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copied!<p>If this happens always, it literally means that the machine exists but that it has no services listening on the specified port, or there is a firewall stopping you.</p> <p>If it happens occasionally - you used the word "sometimes" - and retrying succeeds, it is likely because the server has a full 'backlog'.</p> <p>When you are waiting to be <code>accept</code>ed on a listening socket, you are placed in a backlog. This backlog is finite and quite short - values of 1, 2 or 3 are not unusual - and so the OS might be unable to queue your request for the 'accept' to consume.</p> <p>The backlog is a parameter on the <code>listen</code> function - all languages and platforms have basically the same API in this regard, even the <a href="http://msdn.microsoft.com/en-us/library/system.net.sockets.socket.listen.aspx" rel="noreferrer">C# one</a>. This parameter is often configurable if you control the server, and is likely read from some settings file or the registry. Investigate how to configure your server.</p> <p>If you wrote the server, you might have heavy processing in the accept of your socket, and this can be better moved to a separate worker-thread so your accept is always ready to receive connections. There are various architecture choices you can explore that mitigate queuing up clients and processing them sequentially.</p> <p><em>Regardless</em> of whether you can increase the server backlog, you do need <strong><em>retry</em></strong> logic in your client code to cope with this issue - as even with a long backlog the server might be receiving lots of other requests on that port at that time.</p> <p>There is a rare possibility where a NAT router would give this error should it's ports for mappings be exhausted. I think we can discard this possibility as too much of a long shot though, since the router has 64K simultaneous connections to the same destination address/port before exhaustion.</p>

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