How to connect from Android via VPN using IPSec?

Question

Android

Given: Android (4.x, 5.x - it doesn't matter, the tests use an Nvidia Tegra 7 tablet with Android 5.1), a Linux server with strongswan 5.4.0 or a Mikrotik RB1100AHx2.
Find: VPN between these devices (Linux tablet or Mikrotik tablet)
I tried both the standard client and third-party ones - strongswan for android, thegreenbow (yes, he makes VPN clients for android), fortigate. Various errors occur, but the connection cannot be established in any way.
The question actually in what - who as solves the similar problem. I specify at once - pure IPSec. No L2TP. No OpenVPN.

Answer 1

ipsec is just encryption over ip (like https over http ).