What is one purpose of the TCP three-way-handshake?
The three-way handshake in Transmission Control Protocol (also
called the three message handshake) is the method used to establish
and tear down network connections. This handshaking technique is
referred to as the 3-way handshake or as "SYN-SYN-ACK" (or more
accurately SYN, SYN-ACK, ACK). The TCP handshaking mechanism is
designed so that two computers attempting to communicate can
negotiate the parameters of the network connection before beginning
communication. This process is also designed so that both ends can
initiate and negotiate separate connections at the same time.