If you have a black hole router, that drops packets larger than the Maximum Transmission Unit (MTU) size, when the don't fragment bit is set, use Regedt32 to navigate to:
On the Edit menu, Add Value name EnablePMTUBHDetect, as a type REG_SZ. Set the String to 1. If EnablePMTUBHDetect is missing, or set to 0, the default behavior is restored.
NOTE: Setting EnablePMTUBHDetect to 1 causes TCP/IP to disable the don't fragment bit when
several retransmissions go unacknowledged.