RFC 4891:Using IPsec to Secure IPv6-in-IPv4 Tunnel...
RFC-Ref

security architecture


Click on the red underlined text to get to the source

... deployment may choose to use either version of the security architecture. IKEv2 ...


... Kent, S. and R. Atkinson, "Security Architecture for the Internet Protocol", RFC 2401(-> 4301prop), November 1998. ...
... Kent, S. and K. Seo, "Security Architecture for the Internet Protocol", RFC 4301prop, December 2005. ...



Google
Web
RFC-Ref