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TCP Priority Data Transfer: "Urgent" Function
(Page 2 of 2)
Prioritizing Data For Transfer
TCP provides a means for a process
to prioritize the sending of data in the form of its urgent
feature. To use it, the process that needs to send urgent data enables
the function and sends the urgent data to its TCP layer. TCP then creates
a special TCP segment that has the URG control bit set to 1.
It also sets the Urgent Pointer field to an offset value that
points to the last byte of urgent data in the segment. So, for example,
if the segment contained 400 bytes of urgent data followed by 200 bytes
of regular data, the URG bit would be set and the Urgent Pointer
field would have a value of 400.
Upon receipt of a segment with the
URG flag set to 1, the receiving device looks at the Urgent
Pointer and from its value determines which data in the segment
is urgent. It then forwards the urgent data to the process with an indication
that the data is marked as urgent by the sender. The rest of the data
in the segment is processed normally.
Since we typically want to send urgent
data, well, urgently, it makes sense that when such data
is given to TCP, the push function is usually also invoked.
This ensures that the urgent data is sent as soon as possible by the
transmitting TCP and also forwarded up the protocol stack right away
by the receiving TCP. Again, we need to remember that this does not
guarantee the contents of the urgent segment. Using the push
function may mean the segment contains only urgent data
with no non-urgent data following, but again, an application cannot
assume that this will always be the case.
Key Concept: To deal with situations where a certain part of a data stream needs to be sent with a higher priority than the rest, TCP incorporates an urgent function. When critical data needs to be sent, the application signals this to its TCP layer, which transmits it with the URG bit set in the TCP segment, bypassing any lower-priority data that may have already been queued for transmission.
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Version 3.0 - Version Date: September 20, 2005
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