In order for a connection to become established, a handshake of SYNC messages and acknowledgments of the SYNC message are sent between two hosts. The connection is considered established (i.e. ready to send and receive data) when both ends have sent a SYNC message and have acknowledged the remote host's SYNC message.

A connection may be established in one of two ways:

For passive connections, a socket is allocated by calling function socket. Its local address is set up by calling bind, then listen is called. The function listen sets the socket to be a master socket/listener socket. The opening handshaking is started when the remote host sends a Sync message. When a Sync message is received for that socket, a new socket (slave socket) is allocated to process the connection. A SYNC message and an acknowledgment of the SYNC received will be sent to the remote host. The application needs to call accept after listen. The function accept will return the slave socket when the connection is established.
For active connections, a socket is allocated by calling socket. Then, connect is called. Calling bind is optional. The function connect will initiate the handshaking by sending a SYNC message to the remote host. In blocking mode, connect will return when the connection has become established.

Note that if the socket is in non-blocking mode, connect will return immediately after sending the SYNC message and select may be called to block until the connection is established.

Left TCP

Down Closing a TCP Connection