IPsec
Updated
IPsec is the suite of protocols that authenticates and encrypts IP packets between two points, often used to build site-to-site tunnels across the internet. Its architecture is described in RFC 4301 and its key exchange, IKEv2, in RFC 7296.
In practice
An IPsec tunnel is the quickest way to bring a private APN into a site or a cloud, but the encrypted tunnel still crosses the internet.