# TCP Working: 3-Way Handshake & Reliable Communication

## Introduction: What if the Internet Had No Rules?

Imagine you want to send a message to your friend.

You shout the message.  
Your friend may or may not hear it.  
Even if they hear, they might hear it in the wrong order.  
Some words may be lost.  
Some words may be repeated.

That’s exactly what would happen on the internet **if there were no rules**.

The internet is just millions of computers sending data to each other.  
To make sure this data reaches the correct destination, **in the correct order**, and **without loss**, we need proper rules.

These rules are called **protocols**.

One of the most important protocols is **TCP**.

## What is TCP and Why Is It Needed?

**TCP** stands for **Transmission Control Protocol**.

TCP is a communication protocol that makes sure:

* Data reaches the correct destination
    
* Data is not lost
    
* Data arrives in the correct order
    
* Data is not duplicated
    
* Sender and receiver both agree before talking
    

In simple words:

> **TCP makes the internet reliable.**

Whenever you:

* Open a website
    
* Log in to an app
    
* Submit a form
    
* Send an email
    

TCP is working silently in the background.

## Problems TCP Is Designed to Solve

Before understanding how TCP works, let’s understand **what problems exist without TCP**.

### 1\. Data Loss

Packets can get lost due to:

* Network congestion
    
* Weak connections
    
* Hardware failures
    

### 2\. Wrong Order of Data

Data is broken into small packets.  
Packets can arrive **out of order**.

Example:

```plaintext
Sent: Hello World
Received: World Hello
```

### 3\. Duplicate Data

Sometimes the same packet can arrive multiple times.

### 4\. No Confirmation

Sender doesn’t know:

* Did the receiver get the data?
    
* Should I resend?
    

TCP solves **all these problems**.

## Introducing TCP as a Reliable Protocol

TCP works like a **formal conversation**.

Before sending data:

* TCP checks if the other side is ready
    
* Both sides agree to talk
    
* Rules are decided
    

Only then does actual data transfer begin.

This agreement process is called the **TCP 3-Way Handshake**.

## What Is the TCP 3-Way Handshake?

The **3-Way Handshake** is the process by which:

* A TCP connection is **established**
    
* Both client and server confirm:
    
    * “I am ready”
        
    * “I can hear you”
        
    * “Let’s start communication”
        

It has **three steps**:

1. SYN
    
2. SYN-ACK
    
3. ACK
    

Think of it as a **polite conversation**.

## Working of TCP 3-Way Handshake (Clear Explanation)

Before two computers can send data to each other on the internet, they must first **establish a connection**.  
They cannot start sending data randomly. Both sides need to confirm that the other is ready.

TCP does this using a process called the **3-Way Handshake**.

![](https://cdn.hashnode.com/res/hashnode/image/upload/v1693845931255/f643190a-25d0-4d26-9095-951ef5d13707.png align="left")

### Step 1: *Hey, Are You There?* - SYN

![](https://cdn.hashnode.com/res/hashnode/image/upload/v1693846412502/e698c308-d4ab-402b-b212-1fd2df78ce28.png align="left")

Imagine you want to talk to your friend using a walkie-talkie.

Before you start speaking, you press the button and say:

> **Hey, are you there?**

This is exactly what happens in the first step of TCP.

* The **client** sends a message to the **server**
    
* This message is called **SYN** (Synchronize)
    
* It means:  
    **I want to start a connection with you.**
    

Along with SYN, the client also sends a **sequence number**, usually starting from **SEQ = 0**.

You can think of this sequence number as:

* A starting page number of a notebook
    
* Or a reference point for future messages
    

**At this stage:**

* No real data is sent
    
* Only a connection request is made
    

### Step 2: *Yes, I’m Here!* - SYN + ACK

![](https://cdn.hashnode.com/res/hashnode/image/upload/v1693846450173/51c105df-4082-4d9c-851f-98e0f3a044d4.png align="left")

Now imagine your friend hears your message and replies:

> **Yes, I’m here. I heard you. Can you hear me too?**

This is the second step of the TCP handshake.

* The **server** replies to the client
    
* It sends a message with **SYN + ACK**
    

This means two things:

* **ACK**: I received your SYN
    
* **SYN**: I also want to start communication with you
    

The server also sends:

* Its own **sequence number** (for example, SEQ = 0)
    
* An acknowledgment number **ACK = 1**, meaning:
    
    > I received your sequence number 0, now I expect 1 next
    

**Now both sides know:**

* The other system exists
    
* The connection path works
    
* Sequence numbers are exchanged
    

### Step 3: *Yes, Let’s Start Talking* - ACK

![](https://cdn.hashnode.com/res/hashnode/image/upload/v1693846480334/3a7441c0-15ce-4e79-a988-f6c8060a11ac.png align="left")

Finally, you reply to your friend:

> **Yes, I can hear you too. Let’s talk.**

This is the last step of the TCP handshake.

* The **client** sends an **ACK** message to the server
    
* It confirms:
    
    > I received your SYN and your sequence number
    

The client now sends:

* **ACK = 1**
    
* **SEQ = 1**
    

**At this point:**

* Both client and server fully trust the connection
    
* Both know the starting sequence numbers
    
* The connection is officially established
    

### TCP 3-Way Handshake Completed

After these three steps:

1. SYN
    
2. SYN-ACK
    
3. ACK
    

The TCP connection is **successfully established**, and now:

* Actual data transfer can begin
    
* Data can be sent reliably
    
* Packets can be tracked using sequence numbers
    

### Why This Handshake Is Important

The TCP 3-Way Handshake ensures that:

* Both computers are ready
    
* Data won’t be sent into the void
    
* Sequence numbers are synchronized
    
* Communication starts in a controlled and reliable way
    

Without this handshake:

* Data could be lost
    
* Messages could arrive out of order
    
* Communication would be unreliable
    

## How Data Transfer Works in TCP

Once the connection is established:

* Data is broken into **small packets**
    
* Each packet has:
    
    * Sequence number
        
    * Data
        
* Receiver sends **ACK** for received packets
    

Example:

```plaintext
Packet 1 → ACK 1
Packet 2 → ACK 2
Packet 3 → ACK 3
```

If ACK is missing → sender knows something went wrong.

## What Are Sequence Numbers ?

Sequence numbers help TCP:

* Keep data in correct order
    
* Detect missing packets
    

Example:

```plaintext
Sent packets: 1, 2, 3, 4
Received: 1, 2, 4
Missing: 3
```

TCP notices packet 3 is missing and asks for it again.

## How TCP Ensures Reliability

TCP is reliable because it uses:

### 1\. Acknowledgements (ACKs)

Receiver confirms each packet.

### 2\. Retransmission

If ACK is not received, packet is sent again.

### 3\. Ordered Delivery

Sequence numbers ensure correct order.

### 4\. Error Checking

Corrupted packets are discarded and resent.

## Handling Packet Loss

Let’s say:

* Packet 5 is lost
    
* Receiver keeps sending ACK for packet 4
    

Sender understands:

> Packet 5 did not reach

Sender resends packet 5.

This is called **<mark>retransmission</mark>**<mark>.</mark>

## As a web developer, TCP matters because:

* HTTP runs on top of TCP
    
* Login systems depend on TCP reliability
    
* APIs require ordered, correct data
    
* Databases depend on TCP connections
    

Without TCP:

* Forms would submit incorrectly
    
* Pages would load half-broken
    
* APIs would fail randomly
    

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