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copied!<p>I assume that you are actually correctly receiving the urgent data in the application that's failing and that the data is as you expect it to be?</p> <p>There are many reasons for this to fail, especially if you're attempting it in a cross platform situation: In TCP there are two conflicting descriptions of how urgent data works, <a href="http://tools.ietf.org/html/rfc0793#page-15" rel="nofollow">RFC 793</a> which details TCP says that the Urgent Pointer indicates the byte that follows the urgent data but <a href="http://tools.ietf.org/html/rfc1122#page-84" rel="nofollow">RFC 1122</a> corrects this and states that the Urgent Pointer indicates the final byte of urgent data. This leads to interoperability issues if one peer uses the RFC 793 definition and the other uses the RFC 1122 definition.</p> <p>So, first confirm that your application is actually getting the correct byte of urgent data. Yes, I said byte, there's more compatibility complexity in that Windows only supports a single byte of out of band data whereas RFC 1122 specifies that TCP MUST support sequences of urgent data bytes of any length. Windows also doesn't specify how or if it will buffer subsequent out of band data, so if you are slow in reading a byte of urgent data and another byte of urgent data arrives then one of the bytes may be lost; though our tests have shown that Windows does buffer urgent data. This all makes the use of out of band signalling using urgent data somewhat unreliable on Windows with TCP.</p> <p>And then there are all the other issues that come about if you happen to be using overlapped I/O.</p> <p>I've covered this in a little more depth, albeit from a C++ perspective, here: <a href="http://www.serverframework.com/asynchronousevents/2011/10/out-of-band-data-and-overlapped-io.html" rel="nofollow">http://www.serverframework.com/asynchronousevents/2011/10/out-of-band-data-and-overlapped-io.html</a></p>

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