 4
Send packets 1,2,3,4
Receive ACK for packet 1
Slide window
Send packet 5

This example shows sending 4 packets within the window, receiving an ACK for the first packet, sliding the window, and sending the next packet.

Analysis Table

Step	Action	Window Start	Window End	Packets Sent	ACK Received	Window Slides	Next Packet to Send
1	Initialize window	1	4	None	None	No	1
2	Send packets within window	1	4	1,2,3,4	None	No	5
3	Receive ACK for packet 1	1	4	1,2,3,4	1	Yes	5
4	Slide window forward by 1	2	5	1,2,3,4	1	Yes	5
5	Send next packet within window	2	5	2,3,4,5	1	No	6
6	Receive ACK for packet 2	2	5	2,3,4,5	2	Yes	6
7	Slide window forward by 1	3	6	2,3,4,5	2	Yes	6
8	Send next packet within window	3	6	3,4,5,6	2	No	7
9	No more ACKs received	3	6	3,4,5,6	None	No	7
10	Window full, wait for ACK	3	6	3,4,5,6	None	No	7

💡 Sender waits because window is full and no new ACKs received to slide window.

State Tracker

Variable	Start	After Step 2	After Step 4	After Step 5	After Step 7	Final
Window Start	1	1	2	2	3	3
Window End	4	4	5	5	6	6
Packets Sent	None	1,2,3,4	1,2,3,4	2,3,4,5	2,3,4,5	3,4,5,6
ACK Received	None	None	1	1	2	2
Next Packet to Send	1	5	5	6	6	7

Key Insights - 3 Insights

Why does the sender stop sending new packets even if it has more data?

Because the window is full (see Step 10 in execution_table), the sender must wait for ACKs to slide the window before sending more packets.

What does sliding the window mean in this context?

Sliding the window means moving the window start and end forward after receiving ACKs (see Steps 3 and 4), allowing the sender to send new packets.

Why is the next packet to send updated only after sliding the window?

Because the sender can only send packets within the current window range; sliding the window opens space for new packets (see Steps 4 and 5).

Visual Quiz - 3 Questions

Look at the execution_table at Step 3. What packet ACK is received?

A Packet 4

B Packet 1

C Packet 2

D No ACK received

💡 Hint

Check the 'ACK Received' column at Step 3 in execution_table.

At which step does the window slide forward for the first time?

A Step 4

B Step 2

C Step 5

D Step 10

💡 Hint

Look for 'Window Slides' marked 'Yes' and the window start changing in execution_table.

If the sender receives no ACKs after Step 8, what happens next?

A Sender continues sending new packets

B Sender slides window forward anyway

C Sender waits because window is full

D Sender resets the connection

💡 Hint

See Steps 9 and 10 where no ACKs are received and the sender waits.

Concept Snapshot

TCP sliding window controls data flow by limiting unacknowledged packets.
Sender can send packets within the window size.
ACKs slide the window forward, allowing more packets to be sent.
If window is full, sender waits for ACK before sending more.
This prevents overwhelming the receiver and network.

Full Transcript

TCP flow control uses a sliding window to manage how much data the sender can transmit before needing an acknowledgment. The sender starts with a window size that defines how many packets it can send without waiting. It sends packets within this window and waits for acknowledgments (ACKs) from the receiver. When an ACK is received for the earliest packet, the window slides forward by one, allowing the sender to send the next packet. If the window is full and no ACKs are received, the sender must wait before sending more data. This mechanism ensures smooth data flow and prevents network congestion or receiver overload.