We have now seen where TCP comes from and the standards that describe it. As I said in the introduction to this section, TCP is a complicated protocol, so it will take some time to explain how it works. The most logical first “bite” in consuming this particular “elephant” is to look at exactly what TCP does. From there, we can describe its characteristics and then get into the details of its operation.

Despite the complexity of TCP, its basic operation can be reasonably simplified by describing its primary functions. The following are what I believe to be the five main tasks that TCP performs:

• Addressing/Multiplexing: TCP is used by many different applications for their transport protocol. Therefore, like its simpler sibling UDP, an important job for TCP is multiplexing the data received from these different processes so they can be sent out using the underlying network-layer protocol. At the same time, these higher-layer application processes are identified using TCP ports. The section on TCP/IP transport layer addressing contains a great deal of detail on how this addressing works.
  
  
• Connection Establishment, Management and Termination: TCP provides a set of procedures that devices follow to negotiate and establish a TCP connection over which data can travel. Once opened, TCP includes logic for managing connections and handling problems that may result with them. When a device is done with a TCP connection, a special process is followed to terminate it.
  
  
• Data Handling and Packaging: TCP defines a mechanism by which applications are able to send data to it from higher layers. This data is then packaged into messages to be sent to the destination TCP software. The destination software unpackages the data and gives it to the application on the destination machine.
  
  
• Data Transfer: Conceptually, the TCP implementation on a transmitting device is responsible for the transfer of packaged data to the TCP process on the other device. Following the principle of layering, this is done by having the TCP software on the sending machine pass the data packets to the underlying network-layer protocol, which again normally means IP.
  
  
• Providing Reliability and Transmission Quality Services: TCP includes a set of services and features that allow an application to consider the sending of data using the protocol to be “reliable”. This means that normally, a TCP application doesn't have to worry about data being sent and never showing up, or arriving in the wrong order. It also means other common problems that might arise if IP were used directly are avoided.
  
  
• Providing Flow Control and Congestion Avoidance Features: TCP allows the flow of data between two devices to be controlled and managed. It also includes features to deal with congestion that may be experienced during communication between devices.

Clearly, TCP is responsible for a fairly significant number of key functions. This list may not seem that impressive. The reason is that this is just a high-level look at the protocol, and these functions are summarized in the list above; when we look at them in detail we will see that each one actually involves a rather significant amount of work for TCP to do.