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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

Forcing Immediate Data Transfer

Naturally, the designers of TCP realized that a way was needed to handle these situations. When an application has data that it needs to have sent across the internetwork immediately, it sends the data to TCP, and then uses the TCP push function. This tells the sending TCP to immediately “push” all the data it has to the recipient's TCP as soon as it is able to do so, without waiting for more data.

When this function is invoked, TCP will create a segment (or segments) that contains all the data it has outstanding, and will transmit it with the PSH control bit set to 1. The destination device's TCP software, seeing this bit sent, will know that it should not just take the data in the segment it received and buffer it, but rather push it through directly to the application.

It's important to realize that the push function only forces immediate delivery of data. It does not change the fact that TCP provides no boundaries between data elements. It may seem that an application could send one record of data and then “push” it to the recipient; then send the second record and “push” that, and so on. However, the application cannot assume that because it sets the PSH bit for each piece of data it gives to TCP, that each piece of data will be in a single segment. It possible that the first “push” may contain data given to TCP earlier that wasn't yet transmitted, and it's also possible that two records “pushed” in this manner may end up in the same segment anyway.

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