Is there any proper collection/reference about vendor-independent VPN tunnels and how they're constructed (bonus if they talk a bit also about vendor gimmicks)

I'm talking about the components of a VPN and how they stack or get combined together.

What is, by RFC, mandatory specifications and what isnt (like lifetime) ?

I've only been able to find Vendor snippets who explain a bit more than the config, IETF Drafts who are dozens about each component, and things that make not much sense (cisco VTI IPSEC/GRE)

Basically, i'm really trying to find a dummy guide that's comprehensive.