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

on-line IPR repository


Click on the red underlined text to get to the source

... such proprietary rights by implementers or users of this specification can be obtained from the IETF on-line IPR repository at http://www.ietf.org/ipr. ...



Google
Web
RFC-Ref