How TCP connection is established and terminated.

1. The requesting end (normally called the client) sends a SYN segment specifying the port number of the server that the client wants to connect to, and the client's initial sequence number (ISN, 1415531521 in this example). This is segment 1.
2. The server responds with its own SYN segment containing the server's initial sequence number (segment 2). The server also acknowledges the client's SYN by ACKing the client's ISN plus one. A SYN consumes one sequence number.
3. The client must acknowledge this SYN from the server by ACKing the server's ISN plus one (segment 3).

These three segments complete the connection establishment. This is often called the three-way handshake.


Figure 18.3 Time line of connection establishment and connection termination. The side that sends the first SYN is said to perform an active open. The other side, which receives this SYN and sends the next SYN, performs a passive open

Reference: http://www.pcvr.nl/tcpip/tcp_conn.htm