🔗 Understanding the TCP 3-Way Handshake:
The TCP 3-Way Handshake is a fundamental process that establishes a reliable connection between two devices over a TCP/IP network. It involves three steps: SYN (Synchronize), SYN-ACK (Synchronize-Acknowledge), and ACK (Acknowledge). During the handshake, the client and server exchange initial sequence numbers and confirm the connection establishment.
What Is TCP?
TCP (Transmission Control Protocol) is a foundational internet protocol designed to:
✅ Deliver data reliably (no lost or duplicated packets)
✅ Ensure in-order delivery (packets arrive in the order they were sent)
✅ Provide error-checking (detect and fix corrupted data)
Unlike UDP (which just throws data out, hoping it arrives), TCP carefully sets up, manages, and closes connections.
What’s Inside a TCP Segment?
Before understanding the handshake, it’s useful to know what each TCP packet (called a segment) contains:
The header of a TCP segment can range from 20-60 bytes. 40 bytes are for options. If there are no options, a header is 20 bytes else it can be of upmost 60 bytes.
Header fields:
| Field | Explanation |
|---|---|
| Source Port | 16-bit number identifying the sending application’s port (e.g., your browser’s port) |
| Destination Port | 16-bit number identifying the receiving application’s port (e.g., web server port 80) |
| Sequence Number | 32-bit number marking the first byte in this segment; helps reorder packets and track delivery |
| Acknowledgment Number | 32-bit number showing the next expected byte; confirms successful receipt of earlier data |
| Header Length (HLEN) | 4-bit number indicating header size (in 4-byte words); minimum 5 (20 bytes), maximum 15 (60 bytes) |
| Control Flags | 6 key flags controlling the connection: URG, ACK, PSH, RST, SYN, FIN |
| Window Size | Communicates how much data the receiver can accept at once (flow control) |
| Checksum | Ensures data integrity; calculated by sender, verified by receiver |
| Urgent Pointer | If URG is set, points to urgent data requiring immediate processing |
Key Flags Overview:
- SYN → Start a connection (Synchronize)
- ACK → Acknowledge received data
- FIN → Finish/close the connection
- RST → Reset the connection
- PSH → Push data immediately to the application
- URG → Urgent data included
Step-by-Step: TCP 3-Way Handshake
The handshake synchronizes sequence numbers and ensures both client and server are ready to communicate.
1. SYN → Client → Server
The client sends:
- SYN flag set
- Picks an initial Sequence Number (e.g., 1000)
🗣 Client says:
"I want to connect. Here’s my starting number (1000)."
2. SYN-ACK → Server → Client
The server responds:
- SYN + ACK flags set
- Picks its own Sequence Number (e.g., 5000)
- Acknowledges the client’s number (
ACK = 1001)
🗣 Server says:
"Got your request (ack 1001). My number is 5000."
3. ACK → Client → Server
The client confirms:
- ACK flag set
- Acknowledges the server’s number (
ACK = 5001)
🗣 Client says:
"Got your number (ack 5001). Let’s start!"
Connection is now fully established — data can start flowing both ways.
Real-World Example
Imagine Alice wants to chat with Bob:
1️⃣ Alice: “Hi Bob! I want to start a conversation. Here’s message #1000.” (SYN)
2️⃣ Bob: “Hi Alice! Got your hello (ack 1001). My next message is #5000.” (SYN-ACK)
3️⃣ Alice: “Great! Got your hello (ack 5001). Let’s talk!” (ACK)
Now they’re ready to exchange full conversations.
Puzzle Piece Analogy for Sequence Numbers
Imagine you and your friend are mailing puzzle pieces back and forth:
- You (client) send piece #1000.
- Your friend (server) says: “Okay, I got #1000, send me #1001 next.”
Meanwhile, your friend sends you #5000, and you say:
“Okay, got it, send #5001 next.”
👉 Each side tracks its own sequence counter:
- You track what you’re sending and what you expect back.
- They track what they’re sending and what they expect back.
Why Are Sequence and Acknowledgment Numbers Important?
These numbers:
✅ Ensure no data is lost or duplicated
✅ Allow both sides to reassemble packets correctly, even if they arrive out of order
✅ Provide a reliable stream over an unreliable network
Without them, TCP wouldn’t know if pieces were missing or mixed up.
Final Notes
- The 3-Way Handshake runs once per connection (unless restarted).
- Sequence numbers keep increasing with each data exchange.
- TCP underpins reliable services like websites, emails, video streaming, and file transfers.
Summary Table
| Step | Who | Action |
|---|---|---|
| 1 → SYN | Client | Requests connection; sends initial sequence number |
| 2 → SYN-ACK | Server | Acknowledges client; sends its own sequence number |
| 3 → ACK | Client | Confirms receipt; connection fully established |
Top comments (4)
Definitely adding this to my to-read list!
Amazing content
Usefull content
good article, simple and easy to follow. I suggest to consider adding the full trace for client and sever communication lets say sending a file of size 1MB including the closing of the socket.