Three-way handshake in Computer Networks - Step-by-Step Execution

Concept Flow - Three-way handshake

Client sends SYN

↓

Server receives SYN

↓

Server sends SYN-ACK

↓

Client receives SYN-ACK

↓

Client sends ACK

↓

Server receives ACK

↓

Connection Established

The three-way handshake is a process where the client and server exchange three messages (SYN, SYN-ACK, ACK) to establish a reliable connection.

Execution Sample

Computer Networks

Client -> Server: SYN
Server -> Client: SYN-ACK
Client -> Server: ACK

This sequence shows the three messages exchanged to start a TCP connection.

Analysis Table

Step	Message Sent	Sender	Receiver	Purpose	State After
1	SYN	Client	Server	Request connection	SYN sent
2	SYN-ACK	Server	Client	Acknowledge SYN and send own SYN	SYN-ACK sent
3	ACK	Client	Server	Acknowledge SYN-ACK	Connection established
4	-	-	-	-	Handshake complete, ready to transfer data

💡 After step 3, the connection is established and handshake ends.

State Tracker

Variable	Start	After Step 1	After Step 2	After Step 3	Final
Client State	Closed	SYN sent	Waiting ACK	ACK sent	Established
Server State	Closed	SYN received	SYN-ACK sent	Waiting ACK	Established

Key Insights - 3 Insights

Why does the server send SYN-ACK instead of just ACK?

The server sends SYN-ACK to both acknowledge the client's SYN and to send its own SYN to the client, indicating it wants to establish a connection too. This is shown in step 2 of the execution_table.

What happens if the client never sends the final ACK?

The server stays waiting for the ACK and does not establish the connection. This is implied after step 2 in the execution_table where the client is expected to send ACK.

Is the connection established after the first SYN message?

No, the connection is only established after the client sends the final ACK (step 3). Before that, both sides are still in the process of setting up.

Visual Quiz - 3 Questions

Look at the execution_table, what message does the client send at step 3?

A SYN-ACK

B ACK

C SYN

D FIN

💡 Hint

Check the 'Message Sent' column at step 3 in the execution_table.

At which step does the server send a message to the client?

A Step 1

B Step 3

C Step 2

D Step 4

💡 Hint

Look at the 'Sender' column in the execution_table to find when the server sends a message.

If the client never sends the ACK, what state does the server remain in after step 2?

A Waiting ACK

B Closed

C Established

D SYN sent

💡 Hint

Refer to the 'State After' column after step 2 in the variable_tracker.

Concept Snapshot

Three-way handshake:
1. Client sends SYN to request connection.
2. Server replies with SYN-ACK to acknowledge and request connection.
3. Client sends ACK to confirm.
Connection established after these three steps.
Ensures both sides are ready to communicate.

Full Transcript

The three-way handshake is a process used in computer networks to establish a reliable connection between a client and a server. It involves three steps: first, the client sends a SYN message to the server to request a connection. Second, the server responds with a SYN-ACK message to acknowledge the request and send its own connection request. Third, the client sends an ACK message to confirm the connection. After these three messages are exchanged, the connection is established and both sides can start communicating. This handshake ensures that both client and server are synchronized and ready to transfer data.