TCP flow control (sliding window) in Computer Networks - Interactive Code Practice

Practice - 5 Tasks

1fill in blank

easy

Complete the sentence to describe the purpose of TCP flow control.

Computer Networks

TCP flow control uses a [1] to manage the amount of data sent before receiving an acknowledgment.

Drag options to blanks, or click blank then click option'

Achecksum

Brouting table

Csliding window

DIP address

Attempts:

3 left

💡 Hint

Common Mistakes

Confusing flow control with error checking methods like checksum.

Thinking routing tables are involved in flow control.

TCP flow control uses a sliding window to control how much data can be sent before waiting for an acknowledgment from the receiver.

2fill in blank

medium

Complete the sentence to explain what the window size represents in TCP flow control.

Computer Networks

The window size in TCP flow control represents the [1] of data the sender can transmit without waiting for an acknowledgment.

Drag options to blanks, or click blank then click option'

Amaximum amount

Bminimum amount

Caverage amount

Dtotal amount

Attempts:

3 left

💡 Hint

Common Mistakes

Confusing maximum with minimum or average.

Thinking the window size is the total data to be sent.

The window size is the maximum amount of data the sender can send before needing an acknowledgment, helping to prevent overwhelming the receiver.

3fill in blank

hard

Fix the error in the statement about sliding window movement.

Computer Networks

The sliding window moves forward when the sender receives a [1] from the receiver.

Drag options to blanks, or click blank then click option'

Anegative acknowledgment

Bpositive acknowledgment

Ctimeout

Dreset signal

Attempts:

3 left

💡 Hint

Common Mistakes

Thinking the window moves on timeout or negative acknowledgment.

Confusing reset signals with acknowledgments.

The sliding window advances when the sender receives a positive acknowledgment (ACK), indicating data was received successfully.

4fill in blank

hard

Fill both blanks to complete the explanation of how the sliding window adjusts.

Computer Networks

The sender adjusts the window size based on the [1] value sent by the receiver and [2] network conditions.

Drag options to blanks, or click blank then click option'

Aadvertised window

Bpacket loss

Ccongestion

Dtimeout

Attempts:

3 left

💡 Hint

Common Mistakes

Mixing up advertised window with timeout or packet loss.

Ignoring network congestion effects.

The sender uses the advertised window size from the receiver and considers network congestion to adjust the sliding window for efficient flow control.

5fill in blank

hard

Fill all three blanks to complete the dictionary comprehension representing TCP window updates.

Computer Networks

window_updates = [1]: [2] for [3] in packets if packets[[3]] > 0

Drag options to blanks, or click blank then click option'

Aseq_num

Bwindow_size

Dpacket

Attempts:

3 left

💡 Hint

Common Mistakes

Using packet as key instead of sequence number.

Not filtering packets with positive size.

This comprehension creates a dictionary where each sequence number (seq_num) maps to its window size, including only packets with positive size.

Practice

1. What is the main purpose of the sliding window in TCP flow control?

easy

A. To control the amount of data sent before receiving an acknowledgment

B. To encrypt data during transmission

C. To assign IP addresses to devices

D. To route packets between networks

Solution

1. Step 1: Understand the role of sliding window in TCP

   The sliding window limits how much data the sender can transmit before waiting for an acknowledgment from the receiver.

2. Step 2: Identify the purpose among options

   Only To control the amount of data sent before receiving an acknowledgment describes controlling data flow using acknowledgments, which matches the sliding window's purpose.

3. Final Answer:

   To control the amount of data sent before receiving an acknowledgment -> Option A

4. Quick Check:

   Sliding window controls data flow = To control the amount of data sent before receiving an acknowledgment [OK]

Hint: Sliding window limits unacknowledged data sent [OK]

Common Mistakes:

• Confusing flow control with encryption
• Mixing routing with flow control
• Thinking sliding window assigns IPs

2. Which of the following correctly describes the sliding window size in TCP?

easy

A. It is fixed and never changes during the connection

B. It is set by the IP address of the sender

C. It is always equal to the maximum segment size (MSS)

D. It increases or decreases based on acknowledgments received

Solution

1. Step 1: Recall how sliding window size behaves

   The window size changes dynamically as acknowledgments arrive, allowing the sender to send more data.

