The Three-way handshake begins with the initiator sending a TCP segment with the SYN control bit flag set.
TCP allows one side to establish a connection. The other side may either accept the connection or refuse it. If we consider this from application layer point of view, the side that is establishing the connection is the client and the side waiting for a connection is the server.
TCP identifies two types of OPEN calls:
Active Open. In an Active Open call a device (client process) using TCP takes the active role and initiates the connection by sending a TCP SYN message to start the connection.
Passive Open A passive OPEN can specify that the device (server process) is waiting for an active OPEN from a specific client. It does not generate any TCP message segment. The server processes listening for the clients are in Passive Open mode.
Step 1. Device A (Client) sends a TCP segment with SYN = 1, ACK = 0, ISN (Initial Sequence Number) = 2000.
An Initial Sequence Number (ISN) is a random Sequence Number, allocated for the first packet in a new TCP connection.
The Active Open device (Device A) sends a segment with the SYN flag set to 1, ACK flag set to 0 and an Initial Sequence Number 2000 (For Example), which marks the beginning of the sequence numbers for data that device A will transmit. SYN is short for SYNchronize. SYN flag announces an attempt to open a connection.
Step 2. Device B (Server) receives Device A’s TCP segment and returns a TCP segment with SYN = 1, ACK = 1, ISN = 5000 (Device B’s Initial Sequence Number), Acknowledgment Number = 2001 (2000 + 1, the next sequence number Device B expecting from Device A).
Step 3. Device A sends a TCP segment to Device B that acknowledges receipt of Device B’s ISN, With flags set as SYN = 0, ACK = 1, Sequence number = 2001, Acknowledgment number = 5001 (5000 + 1, the next sequence number Device A expecting from Device B)
This handshaking technique is referred to as TCP Three-way handshake or SYN, SYN-ACK, ACK.
After the Three-way handshake, the connection is open and the participant computers start sending data using the agreed sequence and acknowledge numbers.