This is an old revision of the document!
https://www.freeswan.org/
https://github.com/nm-l2tp/NetworkManager-l2tp
https://github.com/systemd/systemd/pull/11908
https://access.redhat.com/documentation/en-us/red_hat_enterprise_linux/8/html/securing_networks/configuring-a-vpn-with-ipsec_securing-networks