Three-way handshake in Computer Networks - Practice Problems & Coding Challenges

Challenge - 5 Problems

🧠 Conceptual

intermediate

Purpose of the Three-way Handshake

What is the main purpose of the three-way handshake in TCP connections?

A To encrypt data before transmission to ensure security

B To establish a reliable connection by synchronizing sequence numbers between client and server

C To check the bandwidth availability between two hosts

D To immediately transfer data without any delay

💡 Hint

Think about what needs to happen before data can be sent reliably.

The three-way handshake is used to establish a reliable connection by synchronizing sequence numbers and confirming both sides are ready to communicate.

📋 Factual

intermediate

Sequence of Messages in Three-way Handshake

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

A ACK, ACK, SYN

B ACK, SYN, SYN-ACK

C SYN-ACK, SYN, ACK

D SYN, SYN-ACK, ACK

💡 Hint

The first message initiates the connection.

The client sends SYN to start, server replies with SYN-ACK, and client responds with ACK to complete the handshake.

🔍 Analysis

advanced

Impact of Missing Final ACK in Three-way Handshake

What happens if the final ACK message from the client is lost during the three-way handshake?

A The client will close the connection immediately

B The connection is immediately established without any issues

C The server will retransmit SYN-ACK and wait until it receives the ACK to establish the connection

D The server will send data without waiting for the ACK

💡 Hint

Consider how TCP ensures reliability in message delivery.

If the final ACK is lost, the server retransmits SYN-ACK and waits for the ACK to confirm the connection before proceeding.

❓ Comparison

advanced

Difference Between Three-way Handshake and Four-way Handshake

Which statement correctly distinguishes the TCP three-way handshake from the four-way handshake used in connection termination?

A The three-way handshake establishes a connection, while the four-way handshake gracefully closes it

B The four-way handshake is faster and replaces the three-way handshake in modern TCP

C Both handshakes are used to establish connections but differ in encryption methods

D The three-way handshake is used only for UDP connections

💡 Hint

Think about connection setup versus connection teardown.

The three-way handshake is for starting a TCP connection; the four-way handshake is for closing it properly.

❓ Reasoning

expert

Why is the Three-way Handshake Considered Reliable?

Why does the TCP three-way handshake provide a reliable way to establish a connection over an unreliable network?

A Because it uses acknowledgments and sequence numbers to confirm both sides are ready and synchronized

B Because it encrypts all messages to prevent data loss

C Because it sends data multiple times regardless of acknowledgments

D Because it uses UDP packets which are faster and more reliable

💡 Hint

Consider how TCP handles message loss and ordering.

The handshake uses sequence numbers and acknowledgments to ensure both sides agree on connection parameters, making the connection reliable despite network issues.

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