TCP congestion control in Computer Networks - Interactive Code Practice

Practice - 5 Tasks

1fill in blank

easy

Complete the sentence to describe the main goal of TCP congestion control.

Computer Networks

TCP congestion control aims to [1] network congestion by adjusting the rate of data transmission.

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

Adelay

Bincrease

Cignore

Dreduce

Attempts:

3 left

💡 Hint

Common Mistakes

Choosing 'increase' because it sounds like improving speed.

Choosing 'ignore' because congestion seems unrelated.

TCP congestion control reduces network congestion by controlling how fast data is sent.

2fill in blank

medium

Complete the sentence to identify the TCP algorithm phase that increases the congestion window size exponentially.

Computer Networks

The [1] phase rapidly increases the congestion window size to probe network capacity.

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

Aslow start

Bcongestion avoidance

Cfast recovery

Dtimeout

Attempts:

3 left

💡 Hint

Common Mistakes

Confusing slow start with congestion avoidance.

Choosing fast recovery which happens after packet loss.

The slow start phase increases the congestion window exponentially to quickly find the network capacity.

3fill in blank

hard

Fix the error in the description of TCP congestion control phases.

Computer Networks

After detecting packet loss, TCP enters [1] phase to reduce the congestion window and recover quickly.

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

Afast recovery

Bslow start

Ccongestion avoidance

Didle

Attempts:

3 left

💡 Hint

Common Mistakes

Choosing slow start which resets the window too much.

Choosing congestion avoidance which is a steady phase.

Fast recovery is the phase TCP enters after packet loss to reduce congestion window and recover quickly.

4fill in blank

hard

Fill both blanks to complete the formula for updating the congestion window size during congestion avoidance.

Computer Networks

cwnd = cwnd [1] ([2] / cwnd) per acknowledgment received

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

A+

B1

C-

D*

Attempts:

3 left

💡 Hint

Common Mistakes

Using multiplication instead of addition.

Subtracting instead of adding.

During congestion avoidance, the congestion window increases by 1 full-sized segment per round-trip time, approximated as adding 1/cwnd per ACK.

5fill in blank

hard

Fill all three blanks to complete the description of TCP congestion control response to packet loss.

Computer Networks

On packet loss, TCP sets [1] to half of its current value, resets [2] to 1, and enters [3] phase.

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

Assthresh

Bcwnd

Cslow start

Dfast recovery

Attempts:

3 left

💡 Hint

Common Mistakes

Confusing cwnd and ssthresh roles.

Choosing fast recovery instead of slow start after loss.

When packet loss occurs, TCP sets the slow start threshold (ssthresh) to half the congestion window, resets the congestion window (cwnd) to 1, and enters slow start phase to cautiously probe the network.

Practice

1. What is the main purpose of TCP congestion control in computer networks?

easy

A. To prevent network overload by adjusting data sending rates

B. To encrypt data packets for security

C. To assign IP addresses to devices

D. To increase the size of data packets

Solution

1. Step 1: Understand TCP congestion control role

   TCP congestion control manages how much data is sent to avoid overwhelming the network.

2. Step 2: Compare options with this role

   Only To prevent network overload by adjusting data sending rates describes adjusting sending rates to prevent overload, matching the purpose.

3. Final Answer:

   To prevent network overload by adjusting data sending rates -> Option A

4. Quick Check:

   TCP congestion control = prevent overload [OK]

Hint: Focus on network traffic management, not security or addressing [OK]

Common Mistakes:

• Confusing congestion control with encryption
• Thinking it assigns IP addresses
• Believing it increases packet size

2. Which of the following is a correct description of the TCP Slow Start algorithm?

easy

A. It ignores network feedback and sends data at a fixed rate

B. It immediately sends the maximum possible data to the network

C. It decreases the data sending rate after every packet sent

D. It gradually increases the data sending rate to avoid congestion

Solution

1. Step 1: Recall Slow Start behavior

   Slow Start begins with a low sending rate and increases it gradually to detect network capacity.

2. Step 2: Match options to this behavior

   It gradually increases the data sending rate to avoid congestion correctly states the gradual increase to avoid congestion, unlike others.

3. Final Answer:

   It gradually increases the data sending rate to avoid congestion -> Option D

4. Quick Check:

   Slow Start = gradual increase [OK]

Hint: Slow Start means start slow, then speed up carefully [OK]

Common Mistakes:

• Thinking Slow Start sends max data immediately
• Believing it decreases rate after each packet
• Assuming it ignores network feedback

3. Consider this simplified TCP congestion window size sequence during Slow Start: it starts at 1, then doubles each round (1, 2, 4, 8, ...). What will be the window size after 4 rounds?

medium

A. 16

B. 8

C. 4

D. 32

Solution

1. Step 1: Understand doubling pattern

   The window size doubles each round: start at 1, then 2, 4, 8, 16...

2. Step 2: Calculate after 4 rounds

   Start (0 rounds): 1, After 1st round: 2, 2nd: 4, 3rd: 8, 4th: 16.

3. Final Answer:

   16 -> Option A

4. Quick Check:

   1x2⁴ = 16 [OK]

Hint: Double window size each round; 2^4 = 16 [OK]

Common Mistakes:

• Counting rounds incorrectly (off by one)
• Choosing 8 instead of 16
• Confusing doubling with adding

4. A TCP connection uses Slow Start but never reduces its congestion window size after packet loss. What problem will most likely occur?

medium

A. The connection will send data too slowly

B. The connection will close immediately

C. The connection will ignore congestion and cause network overload

D. The connection will encrypt data incorrectly

Solution

1. Step 1: Understand congestion window reduction role

   Reducing window size after packet loss helps avoid network overload by slowing sending rate.

2. Step 2: Analyze effect of not reducing window

   If window size never reduces, the sender keeps sending too much data, causing congestion and overload.

3. Final Answer:

   The connection will ignore congestion and cause network overload -> Option C

4. Quick Check:

   No window reduction = overload risk [OK]

Hint: Always reduce window on loss to prevent overload [OK]

Common Mistakes:

• Thinking it causes slow sending
• Assuming connection closes automatically
• Confusing congestion control with encryption

5. In a network with frequent packet loss, which TCP congestion control strategy best balances throughput and congestion avoidance?

hard

A. Use Slow Start only, ignoring packet loss

B. Use Fast Retransmit and Fast Recovery to quickly recover from losses

C. Send data at maximum rate regardless of network state

D. Disable congestion control to improve speed

Solution

1. Step 1: Identify strategies for packet loss

   Fast Retransmit and Fast Recovery help detect and recover from packet loss quickly without restarting Slow Start.

2. Step 2: Evaluate options for balancing throughput and congestion

   Use Fast Retransmit and Fast Recovery to quickly recover from losses uses these algorithms to maintain good speed while avoiding overload, unlike others that ignore loss or disable control.

3. Final Answer:

   Use Fast Retransmit and Fast Recovery to quickly recover from losses -> Option B

4. Quick Check:

   Fast Retransmit + Recovery = balance throughput & avoid congestion [OK]

Hint: Fast Retransmit + Recovery handle loss efficiently [OK]

Common Mistakes:

• Ignoring packet loss effects
• Assuming max rate always best
• Disabling congestion control harms network