ffer IPv6- enabled SIP-based services as described in the next section. 3.3.4 Migration Scenarios 3.3.4.1 First Alternative The spirit of this alternative is to be ready to support IPv6 in case IP net- works are evolving toward an introduction of IPv6 transfer capabilities (i.e., the density of IPv6-enabled nodes becomes important and not negligible). This first migration scenario consists of exploiting the existing IPv4-enabled con- versational service infrastructure(s) as it is and adding new functions where required to support IPv6-enabled customers and co-exist with IPv4 ones. The main characteristic of this alternative is to keep the core of the existing con- versational services platform untouched, while IPv6 functions are enabled at the access and border segments only when required. This alternative is per- tinent to be considered and studied at earlier stages of IPv6 migration. This alternative should be seen as a transition phase in order to prepare further C 2009 by Taylor & Francis Group, LLC SIP Handbook: Services, Technologies, and Security migration operations, especially in the core segment. This migration scenario can be maintained until the number of IPv6-enabled users becomes significant and will induce significant amount of IPv6 traffic. This alternative is viable, and even interesting to investigate if the service provider has a clear strategy for progressive deployment of IPv6. In other words, this means that the service provider has clear views and roadmaps to progressively support and activate IPv6 capabilities within its service infras- tructure. The service chain can be divided into three major segments: access, core, and border segments. Within this alternative, the support of IPv6 will be mainly enabled in the extremities segments, i.e., access and border segments. This will be enabled in the access segment since it is where IP addresses are assigned and SIP user agents are provisioned with service-specific data. Also, the border segment can be impacted, since the service provider shoul