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TCP's urgent data mechanism provides an application-to-application
notification mechanism, which can be used by an application to mark
parts of the data stream as being more urgent than the normal
stream. It is up to the receiving application to interpret the meaning
of the urgent data.
In many TCP implementations, including the BSD implementation, the
urgent data feature increases the complexity of the implementation
because it requires an asynchronous notification mechanism in an
otherwise synchronous API. As our implementations already use an
asynchronous event based API, the implementation of the urgent data
feature does not lead to increased complexity.
Adam Dunkels
2003-03-03