Three-way handshake in Computer Networks - Interactive Code Practice

Practice - 5 Tasks

1fill in blank

easy

Complete the code to identify the first step in the TCP three-way handshake.

Computer Networks

The client sends a [1] packet to initiate the connection.

Drag options to blanks, or click blank then click option'

ASYN

BACK

CFIN

DRST

Attempts:

3 left

💡 Hint

Common Mistakes

Confusing SYN with ACK or FIN packets.

Thinking the handshake starts with an ACK.

The first step in the TCP three-way handshake is the client sending a SYN (synchronize) packet to the server to request a connection.

2fill in blank

medium

Complete the code to identify the second step in the TCP three-way handshake.

Computer Networks

The server responds with a [1] packet to acknowledge the client's request.

Drag options to blanks, or click blank then click option'

AFIN

BACK

CRST

DSYN-ACK

Attempts:

3 left

💡 Hint

Common Mistakes

Choosing only SYN or only ACK instead of SYN-ACK.

Confusing this step with connection termination.

The server replies with a SYN-ACK packet, which means it acknowledges the client's SYN and also sends its own SYN to synchronize.

3fill in blank

hard

Fix the error in the description of the third step of the TCP handshake.

Computer Networks

The client sends a [1] packet to confirm the connection and complete the handshake.

Drag options to blanks, or click blank then click option'

ASYN

BFIN

CACK

DRST

Attempts:

3 left

💡 Hint

Common Mistakes

Using SYN or FIN instead of ACK for the final step.

Confusing connection setup with teardown.

The client sends an ACK packet to acknowledge the server's SYN-ACK, completing the three-way handshake.

4fill in blank

hard

Fill both blanks to complete the explanation of the TCP three-way handshake steps.

Computer Networks

Step 1: Client sends [1]. Step 2: Server replies with [2].

Drag options to blanks, or click blank then click option'

ASYN

BACK

CSYN-ACK

DFIN

Attempts:

3 left

💡 Hint

Common Mistakes

Mixing up SYN and ACK in the first step.

Using FIN instead of SYN-ACK in the second step.

The client starts by sending a SYN packet. The server responds with a SYN-ACK packet to acknowledge and synchronize.

5fill in blank

hard

Fill all three blanks to describe the full TCP three-way handshake process.

Computer Networks

1) Client sends [1]. 2) Server replies with [2]. 3) Client sends [3] to establish connection.

Drag options to blanks, or click blank then click option'

ASYN

BACK

CSYN-ACK

DFIN

Attempts:

3 left

💡 Hint

Common Mistakes

Swapping the order of SYN-ACK and ACK.

Using FIN instead of ACK in the last step.

The handshake starts with the client sending a SYN. The server responds with SYN-ACK. Finally, the client sends an ACK to complete the connection setup.

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