Three-way handshake in Computer Networks - Cheat Sheet & Quick Revision

Recall & Review

beginner

What is the purpose of the three-way handshake in networking?

The three-way handshake is used to establish a reliable connection between two devices before data transfer begins. It ensures both sides are ready and agree on communication parameters.

beginner

Name the three steps involved in the three-way handshake.

1. SYN: The client sends a synchronization request.
2. SYN-ACK: The server acknowledges and sends its own synchronization.
3. ACK: The client acknowledges the server's response, completing the connection setup.

intermediate

Why is the three-way handshake important for TCP connections?

It helps synchronize sequence numbers and confirms both devices are ready, which prevents data loss and ensures reliable communication.

intermediate

What could happen if the three-way handshake is skipped?

Skipping the handshake can cause data to be sent before the connection is ready, leading to lost or corrupted data and unreliable communication.

beginner

In the three-way handshake, what does SYN and ACK stand for?

SYN stands for Synchronize, which is a request to start communication.
ACK stands for Acknowledgment, which confirms receipt of a message.

What is the first message sent in the three-way handshake?

A SYN-ACK

B ACK

C SYN

D FIN

The client starts the handshake by sending a SYN message to initiate the connection.

What does the server send in response to the client's SYN message?

A ACK

B SYN

C FIN

D SYN-ACK

The server replies with a SYN-ACK message to acknowledge the client's SYN and send its own synchronization.

Which protocol commonly uses the three-way handshake?

A TCP

B IP

C UDP

D HTTP

TCP uses the three-way handshake to establish reliable connections.

What is the main goal of the three-way handshake?

A To establish a reliable connection

B To send data

C To close a connection

D To encrypt data

The handshake sets up a reliable connection before data transfer.

What happens after the client sends the final ACK in the handshake?

A Connection is closed

B Connection is established and data transfer begins

C Server sends FIN

D Client waits for timeout

After the final ACK, both sides have agreed and the connection is ready for data transfer.

Explain the three steps of the three-way handshake and why each step is necessary.

Think about how two devices agree to communicate reliably.

Describe what could go wrong if the three-way handshake did not happen before data transfer.

Consider what happens if devices start talking without agreeing first.

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