DEV Community: Siddhant Bansal

Understanding TCP 3-Way Handshake: The Heartbeat of Internet Communication

Siddhant Bansal — Sat, 03 May 2025 11:25:54 +0000

🔗 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

TCP vs UDP: Which One Should You Choose for Your Next App?

Siddhant Bansal — Sun, 27 Apr 2025 09:13:50 +0000

Recently, while diving deeper into Computer Networks, I stumbled upon a really interesting question!
"When should we use TCP and when should we use UDP?"

At first, it felt simple — two protocols, just pick one, right?

But as I explored more, I realized this question is not just technical — it’s very practical and even might frequently be asked in interviews!

So I thought, why not share this learning in a way that’s easy to understand for everyone?

First Things First: Where Do TCP and UDP Fit?

Before we compare TCP and UDP, it's important to know where they belong.

Both TCP and UDP are part of the Transport Layer of the TCP/IP model .

The TCP/IP model is a fundamental framework that guides how data moves across networks like the Internet ensuring reliable communication between devices.

The layers are:

Application Layer (Presentation + Session) — Your apps (browser, mail client, etc.)
Transport Layer — Responsible for delivering data (hello, TCP and UDP!)
Network Layer — Deals with routing (IP addresses, paths).
Network Access Layer (Data Link + Physical) — Physical sending of data (Wi-Fi, Ethernet, etc.)

Understanding this made me realize:-
TCP and UDP are like two different delivery systems sitting right between your app and the network!

Then, What Exactly is TCP?

As I dug further, I found that TCP stands for Transmission Control Protocol.

What makes TCP special:
🔹 Reliable Data Transfer
TCP/IP ensures that data sent from one device reaches another accurately and reliably without loss or corruption.

🔹 Divides Data into Packets
To maintain accuracy during transmission, TCP/IP breaks large data into smaller packets, which can be easily transmitted over networks.

🔹 Reassembles Data at Destination
At the receiver's end, TCP/IP reassembles the packets back into the original message, ensuring the information remains intact.

🔹 Supports Multiple Physical Media
TCP/IP is flexible — it can work over a wide variety of physical media and network technologies (like Ethernet, Wi-Fi, fiber optics, etc.).

🔹Real-world Internet Backbone
TCP/IP is the foundation of the real-world internet, making it possible for devices across the globe to communicate, regardless of the underlying hardware.

🔹No Strict Physical Layer Specification
Unlike some models, TCP/IP does not bind itself to one specific Physical Layer. This allows it to adapt easily to different hardware setups.

Imagine making a phone call:

You first dial, wait for the person to pick up, confirm they're listening — only then you start speaking.

That's TCP for us.

🔵 Real-world Examples of TCP:

Web browsing (HTTP/HTTPS) — You can’t afford missing chunks of a webpage!
Emails (SMTP/IMAP/POP3) — You need your full email, not half of it!
File transfers (FTP) — Every file must be accurate.

And What About UDP?

On the other hand, UDP stands for User Datagram Protocol.

This one is a connectionless protocol.

Here’s what makes UDP different:

No need to establish a formal connection, data is sent immediately.
No guarantee that the data will arrive in order (or even arrive at all).
It’s faster because there’s no overhead of connection or rechecking.

Think of UDP like sending a letter by post:

You drop it into a mailbox and hope it reaches, but you don’t wait for a "delivered" message.

🟠 Real-world Examples of UDP:

Live video streaming (YouTube Live, Twitch) — A few lost frames? No big deal!
Voice calls (Zoom, Skype) — Speed matters more than a perfect connection.
Online gaming (PUBG, Fortnite) — Instant updates are crucial.

Breaking It Down: TCP vs UDP

Feature	TCP	UDP
Connection	Connection-Oriented	Connectionless
Reliability	High (Reliable Delivery)	Low (Best-Effort Delivery)
Speed	Slower	Faster
Packet Ordering	Yes	No
Overhead	More (acknowledgments, retransmissions)	Less (minimal control)
Common Use Cases	Websites, Emails, File Transfers	Gaming, Live Video, Calls

Now, The Big Question: Which One Should You Choose?

After exploring, here’s the conclusion I came to:

✅ Use TCP when:

You need accuracy and reliability.
Example: Banking apps, payment gateways, websites, file downloads.

✅ Use UDP when:

You need speed and can tolerate minor data loss.
Example: Online games, video calls, real-time streaming.

In short:

TCP = Reliability over Speed

UDP = Speed over Reliability

Final Thoughts

While learning about TCP and UDP, I realized something important,
Choosing the right protocol is not just about technical aspects; it's about user experience.

Imagine losing a few words during a live speech is tolerable.

But losing 20% of your online banking transaction data? Absolutely not!

Both TCP and UDP have their own strengths and weaknesses.

Your choice depends completely on your app’s needs.

I hope this deep dive helped you learn something new today!