Illustrated TCP/IP by Matthew G. Naugle Wiley Computer Publishing, John Wiley & Sons, Inc. ISBN: 0471196568   Pub Date: 11/01/98

Chapter 196
TCP Connection Establishment

Unlike its brother UDP, TCP provides for reliable connections. A TCP connection between two stations on a network must be established before any data is allowed to pass between the two. Applications such as TELNET and FTP communicate using TCP through a series of function calls. This may seem a little confusing now, but the functions are very simple. These calls include OPEN and CLOSE a connection, SEND and RECEIVE (information) to that connection, and STATUS to receive information for a connection.

When a connection to a remote station is needed, an application will request TCP to place an OPEN call. There are two types of OPEN call: passive and active. A passive OPEN is a call to allow connections to be accepted from a remote station. This usually occurs when an application starts on a network station (such as TELNET, FTP), and it will indicate to TCP that it is willing to accept connections from other stations on the network. TCP will note the application through its port assignment and will allow connections to come in. The number of connections allowed depends on the number of passive OPENs issued. This passive end of the TCP actions is known as the responder TCP. It will open up connection slots to accept any incoming connection request. This may be thought of as the server end of TCP. These passive OPEN calls do not wait for any particular station request.

An active OPEN is made when a connection attempt to a remote network station is needed. Referring to the slide, station A wishes to connect to station B. Station A issues an active OPEN call to station B. In order for the connection to be made, station B must already have issued a passive OPEN request to allow incoming connections to be established. In the connection attempt packet is the port number that station A wishes to use on station B. Station B’s operating system will spawn a separate process on its system to maintain that connection. This process will act as if it is running locally on that station. TCP will then await another incoming connection request. This process is similar to the way a multitasking operating system handles multiple applications.

TCP Connection Establishment