TCP connection termination (four-way handshake) in Computer Networks - Practice Problems & Coding Challenges

Challenge - 5 Problems

🧠 Conceptual

intermediate

Understanding the TCP four-way handshake steps

Which sequence correctly describes the four steps in the TCP connection termination process?

A FIN from server, ACK from client, SYN from client, ACK from server

B FIN from client, ACK from server, FIN from server, ACK from client

C ACK from client, FIN from server, ACK from server, FIN from client

D SYN from client, SYN-ACK from server, ACK from client, FIN from server

💡 Hint

Remember the termination starts with a FIN to signal closing.

The TCP four-way handshake for termination involves one side sending a FIN to close its sending side, the other side acknowledges it, then sends its own FIN, and finally the first side acknowledges that FIN.

📋 Factual

intermediate

Role of ACK in TCP connection termination

What is the role of the ACK packet in the TCP four-way handshake termination?

A It carries data to keep the connection alive

B It initiates the connection termination

C It resets the connection immediately

D It acknowledges receipt of the FIN packet from the other side

💡 Hint

Think about what happens after a FIN is sent.

After a FIN is sent to indicate the sender wants to close, the receiver sends an ACK to confirm it received the FIN.

🔍 Analysis

advanced

Why does TCP use a four-way handshake instead of a three-way handshake for termination?

Which explanation best describes why TCP uses a four-step process to terminate a connection?

A Because each side must independently close its sending channel, requiring separate FIN and ACK packets

B Because the connection must be reset twice to ensure no data loss

C Because the server always closes the connection first, then the client follows

D Because the handshake includes an extra SYN packet to confirm termination

💡 Hint

Consider the full-duplex nature of TCP connections.

TCP connections are full-duplex, meaning data can flow both ways independently. Each side must close its sending side separately, so a FIN and ACK are exchanged for each direction, totaling four steps.

❓ Comparison

advanced

Difference between TCP connection termination and reset

Which statement correctly contrasts TCP connection termination with a TCP reset (RST)?

A Both termination and reset use the same four-way handshake process

B Termination resets the connection immediately; reset uses a four-way handshake

C Termination uses a four-way handshake to close gracefully; reset immediately aborts the connection without handshake

D Reset is used only to establish connections; termination is for closing

💡 Hint

Think about graceful vs abrupt connection closing.

Termination allows both sides to finish sending data and close cleanly using the four-way handshake. Reset (RST) immediately aborts the connection, often due to errors or unexpected conditions.

❓ Reasoning

expert

State of TCP connection after first FIN and ACK exchange

After the first FIN is sent by one side and the other side replies with an ACK, what is the state of the connection on the side that sent the FIN?

A It is in FIN-WAIT-2 state, waiting for the other side's FIN

B It is in FIN-WAIT-1 state, waiting for the other side's FIN

C It is CLOSED because it finished termination

D It is ESTABLISHED because connection is still open

💡 Hint

Recall the TCP state transitions during termination.

After sending FIN and receiving ACK, the sender moves to FIN-WAIT-2 state, waiting for the FIN from the other side to complete termination.

Practice

1. What is the main purpose of the TCP four-way handshake during connection termination?

easy

A. To allow both sides to close their sending channels independently

B. To establish a new connection between two hosts

C. To encrypt data before transmission

D. To synchronize sequence numbers at the start of communication

Solution

1. Step 1: Understand TCP connection termination

   TCP uses a four-way handshake to close connections, allowing each side to finish sending data independently.

2. Step 2: Identify the purpose of the handshake

   This process ensures no data is lost and both sides agree to close the connection properly.

3. Final Answer:

   To allow both sides to close their sending channels independently -> Option A

4. Quick Check:

   Four-way handshake = independent close [OK]

Hint: Four steps mean both sides close separately [OK]

Common Mistakes:

• Confusing termination with connection setup
• Thinking handshake encrypts data
• Assuming simultaneous close in TCP

2. Which of the following is the correct sequence of TCP flags used in the four-way handshake for connection termination?

easy

A. FIN, SYN, ACK, FIN

B. SYN, ACK, FIN, RST

C. ACK, SYN, FIN, ACK

D. FIN, ACK, FIN, ACK

Solution

1. Step 1: Recall TCP termination flags

   The four-way handshake uses FIN and ACK flags to close the connection properly.

2. Step 2: Identify correct flag order

   The sequence is FIN from one side, ACK from the other, then FIN from the second side, and finally ACK from the first side.

3. Final Answer:

   FIN, ACK, FIN, ACK -> Option D

4. Quick Check:

   Termination = FIN then ACK pairs [OK]

Hint: Termination uses FIN and ACK flags in pairs [OK]

Common Mistakes:

• Including SYN or RST flags in termination
• Mixing order of flags
• Assuming simultaneous FIN flags

3. Consider this sequence of TCP flags exchanged between Host A and Host B during connection termination:
A sends FIN
B replies ACK
B sends FIN
A replies ACK
What is the state of the connection after Host A sends the final ACK?

medium

A. Connection is fully closed

B. Connection is half-closed, waiting for more data

C. Connection is still established

D. Connection is reset abruptly

Solution

1. Step 1: Analyze the four-way handshake steps

   Host A sends FIN, B acknowledges, B sends FIN, and A acknowledges. This completes the four steps.

2. Step 2: Understand connection state after final ACK

   After the last ACK, both sides have agreed to close, so the connection is fully closed.

3. Final Answer:

   Connection is fully closed -> Option A

4. Quick Check:

   Final ACK means full close [OK]

Hint: Final ACK completes termination [OK]

Common Mistakes:

• Thinking connection stays half-open after final ACK
• Confusing reset with normal close
• Assuming data can still be sent

4. A TCP connection termination sequence is observed as:
Host A sends FIN
Host B sends FIN
Host A sends ACK
Which step is missing or incorrect in this sequence?

medium

A. Host B should send RST instead of FIN

B. Host A should send SYN before FIN

C. Host B should send ACK before sending FIN

D. Host A should send FIN twice

Solution

1. Step 1: Review correct four-way handshake order

   After Host A sends FIN, Host B must acknowledge (ACK) before sending its own FIN.

2. Step 2: Identify missing ACK

   The sequence shows Host B sending FIN without ACKing Host A's FIN first, which is incorrect.

3. Final Answer:

   Host B should send ACK before sending FIN -> Option C

4. Quick Check:

   ACK always precedes FIN from other side [OK]

Hint: ACK must come before FIN from other side [OK]

Common Mistakes:

• Skipping ACK step
• Confusing SYN with FIN
• Thinking RST is normal close

5. During a TCP connection termination, if Host A sends a FIN and immediately closes its socket without waiting for the ACK from Host B, what is the likely outcome?

hard

A. Host B will retransmit FIN until it receives ACK, causing delays

B. The connection will close cleanly without issues

C. Host A will receive data after closing the socket

D. Host B will reset the connection immediately

Solution

1. Step 1: Understand proper termination behavior

   In TCP, after sending FIN, the application closes the socket, but the TCP stack manages states like FIN_WAIT_1, FIN_WAIT_2, and TIME_WAIT independently.

2. Step 2: Analyze consequence of immediate close

   The stack processes incoming ACK and FIN from Host B, sending responses as needed, so the connection closes cleanly without issues.

3. Final Answer:

   The connection will close cleanly without issues -> Option B

4. Quick Check:

   Stack handles states after socket close [OK]

Hint: TCP stack handles states post-socket close [OK]

Common Mistakes:

• Thinking immediate close causes reset by peer
• Expecting retransmissions without ACK loss
• Confusing socket close with immediate connection abort