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The TCP/IP Guide  9  TCP/IP Lower-Layer (Interface, Internet and Transport) Protocols (OSI Layers 2, 3 and 4)       9  TCP/IP Transport Layer Protocols            9  Transmission Control Protocol (TCP) and User Datagram Protocol (UDP)                 9  TCP/IP Transmission Control Protocol (TCP)                      9  TCP Basic Operation: Connection Establishment, Management and Termination

TCP Operational Overview and the TCP Finite State Machine (FSM) 1 2 TCP Connection Establishment Process: The "Three-Way Handshake"

Active and Passive OPENs

TCP/IP is based on the client/server model of operation, and TCP connection setup is based on the existence of these roles as well. The client and server each prepare for the connection by performing an OPEN operation. However, there are two different kinds of OPEN:

• Active OPEN: A client process using TCP takes the “active role” and initiates the connection by actually sending a TCP message to start the connection (a SYN message).
  
  
• Passive OPEN: A server process designed to use TCP, however, takes a more “laid-back” approach. It performs a passive OPEN by contacting TCP and saying “I am here, and I am waiting for clients that may wish to talk to me to send me a message on the following port number”. The OPEN is called passive because aside from indicating that the process is listening, the server process does nothing.

A passive OPEN can in fact specify that the server is waiting for an active OPEN from a specific client, though not all TCP/IP APIs support this capability. More commonly, a server process is willing to accept connections from all comers. Such a passive OPEN is said to be unspecified.

Both the client and the server create the TCB for the connection at the time that they perform the OPEN. The client already knows the IP addresses and port numbers for both the client process and the server process it is trying to reach, so it can use these to uniquely identify the connection and the TCB that goes with it.

For the server, the concept of a TCB at this stage of the game is a bit more complex. If the server is in fact waiting for a particular client, it can identify the connection using its own socket and the socket of the client for which it is waiting. Normally, however, the server doesn't know what client is trying to reach it. In fact, it could be contacted by more than one client nearly at the same time.

In this case, the server creates a TCB with an unspecified (zero) client socket number, and waits for an active OPEN to be received. It then binds the socket number of the client to the TCB for the passive OPEN as part of the connection process. To allow it to handle multiple incoming connections, the server process may in fact perform several unspecified passive OPENs simultaneously.

The transmission control block for a connection is maintained throughout the connection and destroyed when the connection is completely terminated and the device returns to the CLOSED state. TCP does include a procedure to handle the situation where both devices perform an active OPEN simultaneously. This is discussed in more detail in the next topic on the connection establishment process.

TCP Operational Overview and the TCP Finite State Machine (FSM) 1 2 TCP Connection Establishment Process: The "Three-Way Handshake"