Three-way handshake in Computer Networks - Mini Project: Build & Apply

Understanding the TCP Three-Way Handshake

📖 Scenario: You are learning how computers establish a reliable connection over the internet using the TCP protocol. This process is called the three-way handshake. It helps two computers agree to communicate before sending data.

🎯 Goal: Build a simple step-by-step description of the TCP three-way handshake process using a list of messages exchanged between a client and a server.

📋 What You'll Learn

Create a list called handshake_steps with the exact three messages exchanged in order.

Add a variable called current_step to track the handshake progress starting at 0.

Use a loop with variable step to go through each message in handshake_steps.

Add a final message indicating the handshake is complete.

💡 Why This Matters

🌍 Real World

The TCP three-way handshake is how computers start a reliable connection before sending data, like when you open a website or send an email.

💼 Career

Understanding this handshake is important for network engineers, cybersecurity professionals, and software developers working with internet protocols.

1

Create the handshake messages list

Create a list called handshake_steps with these exact strings in order: 'Client sends SYN', 'Server sends SYN-ACK', 'Client sends ACK'.

Computer Networks

# Create the handshake_steps list with the three messages
# Your code here

Hint

Remember to use square brackets [] to create a list and put each message inside quotes.

2

Add a variable to track the current step

Add a variable called current_step and set it to 0 to track which handshake message is next.

Computer Networks

handshake_steps = ['Client sends SYN', 'Server sends SYN-ACK', 'Client sends ACK']
# Create current_step variable and set it to 0
# Your code here

Hint

Just create a variable named current_step and assign it the number 0.

3

Loop through handshake messages

Use a for loop with variable step to go through each message in handshake_steps. Inside the loop, update current_step to step.

Computer Networks

handshake_steps = ['Client sends SYN', 'Server sends SYN-ACK', 'Client sends ACK']
current_step = 0
# Loop through handshake_steps with variable step
# Update current_step to step inside the loop
# Your code here

Hint

Use range(len(handshake_steps)) to get indexes and assign current_step inside the loop.

4

Add final handshake completion message

After the loop, add a variable called final_message and set it to the string 'Handshake complete'.

Computer Networks

handshake_steps = ['Client sends SYN', 'Server sends SYN-ACK', 'Client sends ACK']
current_step = 0
for step in range(len(handshake_steps)):
    current_step = step
# Add final_message variable with 'Handshake complete'
# Your code here

Hint

Just create a variable named final_message and assign the exact string 'Handshake complete'.

Practice

1. What is the main purpose of the three-way handshake in computer networks?

easy

A. To disconnect a network session

B. To encrypt data before sending

C. To establish a reliable connection between two computers

D. To assign IP addresses automatically

Solution

1. Step 1: Understand the role of the three-way handshake

   The three-way handshake is a process used in TCP to start a connection reliably.

2. Step 2: Identify the correct purpose

   It ensures both computers agree to communicate before data transfer begins.

3. Final Answer:

   To establish a reliable connection between two computers -> Option C

4. Quick Check:

   Three-way handshake = establish connection [OK]

Hint: Remember: handshake means starting a connection [OK]

Common Mistakes:

• Confusing handshake with encryption
• Thinking it disconnects sessions
• Mixing it up with IP address assignment

2. Which sequence correctly represents the messages exchanged during the TCP three-way handshake?

easy

A. SYN, SYN-ACK, ACK

B. ACK, SYN, SYN-ACK

C. SYN-ACK, ACK, SYN

D. ACK, ACK, SYN

Solution

1. Step 1: Recall the message order in TCP handshake

   The client first sends SYN to request connection.

2. Step 2: Identify the correct sequence

   The server replies with SYN-ACK, then client sends ACK to confirm.

3. Final Answer:

   SYN, SYN-ACK, ACK -> Option A

4. Quick Check:

   Order is SYN -> SYN-ACK -> ACK [OK]

Hint: Think: Client SYN, Server SYN-ACK, Client ACK [OK]

Common Mistakes:

• Mixing up the order of messages
• Forgetting the SYN-ACK step
• Assuming ACK comes first

3. If a client sends a SYN message but does not receive a SYN-ACK response, what will happen next?

medium

A. The client immediately sends an ACK message

B. The client will resend the SYN message after a timeout

C. The connection is established without confirmation

D. The client closes the connection instantly

Solution

1. Step 1: Understand client behavior on no response

   If the client does not get SYN-ACK, it assumes the message was lost or server is unreachable.

2. Step 2: Identify the retry mechanism

   The client waits for a timeout, then resends the SYN to try again.

3. Final Answer:

   The client will resend the SYN message after a timeout -> Option B

4. Quick Check:

   No SYN-ACK -> resend SYN [OK]

Hint: No reply? Client retries SYN after waiting [OK]

Common Mistakes:

• Assuming connection established without SYN-ACK
• Thinking client sends ACK first
• Believing client closes connection immediately

4. A network engineer notices that after sending SYN, the client never receives SYN-ACK. Which of these is the most likely cause?

medium

A. The server is not listening on the target port

B. The client sent an ACK instead of SYN

C. The client skipped sending SYN-ACK

D. The client closed the connection too early

Solution

1. Step 1: Analyze the missing SYN-ACK response

   If no SYN-ACK arrives, the server likely did not respond to SYN.

2. Step 2: Identify common reasons for no response

   One common reason is the server not listening on the requested port, so it ignores SYN.

3. Final Answer:

   The server is not listening on the target port -> Option A

4. Quick Check:

   No SYN-ACK means server not listening [OK]

Hint: No SYN-ACK? Check if server port is open [OK]

Common Mistakes:

• Blaming client for sending wrong message
• Thinking client sends SYN-ACK
• Assuming client closed connection early

5. During the three-way handshake, what could happen if the final ACK message is lost in transit?

hard

A. The server assumes connection is established without ACK

B. The client will resend SYN immediately

C. The connection will close automatically

D. The server will retransmit SYN-ACK until it receives ACK

Solution

1. Step 1: Understand the role of the final ACK

   The final ACK confirms the client received the server's SYN-ACK and completes the handshake.

2. Step 2: Identify server behavior if ACK is lost

   If the server does not get ACK, it will retransmit SYN-ACK to prompt the client again.

3. Final Answer:

   The server will retransmit SYN-ACK until it receives ACK -> Option D

4. Quick Check:

   Lost ACK -> server retries SYN-ACK [OK]

Hint: Lost ACK? Server keeps sending SYN-ACK [OK]

Common Mistakes:

• Thinking client resends SYN instead
• Assuming connection closes automatically
• Believing server accepts connection without ACK