TCP connection termination (four-way handshake) in Computer Networks - Cheat Sheet & Quick Revision

Recall & Review

beginner

What is the purpose of the TCP four-way handshake?

The TCP four-way handshake is used to properly close a TCP connection between two devices, ensuring that both sides have finished sending data and can safely terminate the connection.

beginner

Name the four steps involved in the TCP connection termination process.

The four steps are: 1) FIN sent by the side that wants to close, 2) ACK sent by the other side to acknowledge the FIN, 3) FIN sent by the other side to close its side, 4) ACK sent by the first side to acknowledge the second FIN.

intermediate

Why does TCP use a four-way handshake instead of a simpler method to close connections?

Because TCP connections are full-duplex, each side can send data independently. The four-way handshake allows each side to close its sending channel separately, ensuring no data is lost.

beginner

What does the FIN flag indicate in a TCP segment?

The FIN flag indicates that the sender has finished sending data and wants to close its side of the connection.

intermediate

What state does a TCP connection enter after sending a FIN and before receiving the final ACK?

It enters the FIN_WAIT_1 state, waiting for an acknowledgment from the other side.

Which TCP flag is used to initiate connection termination?

A ACK

B SYN

C RST

D FIN

The FIN flag signals that the sender wants to close its side of the connection.

How many steps are there in the TCP connection termination process?

A 2

B 4

C 5

D 3

TCP connection termination uses a four-step handshake to close both sides properly.

What does the ACK flag do during TCP connection termination?

A Acknowledges receipt of FIN

B Starts the connection

C Resets the connection

D Sends data

ACK acknowledges that the FIN flag was received, confirming the other side's intent to close.

Why can't TCP close the connection with just one FIN and one ACK?

A Because TCP is half-duplex

B Because SYN must be sent first

C Because both sides must close independently

D Because ACK is not reliable

TCP is full-duplex, so each side closes its sending channel separately, requiring two FINs and two ACKs.

After the first FIN is sent and acknowledged, what does the other side do next?

A Sends its own FIN

B Sends data

C Closes the connection immediately

D Sends a RST

The other side sends its own FIN to indicate it is ready to close its sending channel.

Explain the TCP four-way handshake process for connection termination in your own words.

Think about how each side tells the other it is done sending data.

Why is it important for TCP to have a four-step process to close a connection instead of just one step?

Consider how data flows in both directions.

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