concerned, their implications are as follows: - When serving as originating SBC, the SBC will never enclose contact information in the SDP part. In the same way, when serving as termi- nating SBC, the SBC will always receive "blank SDP" offers. - On reception of an answer message, the SBC will treat the enclosed contact information as described in (Rosenberg 2002), and generate an answer accordingly. If both IP versions offers are available, the originat- ing SBC will make a local decision. The decision-making process of that SBC is driven by the service provider policies; however, the terminating SBC is likely to have positioned priority information allowing to enforce IP version coherence all along the call path. In case a service should be migrated from Alternative 1 to an Alternative 2 environment, the procedure allows for coexistence of both SBC behaviors. In addition, and as far as Alternative 1 SBCs are concerned, the enforcement of this procedure makes the migration transparent to end-users. Therefore, Alternative 1 SBCs still perform the same operations as per Alternative 1. The only slight modification is that a preference information is likely to be enclosed in the messages they receive, which contain SDP offers with several IP connectivity type candidates. In such a case, Alternative 1 SBCs may either ignore or exploit this additional information depending on their capabilities and configuration. 3.5 Conclusions This chapter has identified a set of strategic and technical issues arising when introducing IPv6 within conversational services and describes why IPv6 may be problematic for SIP-based service platforms. This chapter proposed three scenarios for migrating SIP-based conversational services to IPv6. It also enu- merated a set of engineering recommendations to ease the migration to IPv6 © 2009 by Taylor & Francis Group, LLC SIP Handbook: Services, Technologies, and Security and interworking between IPv6 and IPv4 realms. Furthermore, this chapter introduced two new procedures for routing optimizat