Intra-Site Automatic Tunnel Addressing Protocol (ISATAP)
ISATAP is the Intra-Site Automatic Tunnel Addressing Protocol which supports automatic deployment of IPv6 in IPv4 sites. ISATAP specifies the operation of IPv6 and the IPv6 Neighbor Discovery (ND) protocol over a specific link layer (that being IPv4 used as a link layer for IPv6) and is therefore an "IPv6-over-foo" specification.

ISATAP is essentially just a Non-Broadcast Multiple Access (NBMA) variant of 6over4 [RFC2529]. As of March 2008, ISATAP has been re-published as an informational-category RFC [RFC5214] which obsoletes the previous experimental-category version [RFC4214]. A development history of RFC4214 is on the ISATAP Issue Tracker Page

As of April 2008, ISATAP is implemented in the Linux operating system kernel, beginning with linux-2.6.25. ISATAP is also implemented in other widely-deployed systems including Microsoft Windows (XP, Vista, 7, 8), cisco IOS, 6WIND 6WINDGate, and FreeBSD/KAME. An ISATAP client application for both Linux and Android systems is now available here.

ISATAP hosts use IPv6 StateLess Address Auto Configuration (SLAAC) by sending a unicast Router Solicitation (RS) message to an ISATAP router which then returns a unicast Router Advertisement (RA) message. This stateless operation is sufficient for many client-initiated IPv6 applications but does not support true native IPv6 operation. A new system called Asymmetric Extended Route Optimization (AERO) has therefore been developed based on the ISATAP NBMA model.

Like ISATAP, AERO is an "ipv6-over-foo" document and specifies the operation of IPv6 and IPv6 ND over AERO links. Unlike ISATAP, AERO is stateful and requires the use of the IPv6 neighbor cache on AERO interfaces. AERO nodes also use the DHCPv6 Prefix Delegation service to obtain native IPv6 prefixes. The benefits of the AERO approach include 1) use of fully-native IPv6 addresses, 2) persistent IPv6 addresses and/or prefixes that never change, 3) mobilitiy support, 4) router-to-router tunneling, 5) mobile routers, 6) path MTU robustness, and many others. An early experimental implementation of AERO is avialable here.

Other ISATAP-related links:
Towards the full Deployment of IPv6 - the Experience at Swisscom Innovations
Ericsson Roams on IPv6
Internet Protocol Version 6 (Microsoft)
ISATAP Tunnel Support for IPv6 (Cisco)
How to use ISATAP on KAME
ISATAP Client for Linux and Android
OS X: access IPv6 in THU with ISATAP
ISATAP - Wikipedia, the free encyclopedia

NEW: A feature-wise comparison of several popular tunneling mechanisms is here.