 - for the users of the network. The transition will have succeeded if the world one day is predominantly (fully is too far in the future to consider) using IPv6, and the average user has not noticed a thing. Definitely for the network administrators, software developers, and hardware vendors the change is not as transparent, and hard work is required. For the rest of this chapter we look at how this journey is planned to be made in 3GPP networks. Early Days In the early days of IPv6 the thinking was that IPv4 and IPv6 would live in parallel until all nodes would be IPv6 enabled and then IPv4 could be gradually disbanded. This Deploying IPv6 in 3GPP Networks: Evolving Mobile Broadband from 2G to LTE and Beyond, First Edition. Jouni Korhonen, Teemu Savolainen and Jonne Soininen. © 2013 John Wiley & Sons, Ltd. Published 2013 by John Wiley & Sons, Ltd. IPv6 Transition Mechanisms for 3GPP Networks approach is the dual-stack transition, as was documented in RFC 1933 [1] back in 1996 and then later updated in RFC 4213 [2]. RFC 1933 also defined the means for setting up manual and automatic IPv6 over IPv4 tunnels in order to allow initial deployments of IPv6 pockets, or clouds, which were to be connected over IPv4 infrastructure. As often happens with great plans, the world did not really start deploying IPv6 on a large scale until IPv4 address pools were effectively dry. Therefore, the idea of getting everybody to IPv6 before IPv4 addresses ran out did not succeed. Of course many IPv6 deployments were made, and backbone networks were updated to a large extent, but the gap to the target was huge. Nevertheless, the dual-stack transition mechanism is still the most favored tool for transition in 3GPP, as we will see in Section 5.4. For a long time the focus on transition mechanisms were how to get IPv6 connectivity in the face of IPv4 infrastructures. This resulted in standardization of many IPv6-over- IPv4 tunneling technologies, and even more proposals about how the tunneling could be done. Besides manually and statical