Flow control (stop-and-wait, sliding window) in Computer Networks - Mini Project: Build & Apply

Understanding Flow Control: Stop-and-Wait and Sliding Window

📖 Scenario: You are learning how data is sent over a network between two computers. To avoid losing data or overwhelming the receiver, the sender uses flow control methods.Two common methods are Stop-and-Wait and Sliding Window. You will create simple models to understand how these methods control data flow.

🎯 Goal: Build simple step-by-step models that show how Stop-and-Wait and Sliding Window flow control methods work in sending data packets.

📋 What You'll Learn

Create a list of data packets to send

Set a window size for the sliding window method

Simulate sending packets using Stop-and-Wait method

Simulate sending packets using Sliding Window method

💡 Why This Matters

🌍 Real World

Flow control methods like stop-and-wait and sliding window are used in real networks to ensure data is sent reliably without overwhelming the receiver.

💼 Career

Understanding flow control is important for network engineers and software developers working on communication protocols and network applications.

1

Create the list of data packets

Create a list called packets containing these exact strings: 'Packet1', 'Packet2', 'Packet3', 'Packet4', 'Packet5'.

Computer Networks

# Create the list of packets
# Your code here

Hint

Use square brackets to create a list and put each packet name as a string inside.

2

Set the sliding window size

Create a variable called window_size and set it to the number 3.

Computer Networks

packets = ['Packet1', 'Packet2', 'Packet3', 'Packet4', 'Packet5']
# Set the window size to 3
# Your code here

Hint

Just assign the number 3 to the variable named window_size.

3

Simulate Stop-and-Wait sending

Write a for loop using the variable packet to go through each item in packets. Inside the loop, write a comment # Send packet and wait for acknowledgment to show the stop-and-wait process.

Computer Networks

packets = ['Packet1', 'Packet2', 'Packet3', 'Packet4', 'Packet5']
window_size = 3
# Simulate stop-and-wait sending
# Your code here

Hint

Use a for loop with packet as the variable to go through packets. Inside, add the comment to explain the stop-and-wait step.

4

Simulate Sliding Window sending

Write a for loop using the variable i to go from 0 to len(packets) - window_size + 1. Inside the loop, create a variable window_packets that slices packets from i to i + window_size. Then add a comment # Send packets in the current window.

Computer Networks

packets = ['Packet1', 'Packet2', 'Packet3', 'Packet4', 'Packet5']
window_size = 3
for packet in packets:
    # Send packet and wait for acknowledgment

# Simulate sliding window sending
# Your code here

Hint

Use a for loop with i to slide over the packets list. Slice the list from i to i + window_size to get the current window.

Practice

1. Which flow control method sends one packet and waits for an acknowledgment before sending the next?

easy

A. Stop-and-Wait

B. Sliding Window

C. Multiplexing

D. Routing

Solution

1. Step 1: Understand Stop-and-Wait flow control

   Stop-and-Wait sends one packet and waits for an acknowledgment before sending the next packet.

2. Step 2: Compare with other methods

   Sliding Window sends multiple packets before waiting, while Multiplexing and Routing are unrelated to flow control.

3. Final Answer:

   Stop-and-Wait -> Option A

4. Quick Check:

   One packet at a time = Stop-and-Wait [OK]

Hint: One packet then wait means Stop-and-Wait [OK]

Common Mistakes:

• Confusing Sliding Window with Stop-and-Wait
• Thinking Multiplexing is a flow control method
• Mixing routing with flow control

2. Which of the following correctly describes the sliding window flow control method?

easy

A. Sends one packet and waits for acknowledgment

B. Sends multiple packets before waiting for acknowledgment

C. Sends packets without any acknowledgment

D. Sends packets only after a timeout

Solution

1. Step 1: Recall sliding window behavior

   Sliding Window sends multiple packets before waiting for acknowledgment to improve speed.

2. Step 2: Eliminate incorrect options

   Sends one packet and waits for acknowledgment describes Stop-and-Wait. The other options do not describe standard flow control methods.

3. Final Answer:

   Sends multiple packets before waiting for acknowledgment -> Option B

4. Quick Check:

   Multiple packets sent = Sliding Window [OK]

Hint: Multiple packets sent before wait means Sliding Window [OK]

Common Mistakes:

• Choosing Stop-and-Wait for sliding window question
• Thinking no acknowledgment is used
• Confusing timeout with acknowledgment wait

3. Consider a sliding window protocol with window size 4. If the sender has sent packets 1, 2, 3, and 4, but only packets 1 and 2 are acknowledged, which packets can the sender send next?

medium

A. Packets 5 and 6

B. Packets 3, 4, and 5

C. Packets 5, 6, 7, and 8

D. Packets 3, 4, 5, and 6

Solution

1. Step 1: Understand sliding window acknowledgments

   Packets 1 and 2 are acknowledged, so the window slides forward by 2 packets.

2. Step 2: Calculate new window range

   Window size is 4, so after acknowledging 1 and 2, sender can send packets 5 and 6 (next two packets).

3. Final Answer:

   Packets 5 and 6 -> Option A

4. Quick Check:

   Window slides by acknowledged packets = Packets 5 and 6 [OK]

Hint: Window slides by acknowledged packets count [OK]

Common Mistakes:

• Trying to resend unacknowledged packets
• Sending too many packets beyond window size
• Not sliding window after acknowledgments

4. A stop-and-wait protocol implementation is not receiving acknowledgments and keeps resending the same packet. What is the most likely cause?

medium

A. The sender is sending multiple packets at once

B. The sender window size is too large

C. The receiver is not sending acknowledgments

D. The network is using sliding window instead

Solution

1. Step 1: Analyze stop-and-wait behavior

   Stop-and-wait waits for acknowledgment before sending next packet; no acknowledgment causes resending.

2. Step 2: Identify cause of no acknowledgment

   If acknowledgments are missing, likely the receiver is not sending them or they are lost.

3. Final Answer:

   The receiver is not sending acknowledgments -> Option C

4. Quick Check:

   No acknowledgment = Resend same packet [OK]

Hint: No ack means receiver not responding [OK]

Common Mistakes:

• Blaming sender window size in stop-and-wait
• Confusing sliding window with stop-and-wait
• Assuming multiple packets sent in stop-and-wait

5. In a sliding window protocol with window size 5, the sender transmits packets 1 to 5. Packets 2 and 3 are lost, but packets 1, 4, and 5 are acknowledged. Which packets should the sender retransmit next to maintain correct flow control?

hard

A. Packets 6 and 7

B. Packets 2, 3, and 6

C. Packets 2, 3, 4, and 5

D. Packets 2 and 3 only

Solution

1. Step 1: Identify lost packets

   Packets 2 and 3 are lost, so they need retransmission.

2. Step 2: Understand acknowledgments impact

   Packets 1, 4, and 5 are acknowledged, so only lost packets 2 and 3 must be resent before sending new packets.

3. Final Answer:

   Packets 2 and 3 only -> Option D

4. Quick Check:

   Retransmit lost packets only = Packets 2 and 3 [OK]

Hint: Retransmit only lost packets, not acknowledged ones [OK]

Common Mistakes:

• Retransmitting already acknowledged packets
• Sending new packets before lost ones
• Confusing lost packets with acknowledged packets