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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 Connection Preparation: Transmission Control Blocks (TCBs) and Passive and Active Socket OPENs 1 2 3 4 TCP Connection Establishment Sequence Number Synchronization and Parameter Exchange

We have discussed in earlier topics in this section the connection orientation of TCP and its operation. Before TCP can be employed for any actually useful purpose—that is, sending data—a connection must be set up between the two devices that wish to communicate. This process, usually called connection establishment, involves an exchange of messages that transitions both devices from their initial connection state (CLOSED) to the normal operating state (ESTABLISHED).

The connection establishment process actually accomplishes several things as it creates a connection suitable for data exchange:

• Contact and Communication: The client and server make contact with each other and establish communication by sending each other messages. The server usually doesn’t even know what client it will be talking to before this point, so it discovers this during connection establishment.
  
  
• Sequence Number Synchronization: Each device lets the other know what initial sequence number it wants to use for its first transmission.
  
  
• Parameter Exchange: Certain parameters that control the operation of the TCP connection are exchanged by the two devices.

TCP Connection Preparation: Transmission Control Blocks (TCBs) and Passive and Active Socket OPENs 1 2 3 4 TCP Connection Establishment Sequence Number Synchronization and Parameter Exchange