TCP Working: 3-Way Handshake & Reliable Communication
What is TCP and why it is needed?
Imagine you’re sending a long handwritten letter through the post:
You want all pages.
In the right order.
With nothing missing or duplicated.
but the internet itself is unreliable:
Data can get lost.
Data can arrive out of order.
Data can get duplicated.
Data can get corrupted.
To solve all these TCP(Transmission Control Protocol) is required.
Think of it like a rulebook computers follow to send data safely and correctly over the internet.
Problems TCP is designed to solve
Data loss → Some packets just disappear mid-way.
Wrong order → Packet 5 arrives before packet 2.
Duplicate packets → Same data arrives twice.
Overloading the receiver → Sender sends too fast → receiver crashes.
No confirmation → Sender has no idea if data arrived or not.
What is the TCP 3-Way Handshake
Before sending actual data, TCP first does a connection setup.
Think of it like a phone call :
Can you hear me?
Yes, I can hear you.
Okay, let’s talk.
That’s the 3-Way Handshake.
SYN → SYN-ACK → ACK
Step 1: SYN (Client → Server)
Client says:
“Hey server, I want to talk. Are you there?”
SYN = Synchronize
Client also sends a starting number (sequence number)
Step 2: SYN-ACK (Server → Client)
Server replies:
“Yes, I’m here. I heard you. Are you ready?”
SYN: Server sends its own starting number
ACK: Server confirms client’s number
Step 3: ACK (Client → Server)
Client replies:
“Yes, I’m ready too.”
Connection is now officially open
Both sides agree on starting points
Now data transfer can begin.
How data transfer works in TCP
Once the connection is open, data is split into small pieces called segments (not sent as one big chunk).
Each segment has a number
This number tells:
Where it belongs
What comes before & after
Receiver sends ACKs, After receiving data, receiver says:
“Got segment #5 ”
Missing data is resent, If segment #6 doesn’t arrive:
Sender resends only #6
Not everything again
This is smart and efficient.
How TCP ensures reliability, order, and correctness
Reliability
Every segment must be acknowledged
If no ACK → resend
Correct order
Sequence numbers keep data in order
Receiver rearranges if needed
No duplication
- Duplicate numbers → ignored
Flow control
- Receiver tells sender:
“Slow down, I’m busy”
Congestion control
TCP detects network traffic
Automatically adjusts speed
How a TCP connection is closed
Think:
“I’m done talking.”
“Okay, me too.”
“Bye.”
“Bye.”
Client sends FIN
- “I’m done sending data”
Server sends ACK
- “Got it”
Server sends FIN
- “I’m also done”
Client sends ACK
- “Confirmed”
Connection closed cleanly.
Conclusion
TCP makes sure data between a client and server is delivered completely, in the correct order, and without guessing. From connection setup to safe data transfer and clean shutdown, TCP handles all the mess so applications can work reliably.
