tion where the serving SGSN changes from a dual-stack capable SGSN to a single-stack IPv4 capable SGSN. If the dual-stack SGSN has had the GTP tunnel to the GGSN over IPv6, and the dual-stack SGSN would only pass the IPv6 address of the GGSN to the IPv4-only capable SGSN, the new serving SGSN would have to drop the connection - even if the GGSN were dual-stack capable. Therefore, 3GPP has included a mechanism in the GTP protocol to pass the network elements' IPv4 and IPv6 addresses. This ensures correct functionality even in an environ- ment where there are mixed IPv4-only capable and dual-stack capable network elements. This functionality would not help in an environment where there were single-stack IPv4 and single-stack IPv6 elements. However, the 3GPP standards do not support this kind of setup, and it has not defined how GTP should be run through translators. It is, how- ever, unlikely that anybody would configure this kind of environment - at least not intentionally. 5.5.6 Roaming We have now learned that transitioning to IPv6 is a relatively difficult task that needs focus and careful planning. Even inside one operator's network, the task can be daunting, and if it is not planned carefully and executed well, multiple problems on many levels can happen. The reassuring thing about transitioning an operator's network is the fact that everything can be controlled by that operator, and that the first operators are already running commercial IPv6 networks. Unfortunately roaming removes the luxury of full control. Basically, the operator does not have to only work with its own network, but has to make sure that its service and network will be able to cooperate with all of the roaming partners' networks and services. Section 4.2.5 discussed a number of techniques of how to turn on roaming restrictions that are specifically targeted to selectively disable Deploying IPv6 in 3GPP Networks IPv6 roaming. In certain cases that can be done in the granularity of an APN per visited operator (or rather per Public Land Mobil