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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 Immediate Data Transfer: "Push" Function 1 2 TCP Reliability and Flow Control Features and Protocol Modifications

TCP treats data to be transmitted as just an unstructured stream of bytes, and this has some important implications on how it used. One aspect of this characteristic is that since TCP doesn't understand the content of the data it sends, it normally treats all the data bytes in a stream as equals. The data is sent to TCP in a particular sequence, and is transmitted in that same order. This makes TCP, in this regard, like those annoying voice mail systems that tell you not to hang up because they will answer calls in the order received.

Of course, while waiting on hold is irritating, this first in, first out behavior is usually how we want TCP to operate. If we are transmitting a message or a file we want to be able to give TCP the bytes that comprise the file to be sent and have TCP transmit that data in the order we gave it. However, just as special circumstances can require the “push” function we saw in the previous topic, there are cases where we may not want to always send all data over in the exact sequence it was given to TCP.

The most common example of this is when it is necessary to interrupt an application's data transfer. Suppose we have an application that sends large files in both directions between two devices. The user of the application realizes that the wrong file is being transferred. When he or she tells the application to stop the file being sent, we want this to be communicated to the other end of the TCP connection immediately. We don't want the “abort” command to just be placed at the end of the line after the file we are trying to send!

TCP Immediate Data Transfer: "Push" Function 1 2 TCP Reliability and Flow Control Features and Protocol Modifications