Starting from Windows Server 2008 and Vista SP2 (Service Pack 2), the incomplete simultaneous or concurrent half-open outgoing TCP connections limit is disabled by default. The new behavior allows as many as unlimited half-open TCP connections to be created or established at any time. Users or administrators who want to enforce and enable the concurrent half-open outbound TCP connection attempts limit in Windows Server 2008 SP2, Windows Vista SP2, Windows Server 2008 R2 and Windows 7 must manually configure and set EnableConnectionRateLimiting registry key to impose the half-open TCP connections limit.

Previously in Windows Server 2008 and in Windows Vista with Service Pack 1 (SP1), the system allows for a maximum of ten half-open TCP connections at any time. When the limit is enabled and turned on in Windows Vista SP2, Windows Server 2008 SP2, Windows Server 2008 R2 or Windows 7 based on guide by KB969710 the limit will be ten connections too.

How to Enable Half-Open TCP Connections Limit in Windows Vista with SP2 or Windows 7

  1. Click Start button.
  2. Type regedit in the Start Search box, and then click regedit.exe in the Programs list.
  3. If you are prompted for an administrator password or for confirmation, type your password, or click Continue.
  4. Locate and then double-click the following registry key:

    HKEY_LOCAL_MACHINE\SYSTEM\CurrentControlSet\Services\
    Tcpip\Parameters\EnableConnectionRateLimiting

    Note: In Windows 7, user has to create a new DWORD (32-bit) Value registry entry under Parameters subkey, and named the new value data as EnableConnectionRateLimiting.

  5. In the Value data box, type 1, and then click OK.
  6. Exit Registry Editor.
  7. Restart the computer (in fact, the change takes effect immediately without the need to reboot).

The registry key works on all 32-bit (x86) and 64-bit (x64) edition of Windows Vista with SP2 and Windows Server 2008 with SP2, and unofficially, Windows 7.

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