troduction of IPv6 should be transparent: Customers should not observe alteration of their connectivity services and the migration should be transpar- ent for them. The current trade is that network providers offer dual connectivity to their customers, i.e., IPv4 and IPv6 access. IPv4 connectivity usage should be grad- ually decreased in favor of IPv6 one. This convergence phase toward a pure IPv6 connectivity will take several years depending on the policies adopted by service providers. For operators adopting an aggressive position with regards to the activation of IPv6, this transition phase could be small compared to passive operators. Nevertheless, the overall Internet IPv6 coverage will take at least one decade. Based on this IPv6 transport migration, "high level" service providers can conclude that two categories of customers will co-exist during the transition phase: - Pure IPv4 customers who don't have an IPv6 access, - Dual-stack customers who benefit from the two access types. C 2009 by Taylor & Francis Group, LLC SIP Handbook: Services, Technologies, and Security As a consequence, and from a VoIP standpoint, the deployed solutions for con- versational services should be designed taking into account these requirements: - Pure IPv4 customers must still be able to access VoIP services: Meaning that the dimensioning of the service platform should be aware of the amount of IPv4 traffic that is still being routed by SIP proxy servers and media gateways. For this category of customers, the SIP service should not be disturbed. The service provider should own the means to prioritize IPv6 traffic: This means that service providers will activate some means in their service platform to promote the usage of IPv6 instead of IPv4 when possible. 3.3.3 Business and Strategy Considerations The enforcement strategy of the IPv6 SIP support and the SIP IPv4-IPv6 interworking objectives depends on the type of the SIP-based offerings man- aged by a given VoIP service provider. For a newcomer who does not manage any existing