Windows 2012 RRAS IPsec VPN does not support NAT-T out-of-the-box.  By default, RRAS only works with public IP addresses -no NAT.  Windows 10 clients cannot connect with L2TP from outside the office.  Windows 2016 does not support L2TP for any client from behind routers running NAT.

Enable NAT-T on both Windows servers and the clients.  NAT-T allows the VPN server to serve clients (e.g., Windows 10, Android, Apple iOS) from behind the NAT device.  Modify MTU. 

Why NAT-T? 

IPsec uses Encapsulating Security Payload (ESP) to encrypt packet headers and payloads.  By default, ESP is not compatible with Port Address Translation (PAT).  This is because TCP uses ports and ESP does not.  

ESP uses protocol (i.e., not port) number 50.   ESP is a protocol without ports.  Network Address Translation (NAT) uses port translation PAT to bind traffic flows with internal hosts.  Therefore, ESP does not work with NAT.

NAT-T allows ESP to work from behind NAT.  It encapsulates ESP protocol 50 inside User Datagram Protocol (UDP) 4500.   N.B, NAT-T is not the same as IPsec over UDP.

NAT-T is enabled on most operating systems (e.g., Android) -Windows is the exception.  Fortunately,  we can enable NAT-T on Windows 10 and Windows 2012 with a few simple changes. 

Windows IPsec clients are supposed to work from any location.  Therefore, only enable NAT-T on the 2012 RRAS server.  

Create a new registry key to enable NAT-T.

                         HKLM\SYSTEM\CurrentControlSet\Services\PolicyAgent\Parameters\

These changes will fix those pesky L2TP-NAT problem.  

Troubleshooting Issues

Make sure clients use the latest edition of Windows 10.  Early versions had quirks where clients simply would not connect via NAT-T.  

   NAT-T does not work with  the following editions:

• version 10240
• version 1511 (i.e. November Update)

   Unconfirmed (may or may not work):  

• version 1607 (i.e., Anniversary Update)

   Confirmed:

• version 1703 (i.e., Creators Update)

   NAT-T works great with the registry fix and Creators Update.

   Workarounds:  

Some folks had to toggle the NAT-T registry value in order to connect (http://bit.ly/2r2CKnF).  I assume this fix was for the November or Anniversary Update.  

MTU

Don't forget to adjust the Max Segment Size (MSS):  
http://www.stevenjordan.net/2016/11/windows-ikev2-mtu.html.  

That's It!