2. Step 2: Match behavior to options

   It increases or decreases based on acknowledgments received correctly states the window size adjusts based on acknowledgments, unlike fixed size or unrelated factors.

3. Final Answer:

   It increases or decreases based on acknowledgments received -> Option D

4. Quick Check:

   Window size changes with ACKs = It increases or decreases based on acknowledgments received [OK]

Hint: Window size changes with ACKs, not fixed [OK]

Common Mistakes:

• Assuming window size is fixed
• Confusing window size with MSS
• Linking window size to IP address

3. Consider a TCP connection with a window size of 4 segments. The sender has sent segments 1 to 4. The receiver acknowledges segment 2. Which segments can the sender send next?

medium

A. Segments 5, 6, and 7

B. Segments 3 and 4 again

C. Segments 5 and 6

D. No new segments until all are acknowledged

Solution

1. Step 1: Understand sliding window movement after ACK

   When segment 2 is acknowledged, the window slides forward, allowing sending of new segments beyond 4.

2. Step 2: Calculate which segments fit in the window

   Window size is 4, segments 1 and 2 acknowledged, so segments 3 and 4 are still unacknowledged. The sender can send segments 5 and 6 to fill the window.

3. Final Answer:

   Segments 5 and 6 -> Option C

4. Quick Check:

   ACK 2 slides window, send next 2 segments = Segments 5 and 6 [OK]

Hint: ACK moves window; send new segments within window size [OK]

Common Mistakes:

• Resending unacknowledged segments unnecessarily
• Sending more segments than window size allows
• Waiting for all segments to be acknowledged

4. A TCP sender has a window size of 3 segments and has sent segments 1, 2, and 3. The receiver sends an acknowledgment for segment 1 only. The sender mistakenly sends segment 4 and 5 immediately. What is the problem with this behavior?

medium

A. It violates the sliding window limit and may cause congestion

B. It improves throughput by sending more data

C. It is correct because ACK 1 means all previous segments are received

D. It causes the receiver to close the connection

Solution

1. Step 1: Analyze window size and ACK received

   The window size is 3, and only segment 1 is acknowledged, so segments 2 and 3 are still unacknowledged.

2. Step 2: Identify why sending segments 4 and 5 is wrong

   Sending segments 4 and 5 exceeds the allowed window size, risking congestion and data loss.

3. Final Answer:

   It violates the sliding window limit and may cause congestion -> Option A

4. Quick Check:

   Sending beyond window size causes issues = It violates the sliding window limit and may cause congestion [OK]

Hint: Don't send beyond window size before ACKs arrive [OK]

Common Mistakes:

• Assuming ACK 1 means all segments received
• Ignoring window size limits
• Thinking sending more data always helps

5. In a TCP connection, the sender's window size is 5 segments. The receiver's buffer can only hold 3 segments. How should the sliding window mechanism adapt to prevent buffer overflow?

hard

A. The sender should keep sending 5 segments regardless

B. The sender should reduce the window size to 3 segments

C. The receiver should increase its buffer size to 5 segments

D. The sender should stop sending any segments

Solution

1. Step 1: Understand receiver buffer limits

   The receiver can only hold 3 segments, so sending more than 3 risks overflow and data loss.

2. Step 2: Adjust sender window size accordingly

   The sliding window size must match or be less than the receiver's buffer to avoid overflow, so the sender should reduce its window size to 3.

3. Final Answer:

   The sender should reduce the window size to 3 segments -> Option B

4. Quick Check:

   Sender window ≤ receiver buffer to prevent overflow = The sender should reduce the window size to 3 segments [OK]

Hint: Match sender window to receiver buffer size [OK]

Common Mistakes:

• Ignoring receiver buffer size
• Assuming receiver buffer can always expand
• Stopping sending completely without reason