Pv6 SIP Support: This means allowing IPv6-enabled cus- tomers, who should be in majority dual stack ones at early stages, to benefit from SIP-based telephony services using their IPv6 connectivity capabilities. This objective is valid when there are IPv6-enabled customers. These cus- tomers could prioritize one of their interfaces, logical or physical, and use IPv6 access for VoIP services or would like to load balance their VoIP traf- fic between their IP connectivity interfaces. In such a case, service providers should be able to provide service access to these users because IPv6 can be used as a criterion for selecting a VoIP service provider. It is obvious that this constraint is more valid for enterprise customers than for residential ones who in general do not care about the enabled technology, apart from those who (want to) own static IP addresses affected to specific usages such as FTP (File Transfer Protocol) servers and for whom the evolution of IP connectivity pro- tocols could impact their IP-enabled services. When enforcing this objective, the following requirements should be met: 1- Ability of the customers to exclusively access SIP-based services via their IPv6 connectivity: This requirement assumes that customers who subscribed to IPv6 connectivity services are able to configure their IP access to use exclu- sively IPv6 for SIP-based service offerings. The service platform should be aware of the nature of the interface that is configured to receive VoIP traffic (i.e., SIP signalling or Real-Time Transport Protocol (RTP) streams) SO as to allow successful communication (to illustrate this requirement, service discov- ery means should be adapted SO as to provide end-users with appropriate infor- mation to access the service. Providing exclusively IPv4 service contact point address would lead to unsuccessful subscription of IPv6-enabled customers to the service.). This VoIP traffic can enclose both signalling and media streams. © 2009 by Taylor & Francis Group, LLC SIP and IPv6 2- Ability of the cust