P version IPv6 promotion IP version towards Originating SBC sends IP version destination SBC ACK with IPv6 information coherence coherence coherence destination SBC and 200 OK with and 200 OK with Originating SBC sends ACK with IPv6 towards destination SBC IPv4 information IPv4 information information towards destination SBC and and 200 OK with IPv4 towards calling towards calling 200 OK with IPv6 information towards information towards IPv4-only UA Originating SBC sends ACK with IPv6 dual-stack UA calling dual-stack UA calling IPv4-only UA Originating SBC sends ACK with IPv6 information towards destination SBC and information towards destination SBC and 200 OK with IPv4 information towards 200 OK with IPv4 information towards calling IPv4-only UA calling IPv4-only UA Destination SBC Destination SBC Destination SBC Destination SBC sends ACK with sends ACK with sends ACK with sends ACK with IPv6 information to IPv4 information to IPv4 information to IPv6 information to called UA called UA* called UA* called UA Mixed media IPv4 media IPv4 media IPv6 media Mixed media exchanges exchanges exchanges exchanges exchanges *Considered as a new offer *Considered as a new offer FIGURE 3.5 Blank SDP procedure applied for Alternative 2 (IPv6 Promotion policy). C 2009 by Taylor & Francis Group, LLC SIP and IPv6 Moreover, the proposed solution allows different implementations of the SIP service, depending on the service provider policy, whether it favors IPv6 promotion (as illustrated in Figure 3.5) or IP version coherence. The main differences with Alternative 1 application are: - The SDP offers made by SBC towards SIP user agents are more "targeted." When several IP connectivity type candidates are enclosed in the SDP offer made by the terminating SBC, a priority information is enclosed, depending on the IP connectivity type of the called SIP user agent, pro- viding complementary information to the originating SBC, which is still responsible for the final decision of the IP version to use for a given call. As far as SBCs are 