TCP – How three-way handshake works?

TCP Protocol is one of the two methods used at the Layer 4 (Segment) of the OSI

Model. TCP ensure that packet reach its destination which makes it a reliable way of

forwarding traffic in the network. It uses sequence number place at the TCP header to

assemble segments and return it to its original form. Before the forwarding of traffic

starts between a client and the server, three-way handshake must need to establish

first. After the three-way handshake is established, both hosts then can start to forward

traffic to each other. Therefore, we will discuss how the three-way handshake occurs in

the network.

From the image below, Host A want to access the web-server via HTTP (port 80) and

HTTP used TCP method to forward traffic.

So, the host will send a SYN Packet destine to the Webserver. Let's say the content of

the SYN Packet is "Hi! I'm host, and I want to talk to you".

Then, once the Webserver receive the packet and check, it will then acknowledge the

message by sending a SYN, ACK packet to the host. Let's say the content of the SYN,

ACK Packet is "Hello! I'm Webserver. Sure!"

Once host receive the packet, it will send ACK Packet destine to the Webserver saying

"That's great!"

After both hosts agreed to talk to each other, they will now start to forward and receive

traffic from each other. That's how TCP three-way handshake is establish in the network

before traffic is forwarded and received from both side.

Sample of a three-way handshake captured by Wireshark application: