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I hope this question isn't out of place here, but can anyone explain to me what the 'TcpNumConnections' registry key does? The description has me confused; I'm not sure what Microsoft means by 'connection' in 'Specifies the maximum number of connections that TCP can have open simultaneously.'

Does this key define the total number of active TCP connections my machine can have open at a time, i.e. any sockets in any non-closed state?

Or does this key define the maximum number of parallel connection attempts, i.e. sockets that are in a LISTEN/SYN_RCVD/SYN_SENT state?

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My understanding of this - established connections only. –  Vick Vega Apr 29 '11 at 20:27

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Not having the source code to Windows it's a little difficult to tell for sure, but my reading of the documentation on Microsoft's site seems to say that this applies only to open TCP connections. To your particular question about SYN_RCVD state connections I'd point you to the TcpMaxHalfOpen parameter. The existence of TcpMaxHalfOpen further makes me think that TcpNumConnections applies only to open connections.

In the end, only the source code is going to tell you for sure. (That, or more in depth documentation.)

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