TCP handshake

TCP (Transmission Control Protocol) uses a three-way handshake (aka TCP-handshake, three message handshake, and/or SYN-SYN-ACK) to set up a TCP/IP connection over an IP based network. Similarly, a four-way handshake is used to terminate the connection.

The three messages transmitted by TCP to negotiate and start a TCP session are nicknamed SYN, SYN-ACK, and ACK for SYNchronize, SYNchronize-ACKnowledgement, and ACKnowledge respectively. The three message mechanism is designed so that two computers that want to pass information back and forth to each other can negotiate the parameters of the connection before transmitting data such as HTTP browser requests.