Reducing the chance of TCP port exhaustion on a Windows Server.

For the first time in my career, I ran into port exhaustion. On a Windows server, our .NET 5.0 application kept exhausting the available TCP ports. The resolution, among other things, involved increasing MaxUserPorts and decreasing TcpTimedWaitDelay.

It looks like this in PowerShell:

Get-ItemProperty -Path "HKLM:\SYSTEM\CurrentControlSet\Services\Tcpip\Parameters"

TcpTimedWaitDelay        : 30
MaxUserPorts             : 65534

And like this in the Registry Editor.

After modifying the registry, we DO need to restart the server.

After doing that, again from PowerShell, we can see fewer active TCP connections in the TimeWait state:

$props = [System.Net.NetworkInformation.IPGlobalProperties];

$props::GetIPGlobalProperties().GetActiveTcpConnections() |
   Group-Object -Property State |
   Select-Object Count, Name;

The output looks like this right now for us:

Note how few connections have the state of TimeWait.

For more information about the various states, IBM has a useful diagram and table and this answer on StackOverflow explains how to set the TcpTimedWaitDelay value safely.  From my beginners perspective, the TIME_WAIT period helps to ensure the conversation (and its echo, so to speak) has ended before reusing the same connection.