TCP Working: 3-Way Handshake & Reliable Communication
How computers talk reliably, step by step
I build clean and simple web experiences and learn something new every day.
Whenever we open a website, send a message, or download a file, data is constantly moving from one computer to another.
But have you ever wondered how this data reaches the correct destination safely and in the right order?
That’s where TCP (Transmission Control Protocol) comes into the picture.
In this blog, I’ll explain how TCP works, why it is important, and how the famous 3-way handshake helps in establishing a reliable connection in a simple and relatable way.
What is TCP and why do we need it?
TCP is a connection-oriented protocol that makes sure data is delivered reliably between two devices.
The internet is not perfect:
Data packets can get lost
They can arrive in the wrong order
Some packets might even get duplicated
TCP handles all of this for us so that applications don’t have to worry about it.
That’s why TCP is used in:
Web browsing (HTTP / HTTPS)
Emails
File transfers
Online payments
Basically, anywhere accuracy matters, TCP is used.
The problem TCP solves
Imagine you are sending a long handwritten letter page by page:
Some pages might get lost
Some might arrive late
Some might reach in the wrong order
Now imagine numbering each page and asking the receiver to confirm which pages they received.
If any page is missing, you resend it.
That is exactly how TCP works, but with data packets.
Before sending data, a connection must be created
TCP does not start sending data immediately.
First, it makes sure that both the sender and receiver are ready to communicate.
This process is called the TCP 3-Way Handshake.
TCP 3-Way Handshake (step by step)
The 3-way handshake is simply a conversation between client and server to start a connection.
Step 1: SYN (Client → Server)
The client sends a message saying:
Hey, I want to connect with you.
This message is called SYN (Synchronize) and it also includes an initial sequence number.
At this point, the client is just checking if the server is available.
Step 2: SYN-ACK (Server → Client)
If the server is ready, it replies with:
SYN: “I’m ready too”
ACK: “I received your request”
This tells the client:
Yes, I’m here and I acknowledge your request.
Step 3: ACK (Client → Server)
Finally, the client sends an ACK back to the server confirming:
I received your response. Let’s start communicating.
After this step, the connection is successfully established.
Now both sides trust each other and are ready to exchange data.
Why is it called a 3-way handshake?
Because three messages are exchanged to make sure:
The client is reachable
The server is reachable
Both are synchronized
This avoids confusion and prevents half-open or fake connections.
How data is transferred using TCP
Once the connection is established:
Data is divided into small packets
Each packet has a sequence number
The receiver sends ACKs for received packets
Lost packets are automatically resent
So even if:
A packet is lost
Packets arrive out of order
TCP fixes everything behind the scenes.
Sequence numbers and acknowledgments (in simple words)
Sequence numbers help TCP arrange packets in the correct order
ACKs (Acknowledgements) tell the sender which data has been received
If the sender doesn’t receive an ACK:
It assumes the packet is lost
It resends the packet
This is how TCP guarantees reliable delivery.
What if a packet is lost?
No panic
TCP handles it automatically.
The receiver notices a missing sequence number
The sender doesn’t get an ACK
The sender resends the missing packet
The user never even notices that something went wrong.
How does TCP close a connection?
Just like starting a connection, TCP also ends it properly.
One side sends a FIN message saying “I’m done”
The other side sends ACK
Then it sends its own FIN
Final ACK is sent and the connection is closed
This ensures that no data is left hanging.
Why TCP is reliable but slightly slower
TCP takes extra steps:
Acknowledgements
Retransmissions
Flow control
Because of this, TCP is slower than UDP.
But in return, it gives accuracy and reliability, which is more important for most applications.
Final thoughts
TCP might look complex at first, but its goal is very simple:
Make sure data reaches the right place, in the right order, without loss.
The 3-way handshake builds trust, sequence numbers maintain order, and acknowledgements ensure delivery.
Once you understand TCP, many internet concepts start making a lot more sense.