TCP congestion control in Computer Networks - Practice Problems & Coding Challenges

Challenge - 5 Problems

🧠 Conceptual

intermediate

Understanding TCP Congestion Window Behavior

What happens to the TCP congestion window size immediately after a packet loss is detected by triple duplicate acknowledgments?

A The congestion window size is reset to one segment and TCP enters slow start phase.

B The congestion window size is halved and TCP enters congestion avoidance phase.

C The congestion window size remains unchanged and TCP continues transmission normally.

D The congestion window size doubles immediately to recover lost throughput quickly.

💡 Hint

Think about how TCP reacts to moderate congestion signals without restarting completely.

When triple duplicate ACKs are received, TCP performs fast retransmit and fast recovery. It halves the congestion window size to reduce load and enters congestion avoidance to cautiously increase the window again.

📋 Factual

intermediate

TCP Slow Start Phase Behavior

During the TCP slow start phase, how does the congestion window size change with each round-trip time (RTT)?

A It increases exponentially, doubling each RTT.

B It remains constant until packet loss occurs.

C It increases linearly by one segment per RTT.

D It decreases exponentially to avoid congestion.

💡 Hint

Slow start aims to quickly find the network capacity by rapidly increasing the window.

In slow start, the congestion window doubles every RTT because it increases by one segment for every ACK received, effectively doubling the window each round.

🔍 Analysis

advanced

Effect of Network Delay on TCP Congestion Control

How does an increase in network round-trip time (RTT) affect the TCP congestion control performance?

A Higher RTT causes TCP to ignore congestion signals and maintain window size.

B Higher RTT causes faster congestion window growth, increasing throughput.

C RTT has no effect on congestion window size or throughput.

D Higher RTT causes slower congestion window growth, reducing throughput.

💡 Hint

Consider how RTT influences the feedback loop speed in TCP.

TCP relies on acknowledgments to adjust its congestion window. Higher RTT means acknowledgments arrive slower, so window growth slows, reducing throughput.

❓ Comparison

advanced

Comparing TCP Tahoe and TCP Reno

Which key difference distinguishes TCP Reno from TCP Tahoe in handling packet loss?

A TCP Tahoe ignores packet loss, TCP Reno reduces transmission rate.

B TCP Tahoe uses selective acknowledgments, TCP Reno does not.

C TCP Reno performs fast recovery after triple duplicate ACKs, while TCP Tahoe always enters slow start.

D TCP Reno resets the congestion window to zero on any loss, TCP Tahoe halves it.

💡 Hint

Think about how each version recovers from packet loss without restarting completely.

TCP Reno improves on Tahoe by using fast recovery to avoid slow start after triple duplicate ACKs, halving the window instead of resetting it.

❓ Reasoning

expert

Impact of Packet Loss Detection Method on TCP Throughput

Consider two TCP connections: one detects packet loss via timeout, the other via triple duplicate ACKs. Which connection typically achieves higher throughput and why?

A The connection detecting loss via triple duplicate ACKs achieves higher throughput because it recovers faster without reducing the congestion window drastically.

B The connection detecting loss via timeout achieves higher throughput because it waits longer before retransmitting, avoiding unnecessary retransmissions.

C Both connections achieve the same throughput since loss detection method does not affect performance.

D The connection detecting loss via timeout achieves higher throughput because it halves the congestion window more aggressively.

💡 Hint

Think about how quickly each method allows TCP to resume sending data after loss.

Triple duplicate ACKs allow fast retransmit and fast recovery, reducing the congestion window less and resuming transmission faster than timeout-based detection, which causes slow start.

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