The Internet Engineering Task Force (IETF) developed formal standards for L2TP/ IPsec user authentication.
Request for Comments (RFC) documents provide an overview of a protocol or service and details about how the protocol should behave. If you’re a novice server administrator, you’ll probably find some of the background information in an RFC helpful.
If you’re an experienced server administrator, you can find all the technical details about a protocol in its RFC document.
You can search for RFC documents by number at www.ietf.org/rfc.html.
For L2TP information, see RFC 2661.
For IPsec information, see RFC 4301 and 4309.
For PPTP information, see RFC 2637.
For Kerberos version 5 information, see RFC 1510.