http://www.rfc-editor.org/rfc/rfc4659.txt
RFC 4659: BGP-MPLS IP Virtual Private Network (VPN) Extension for IPv6 VPN, J. De Clercq, D. Ooms, M. Carugi, F. Le Faucheur2006-09-01T23:00:00-00:00This document defines support of the IPv6 VPN service over both an IPv4 and an IPv6 backbone, and for using various tunneling techniques over the core, including MPLS, IP-in-IP, Generic Routing Encapsulation (GRE) and IPsec protected tunnels. The inter-working between an IPv4 site and an IPv6 site is outside the scope of this document. [STANDARDS-TRACK]