http://www.rfc-editor.org/rfc/rfc6732.txt
RFC 6732: 6to4 Provider Managed Tunnels, V. Kuarsingh, Y. Lee, O. Vautrin2012-09-01T23:00:00-00:006to4 Provider Managed Tunnels (6to4-PMT) provide a framework that can help manage 6to4 tunnels operating in an anycast configuration. The 6to4-PMT framework is intended to serve as an option for operators to help improve the experience of 6to4 operation when conditions of the network may provide sub-optimal performance or break normal 6to4 operation. 6to4-PMT supplies a stable provider prefix and forwarding environment by utilizing existing 6to4 relays with an added function of IPv6 Prefix Translation. This operation may be particularly important in NAT444 infrastructures where a customer endpoint may be assigned a non-RFC1918 address, thus breaking the return path for anycast-based 6to4 operation. 6to4-PMT has been successfully used in a production network, implemented as open source code, and implemented by a major routing vendor. This document is not an Internet Standards Track specification; it is published for informational purposes.