When you're running a MySQL server on Windows with many TCP/IP connections to it, and you're experiencing that quite often your clients get a Can't connect to MySQL server error, the reason might be that Windows does not allow for enough ephemeral (short-lived) ports to serve those connections.
The purpose of TIME_WAIT is to keep a connection accepting packets even after the connection has been closed. This is because Internet routing can cause a packet to take a slow route to its destination and it may arrive after both sides have agreed to close. If the port is in use for a new connection, that packet from the old connection could break the protocol or compromise personal information from the original connection. The TIME_WAIT delay prevents this by ensuring that the port cannot be reused until after some time has been permitted for those delayed packets to arrive.
It is safe to reduce TIME_WAIT greatly on LAN connections because there is little chance of packets arriving at very long delays, as they could through the Internet with its comparatively large distances and latencies.
Windows permits ephemeral (short-lived) TCP ports to the user. After any port is closed, it remains in a TIME_WAIT status for 120 seconds. The port is not available again until this time expires. The default range of port numbers depends on the version of Windows, with a more limited number of ports in older versions:
Windows through Server 2003: Ports in range 1025–5000
Windows Vista, Server 2008, and newer: Ports in range 49152–65535
With a small stack of available TCP ports (5000) and a high number of TCP ports being open and closed over a short period of time along with the TIME_WAIT status you have a good chance for running out of ports. There are two ways to address this problem:
Reduce the number of TCP ports consumed quickly by investigating connection pooling or persistent connections where possible
Tune some settings in the Windows registry (see below)
The following procedure involves modifying the Windows registry. Before you modify the registry, make sure to back it up and make sure that you understand how to restore it if a problem occurs. For information about how to back up, restore, and edit the registry, view the following article in the Microsoft Knowledge Base: http://support.microsoft.com/kb/256986/EN-US/.
Start Registry Editor (Regedt32.exe).
Locate the following key in the registry:
HKEY_LOCAL_MACHINE\SYSTEM\CurrentControlSet\Services\Tcpip\Parameters
On the Edit menu, click Add Value, and then add the following registry value:
Value Name: MaxUserPort Data Type: REG_DWORD Value: 65534
This sets the number of ephemeral ports available to any user. The valid range is between 5000 and 65534 (decimal). The default value is 0x1388 (5000 decimal).
On the Edit menu, click Add Value, and then add the following registry value:
Value Name: TcpTimedWaitDelay Data Type: REG_DWORD Value: 30
This sets the number of seconds to hold a TCP port connection in TIME_WAIT state before closing. The valid range is between 30 and 300 decimal, although you may wish to check with Microsoft for the latest permitted values. The default value is 0x78 (120 decimal).
Quit Registry Editor.
Reboot the machine.
Note: Undoing the above should be as simple as deleting the registry entries you've created.