each type of enclosed connectivity type. The proposal described in (Schulzrinne 2002) provides guidelines to ensure correct processing of this new tag by SIP user agents not supporting the ANAT attribute. But this procedure alone cannot lead to a successful call establishment between heterogeneous nodes. Conse- quently, enhancements to SIP behavior should be investigated, proposed, and promoted. This chapter proposes some insights about some new proposals. Business and Service Considerations Regarding IPv6 VoIP Migration In general, migration operations are always seen as risky tasks to perform by service providers. As far as migration to IPv6 of SIP-based service in concerned, and in order to ease this task, the following sections describe the © 2009 by Taylor & Francis Group, LLC SIP Handbook: Services, Technologies, and Security overall context and list a set of pertinent indicators and parameters to be taken into account. Both service-specific and transport considerations have been mentioned. Moreover, several viable scenarios are identified and their technical hurdles highlighted. 3.3.1 Context and Objectives The following section focuses on the context and environment in which IPv6 migration of SIP-based architectures is to be undertaken. The ultimate target is to identify migration strategies and to sketch a set of recommendations aim- ing at offering a full IPv6-enabled SIP-based telephony service to pure IPv6 customers. This target is a long-term objective and can be reached incremen- tally through one or several intermediate steps, depending on the strategies adopted by each service provider. These intermediate steps' perimeters differ from one service provider to another depending on (1) the business context, (2) the service opportunities targeted when enabling IPv6-related services, and (3) their position with regard to IPv6 (i.e., adopt an aggressive or pas- sive position). Nevertheless, all these intermediate steps alternatives serve the same mid-term objectives listed hereafter: - First Objective-I