TCP connection termination (four-way handshake) in Computer Networks - Mini Project: Build & Apply

TCP Connection Termination (Four-Way Handshake)

📖 Scenario: Imagine you are learning how two computers safely end a conversation over the internet using TCP. This process ensures that both computers agree to stop talking without losing any information.

🎯 Goal: You will build a step-by-step outline of the TCP connection termination process, showing the four messages exchanged between two computers to close a connection properly.

📋 What You'll Learn

Create variables representing the four TCP messages with exact names and values

Add a variable to represent the connection state after the first message

Use a sequence of steps to show the four-way handshake messages in order

Complete the process by showing the final connection state after termination

💡 Why This Matters

🌍 Real World

Understanding TCP connection termination helps network engineers and developers ensure reliable and clean disconnections in internet communications.

💼 Career

Knowledge of TCP handshakes is essential for roles in network administration, cybersecurity, and software development involving network programming.

1

Create the initial TCP termination messages

Create four string variables named fin_from_client, ack_from_server, fin_from_server, and ack_from_client with these exact values respectively: "FIN from client", "ACK from server", "FIN from server", and "ACK from client".

Computer Networks

# Your code here

Hint

Use simple string assignments with the exact variable names and values given.

2

Add the initial connection state after client sends FIN

Create a variable called connection_state and set it to the string "FIN_WAIT_1" to represent the state after the client sends the first FIN message.

Computer Networks

fin_from_client = "FIN from client"
ack_from_server = "ACK from server"
fin_from_server = "FIN from server"
ack_from_client = "ACK from client"
# Your code here

Hint

Assign the exact string "FIN_WAIT_1" to the variable connection_state.

3

Outline the four-way handshake message sequence

Create a list called termination_sequence that contains the four variables fin_from_client, ack_from_server, fin_from_server, and ack_from_client in this exact order.

Computer Networks

fin_from_client = "FIN from client"
ack_from_server = "ACK from server"
fin_from_server = "FIN from server"
ack_from_client = "ACK from client"
connection_state = "FIN_WAIT_1"
# Your code here

Hint

Create a list with the four message variables in the exact order shown.

4

Set the final connection state after termination

Update the variable connection_state to the string "CLOSED" to represent the connection state after the four-way handshake completes.

Computer Networks

fin_from_client = "FIN from client"
ack_from_server = "ACK from server"
fin_from_server = "FIN from server"
ack_from_client = "ACK from client"
connection_state = "FIN_WAIT_1"
termination_sequence = [fin_from_client, ack_from_server, fin_from_server, ack_from_client]
# Your code here

Hint

Change the connection_state variable to the exact string "CLOSED".

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