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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 Message Formatting and Data Transfer

TCP Sliding Window Data Transfer and Acknowledgement Mechanics 1 2 TCP Priority Data Transfer: "Urgent" Function

The fact that TCP takes incoming data from a process as an unstructured stream of bytes gives it great flexibility in meeting the needs of most applications. There is no need for an application to create blocks or messages; it just sends the data to TCP when it is ready for transmission. For its part, TCP has no knowledge or interest in the meaning of the bytes of data in this stream. They are “just bytes” and TCP just sends them without any real concern for their structure or purpose.

This has a couple of interesting impacts on how applications work. One is that TCP does not provide any natural indication of the dividing point between pieces of data, such as database records or files. The application must take care of this. Another result of TCP's byte orientation is that TCP cannot decide when to form a segment and send bytes between devices based on the contents of the data. TCP will generally accumulate data sent to it by an application process in a buffer. It chooses when and how to send data based solely on the sliding window system discussed in the previous topic, in combination with logic that helps to ensure efficient operation of the protocol.

This means that while an application can control the rate and timing with which it sends data to TCP, it cannot inherently control the timing with which TCP itself sends the data over the internetwork. Now, if we are sending a large file, for example, this isn't a big problem. As long as we keep sending data, TCP will keep forwarding it over the internetwork. It's generally fine in such a case to let TCP fill its internal transmit buffer with data and form a segment to be sent when TCP feels it is appropriate.

However, there are situations where letting TCP accumulate data before transmitting it can cause serious application problems. The classic example of this is an interactive application such as the Telnet Protocol. When you are using such a program, you want each keystroke to be sent immediately to the other application; you don't want TCP to accumulate hundreds of keystrokes and then send them all at once. The latter may be more “efficient” but it makes the application unusable, which is really putting the cart before the horse.

Even with a more mundane protocol that transfers files, there are many situations in which we need to say “send the data now”. For example, many protocols begin with a client sending a request to a server—like the hypothetical one in the example in the preceding topic, or a request for a Web page sent by a Web browser. In that circumstance, we want the client's request sent immediately; we don't want to wait until enough requests have been accumulated by TCP to fill an “optimal-sized” segment.

TCP Sliding Window Data Transfer and Acknowledgement Mechanics 1 2 TCP Priority Data Transfer: "Urgent" Function