TCP 3 Way Handshake

To establish a connection, TCP uses a three-way handshake. Before a client attempts to connect with a server, the server must first bind to and listen at a port to open it up for connections: this is called a passive open. Once the passive open is established, a client may initiate an active open. To establish a connection, the three-way (or 3-step) handshake occurs:

1.          SYN: The active open is performed by the client sending a SYN to the server. The client      
                  sets the segment's sequence number to a random value A.

2.          SYN-ACK: In response, the server replies with a SYN-ACK. The acknowledgment number 
                  is set to one more than the received sequence number i.e. A+1, and the sequence 
                  number that the server chooses for the packet is another random number, B.

3.            ACK: Finally, the client sends an ACK back to the server. The sequence number is set to 
                  the received acknowledgement value i.e. A+1, and the acknowledgement number is 
                  set to one more than the received sequence number i.e. B+1.

At this point, both the client and server have received an acknowledgment of the connection. The steps 1, 2 establish the connection parameter (sequence number) for one direction and it is acknowledged. The steps 2, 3 establish the connection parameter (sequence number) for the other direction and it is acknowledged. With these, a full-duplex communication is established.