winxp默認會使用ipsec來加密l2tp，可以通過修改註冊表來取消。win7好像沒這個問題。
HKEY_LOCAL_MACHINE\System\CurrentControlSet\Services\Rasman\Parameters
Add the following registry value to this key:
Value Name: ProhibitIpSec
Data Type: REG_DWORD
Value: 1
https://support.microsoft.com/en-us/help/258261/disabling-ipsec-policy-used-with-l2tp