The TCP three-way handshake in Transmission Control Protocol (also called the TCP-handshake; three message handshake and/or SYN-SYN-ACK) is the method used by TCP set up a TCP/IP connection over an Internet Protocol based network. TCP’s three way handshaking technique is often referred to as “SYN-SYN-ACK” (or more accurately SYN, SYN-ACK, ACK) because there are three messages transmitted by TCP to negotiate and start a TCP session between two computers. The TCP handshaking mechanism is designed so that two computers attempting to communicate can negotiate the parameters of the network TCP socket connection before transmitting data such as SSH and HTTP web browser requests.
This 3-way handshake process is also designed so that both ends can initiate and negotiate separate TCP socket connections at the same time. Being able to negotiate multiple TCP socketconnections in both directions at the same time allows a single physical network interface, such asethernet, to be multiplexed to transfer multiple streams of TCP data simultaneously.
SYNchronize and ACKnowledge messages are indicated by a either the SYN bit, or the ACK bit inside the TCP header, and the SYN-ACK message has both the SYN and the ACK bits turned on (set to 1) in the TCP header.
TCP knows whether the network TCP socket connection is opening, synchronizing, established by using the SYNchronize and ACKnowledge messages when establishing a network TCP socket connection.
When the communication between two computers ends, another 3-way communication is performed to tear down the TCP socket connection. This setup and teardown of a TCP socket connection is part of what qualifies TCP a reliable protocol. TCP also acknowledges that data is successfully received and guarantees the data is reassenbled in the correct order.
Note that UDP is connectionless. That means UDP doesn’t establish connections as TCP does, so UDP does not perform this 3-way handshake and for this reason, it is referred to as an unreliable protocol. That doesn’t mean UDP can’t transfer data, it just doesn’t negotiate how the conneciton will work, UDP just transmits and hopes for the best.