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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)

TCP Common Applications and Server Port Assignments 1 TCP Operational Overview and the TCP Finite State Machine (FSM)

While I have described the Transmission Control Protocol as connection-oriented, this term isn't "just any old characteristic" of TCP. The overall operation of the entire protocol can be described in terms of how TCP software prepares, negotiates, establishes, manages and terminates connections. TCP implementations certainly do more than handle connections, but the other major tasks they perform, such as data handling and providing reliability and flow control, can only occur over a stable connection. This makes connections the logical place to begin in exploring the details of how TCP does its thing.

In this section I describe TCP connections from start to finish. I begin with an overview of TCP's operation by providing a summary of the finite state machine that formally defines the stages of a connection. State machines can be a bit mind-boggling when you read about them in standards, but a simplified version provides an excellent high-level view of the "life" of a connection, so it is a good place to start.

From there, I move on to provide details about TCP's handling of connections. I described how connections are prepared and transmission control blocks (TCBs) set up, and the difference between a passive and an active socket open. I explain the three-way handshake used to create a connection, and the method by which parameters are exchanged and sequence numbers synchronized. I talk about how an established connection is managed, including the method by which TCP handles problem conditions and resets the connection when necessary. Finally, I describe how a connection can be terminated when it is no longer needed.

TCP Common Applications and Server Port Assignments 1 TCP Operational Overview and the TCP Finite State Machine (FSM)