apt-get install strongswan network-manager-strongswan

https://launchpad.net/~seriy-pr/+archive/ubuntu/network-manager-l2tp

Jul  2 14:44:00 xen ipsec_setup: Starting Openswan IPsec 2.6.38...
Jul  2 14:44:01 xen ipsec_setup: Using KLIPS/legacy stack
Jul  2 14:44:01 xen kernel: [   49.125995] sha512_ssse3: Neither AVX nor SSSE3 is available/usable.
Jul  2 14:44:01 xen kernel: [   49.152561] sha256_ssse3: Neither AVX nor SSSE3 is available/usable.
Jul  2 14:44:01 xen kernel: [   49.172064] AVX instructions are not detected.
Jul  2 14:44:01 xen kernel: [   49.189713] AVX instructions are not detected.
Jul  2 14:44:01 xen kernel: [   49.207982] AVX instructions are not detected.
Jul  2 14:44:01 xen kernel: [   49.232719] AVX instructions are not detected.
Jul  2 14:44:01 xen kernel: [   49.261421] AVX instructions are not detected.
Jul  2 14:44:01 xen kernel: [   49.287047] AVX or AES-NI instructions are not detected.
Jul  2 14:44:01 xen kernel: [   49.307848] AVX or AES-NI instructions are not detected.
Jul  2 14:44:01 xen ipsec_setup: No KLIPS support found while requested, desperately falling back to netkey
Jul  2 14:44:01 xen ipsec_setup: NETKEY support found. Use protostack=netkey in /etc/ipsec.conf to avoid attempts to use KLIPS. Attempting to continue with NETKEY
Jul  2 14:44:01 xen kernel: [   49.329498] NET: Registered protocol family 15
Jul  2 14:44:01 xen ipsec_setup: Using NETKEY(XFRM) stack
Jul  2 14:44:01 xen kernel: [   49.403213] Initializing XFRM netlink socket
Jul  2 14:44:01 xen kernel: [   49.501839] sha512_ssse3: Neither AVX nor SSSE3 is available/usable.
Jul  2 14:44:01 xen kernel: [   49.516281] sha256_ssse3: Neither AVX nor SSSE3 is available/usable.
Jul  2 14:44:01 xen kernel: [   49.539742] AVX instructions are not detected.
Jul  2 14:44:01 xen kernel: [   49.561641] AVX instructions are not detected.
Jul  2 14:44:01 xen kernel: [   49.579962] AVX instructions are not detected.
Jul  2 14:44:01 xen kernel: [   49.604578] AVX instructions are not detected.
Jul  2 14:44:01 xen kernel: [   49.621050] AVX instructions are not detected.
Jul  2 14:44:01 xen kernel: [   49.646920] AVX or AES-NI instructions are not detected.
Jul  2 14:44:01 xen kernel: [   49.667841] AVX or AES-NI instructions are not detected.
Jul  2 14:44:01 xen kernel: [   49.686945] netlink: 24 bytes leftover after parsing attributes in process `ip'.
Jul  2 14:44:01 xen ipsec_setup: ...Openswan IPsec started
Jul  2 14:44:01 xen pluto: adjusting ipsec.d to /etc/ipsec.d
Jul  2 14:44:01 xen ipsec__plutorun: adjusting ipsec.d to /etc/ipsec.d
Jul  2 14:44:01 xen NetworkManager[806]: <info> VPN connection 'OTOY Cluster' (Connect) reply received.
Jul  2 14:44:11 xen NetworkManager[806]: <warn> VPN plugin failed: 7
Jul  2 14:44:42 xen NetworkManager[806]: <warn> VPN connection 'OTOY Cluster' (IP Config Get) timeout exceeded.
Jul  2 14:44:42 xen NetworkManager[806]: <info> Policy set 'Wired connection 1' (eth0) as default for IPv4 routing and DNS.
Jul  2 14:44:47 xen NetworkManager[806]: <info> VPN service 'l2tp' disappeared
Jul  2 14:51:35 xen kernel: [  503.672175] netlink: 24 bytes leftover after parsing attributes in process `ip'.
Jul  2 14:51:35 xen kernel: [  503.680945] ip_tables: (C) 2000-2006 Netfilter Core Team
Jul  2 14:51:35 xen kernel: [  503.686441] ip6_tables: (C) 2000-2006 Netfilter Core Team
Jul  2 14:52:04 xen NetworkManager[806]: <info> Starting VPN service 'l2tp'...
Jul  2 14:52:04 xen NetworkManager[806]: <info> VPN service 'l2tp' started (org.freedesktop.NetworkManager.l2tp), PID 3589
Jul  2 14:52:04 xen NetworkManager[806]: <info> VPN service 'l2tp' appeared; activating connections
Jul  2 14:52:04 xen NetworkManager[806]: <info> VPN plugin state changed: starting (3)
Jul  2 14:52:04 xen ipsec_setup: Stopping Openswan IPsec...
Jul  2 14:52:05 xen kernel: [  533.776479] netlink: 24 bytes leftover after parsing attributes in process `ip'.
Jul  2 14:52:05 xen kernel: [  534.105661] NET: Unregistered protocol family 15
Jul  2 14:52:06 xen ipsec_setup: ...Openswan IPsec stopped

sudo apt install
intltool
libtool
network-manager-dev
libnm-util-dev
libnm-glib-dev
libnm-glib-vpn-dev
libnm-gtk-dev
libnm-dev
libnma-dev
ppp-dev
libdbus-glib-1-dev
libsecret-1-dev
libgtk-3-dev
libglib2.0-dev
xl2tpd
strongswan

git clone https://github.com/nm-l2tp/network-manager-l2tp.git
cd network-manager-l2tp
autoreconf -fi
intltoolize

Make sure no errors have occurred.

./configure
  --disable-static --prefix=/usr
  --sysconfdir=/etc --libdir=/usr/lib/x86_64-linux-gnu
  --libexecdir=/usr/lib/NetworkManager
  --localstatedir=/var
  --with-pppd-plugin-dir=/usr/lib/pppd/2.4.7

Make sure no errors occurred.

make
sudo make install

Remove AppArmor settings for IPSec:

sudo apparmor_parser -R /etc/apparmor.d/usr.lib.ipsec.charon
sudo apparmor_parser -R /etc/apparmor.d/usr.lib.ipsec.stroke

sudo apt remove xl2tpd
sudo apt install libpcap0.8-dev

wget https://github.com/xelerance/xl2tpd/archive/v1.3.6/xl2tpd-1.3.6.tar.gz
tar xvzf xl2tpd-1.3.6.tar.gz
cd xl2tpd-1.3.6
make
sudo make install