What is TCP and Why It Is Needed

TCP (Transmission Control Protocol) is a protocol that establishes a connection before sending data.

It makes sure that:

• Data is delivered correctly

• Data arrives in the right order

• Lost data is detected and resent

This is why TCP is used for things like:

• Web browsing

• Emails

• File transfers

In these cases, accuracy matters more than speed.
A broken web page or a corrupted email is unacceptable.

What Happens If Data Is Sent Without Rules

Imagine two people talking without waiting for responses.

One speaks fast, the other misses words, sentences overlap, and parts of the message are lost.
That’s what network communication would look like without TCP.

UDP works like this — fast, but unreliable.
TCP exists to bring discipline and structure to communication.

TCP vs UDP

UDP sends data without checking anything.
It doesn’t establish a connection and doesn’t care if data is lost.

That’s fine for:

• Video calls

• Live matches

• Online gaming

But TCP is used when every bit of data matters.

How TCP Establishes a Connection: The 3-Way Handshake

Before sending actual data, TCP first checks:
“Are you ready to talk?”
“Yes, I am.”
“Okay, let’s start.”

This process is called the 3-Way Handshake.

Think of it like a phone call.

Step-by-Step: SYN, SYN-ACK, ACK

Step 1: SYN (Client → Server)

The client sends a message saying:

“I want to talk.”

This message is called SYN (synchronize).

Step 2: SYN + ACK (Server → Client)

The server replies:

“I received your request, and I’m ready.”

This response is SYN + ACK.

Step 3: ACK (Client → Server)

The client responds:

“I received your response.”

This final message is ACK.

Now both sides agree:

• The connection is established

• Data transfer can begin

Only after this does real data start flowing.

How Data Transfer Works in TCP

Once the connection is open, TCP sends data in small chunks called segments.

In this way the data is transfered:

• Data arrives in order

• Missing data is detected

• Duplicate data is ignored

How TCP Ensures Reliability

TCP constantly checks:

• Was the data received?

• Was it received in the correct order?

• Did anything get lost?

If something is missing:

• TCP resends the data

• Slows down if the network is congested

• Adjusts speed automatically

This is why TCP is reliable but slower than UDP.

How a TCP Connection Is Closed

Ending a TCP connection is more careful than starting one.
TCP does not suddenly cut the connection, because there might be transporting data.

TCP uses a 4-way handshake to close a connection safely.

Step 1: FIN (Client → Server)

When one side finishes sending data, it sends a FIN message.

This means:

“I’m done sending data to you.”

At this point:

• The client will not send more data

• But it can still receive data from the server

Step 2: ACK (Server → Client)

The server replies with an ACK.

This means:

“I received your FIN. I know you are done sending.”

Now:

• The client waits

• The server may still have data left to send

Step 3: FIN (Server → Client)

When the server also finishes sending its remaining data, it sends its own FIN.

This means:

“I’m also done sending data.”

Now both sides have said they are finished sending.

Step 4: ACK (Client → Server)

Finally, the client sends an ACK back to the server.

This means:

“I received your FIN. We are done.”

After this:

• The connection is fully closed

• All resources are released

• No data is lost

Why This Matters for Developers

Whenever you Open a website:

TCP works as:

• Establishing connections

• Managing data flow

• Handling packet loss