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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 Overview, Functions and Characteristics

TCP Functions: What TCP Does 1 2 TCP Fundamentals and General Operation

In the preceding topic we began our high-level look at TCP by examining the most important functions the protocol performs—as well as a few that it does not. In many ways, it is more interesting to look at how TCP does its job than the functions of the job itself. By examining the most important attributes of TCP and its operation, we can get a better handle on the way TCP works. We can also see the many ways that it contrasts to its simpler transport layer sibling, UDP.

The following are the ways that I would best describe the Transmission Control Protocol and how it performs the functions described in the preceding topic:

• Connection-Oriented: TCP requires that devices first establish a connection with each other before they send data. The connection creates the equivalent of a circuit between the units, and is analogous to a telephone call. A process of negotiation occurs to establish the connection, ensuring that both devices agree on how data is to be exchanged.
  
  
• Bidrectional: Once a connection is established, TCP devices send data bidirectionally. Both devices on the connection can send and receive, regardless of which of them initiated the connection.
  
  
• Multiply-Connected and Endpoint-Identified: TCP connections are identified by the pair of sockets used by the two devices in the connection. This allows each device to have multiple connections opened, either to the same IP device or different IP devices, and to handle each connection independently without conflicts.
  
  
• Reliable: Communication using TCP is said to be reliable because TCP keeps track of data that has been sent and received to ensure it all gets to its destination. As we saw in the previous topic, TCP can't really “guarantee” that data will always be received. However, it can guarantee that all data sent will be checked for reception, and checked for data integrity, and then retransmitted when needed. So, while IP uses “best effort” transmissions, you could say TCP tries harder, as the old rent-a-car commercial goes.
  
  
• Acknowledged: A key to providing reliability is that all transmissions in TCP are acknowledged (at the TCP layer—TCP cannot guarantee that all such transmissions are received by the remote application). The recipient must tell the sender “yes, I got that” for each piece of data transferred. This is in stark contrast to typical messaging protocols where the sender never knows what happened to its transmission. As we will see, this is fundamental to the operation of TCP as a whole.
  
  
• Stream-Oriented: Most lower-layer protocols are designed so that to use them, higher-layer protocols must send them data in blocks. IP is the best example of this; you send it a message to be formatted and it puts that message into a datagram. UDP is the same. In contrast, TCP allows applications to send it a continuous stream of data for transmission. Applications don't need to worry about making this into chunks for transmission; TCP does it.
  
  
• Data-Unstructured: An important consequence of TCP's stream orientation is that there are no natural divisions between data elements in the application's data stream. When multiple messages are sent over TCP, applications must provide a way of differentiating one message (data element, record, etc.) from the next.
  
  
• Data-Flow-Managed: TCP does more than just package data and send it as fast as possible. A TCP connection is managed to ensure that data flows evenly and smoothly, with means included to deal with problems that arise along the way.

TCP Functions: What TCP Does 1 2 TCP Fundamentals and General Operation