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TCP Immediate Data Transfer: "Push" Function
(Page 2 of 2)
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.
 Key Concept: TCP includes a special “push” function to handle cases where data given to TCP needs to be sent immediately. An application can send data to its TCP software and indicate that it should be pushed. The segment will be sent right away rather than being buffered. The pushed segment’s PSH control bit will be set to one to tell the receiving TCP that it should immediately pass the data up to the receiving application.
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