TCP congestion control in Computer Networks - Cheat Sheet & Quick Revision

Recall & Review

beginner

What is the main goal of TCP congestion control?

The main goal of TCP congestion control is to prevent network congestion by controlling the rate at which data is sent, ensuring efficient use of network resources and avoiding packet loss.

beginner

Name the four key phases of TCP congestion control.

The four key phases are: Slow Start, Congestion Avoidance, Fast Retransmit, and Fast Recovery.

intermediate

Explain the Slow Start phase in TCP congestion control.

In Slow Start, TCP begins sending data slowly, increasing the congestion window exponentially each round-trip time until it detects packet loss or reaches a threshold called the slow start threshold (ssthresh).

intermediate

What happens during the Fast Retransmit phase?

Fast Retransmit triggers retransmission of lost packets quickly after receiving three duplicate acknowledgments, without waiting for a timeout.

intermediate

How does TCP adjust its congestion window during Congestion Avoidance?

During Congestion Avoidance, TCP increases the congestion window linearly to probe for available bandwidth carefully and avoid congestion.

Which TCP phase increases the congestion window exponentially?

A Fast Retransmit

B Congestion Avoidance

C Slow Start

D Fast Recovery

Slow Start increases the congestion window exponentially to quickly find the network capacity.

What triggers the Fast Retransmit phase in TCP?

A Three duplicate ACKs

B Window size reaching zero

C Timeout expiration

D Receiving a SYN packet

Fast Retransmit is triggered when TCP receives three duplicate acknowledgments indicating packet loss.

During Congestion Avoidance, TCP increases the congestion window:

A Linearly

B Exponentially

C Randomly

D Not at all

TCP increases the congestion window linearly during Congestion Avoidance to avoid congestion.

What is the purpose of the congestion window in TCP?

A To route packets through the network

B To encrypt data packets

C To establish a connection

D To limit the number of packets sent without acknowledgment

The congestion window limits how many packets TCP can send before receiving an acknowledgment.

Which TCP phase immediately follows Slow Start?

A Fast Retransmit

B Congestion Avoidance

C Connection Termination

D Handshake

After Slow Start, TCP enters Congestion Avoidance to increase the window more cautiously.

Describe the four main phases of TCP congestion control and their roles.

Think about how TCP adjusts sending rate to avoid congestion.

Explain how TCP detects and responds to network congestion.

Focus on signals TCP uses and its reaction steps.

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