d others in the IPv6 realm. Handling Technical Issues Raised When Migrating SIP-Based Services to IPv6 This section describes some proposals made by the authors of this chapter to solve some of the critical technical issues that arise when migration SIP-based conversational services to IPv6. These issues are pertinent when both IPv4 and IPv6 conversational realms co-exist in the same IP Telephony Administrative Domain (ITAD) managed by a single service provider. The first technical issue C 2009 by Taylor & Francis Group, LLC SIP and IPv6 is how to discover the connectivity type of SIP user agents. This discovery operation's outcome will be used to drive the routing logic SO as to optimize the invocation of interworking and adaptation functions. The second technical issue is how to tune the media traffic process between service nodes in an optimal scheme, i.e., avoid IP version change. 3.4.1 Telephony Routing Optimization: SIP User Agent's Type Discovery The SIP service platform should be aware of the type of the involved call participants before forwarding session establishment requests. If these means are not supported, SIP proxy servers have no way to predict a session failure even if ANAT procedure is implemented. An alternative may be to add dual- stack elements in the core service platform and to force the signalling and media path to cross these dual-stack elements. This latter alternative is viable for big service providers but not for smaller one especially at the earlier stages of deployment of IPv6. For these reasons, we propose lightweight means to discriminate the IP connectivity types as supported by end users' machines. A SIP ALG will be invoked only if necessary. This section focuses on call routing and optimization means to avoid as possible requesting SIP ALGs and NAT-PT boxes when establishing multimedia sessions between two SIP user agents. 3.4.1.1 Use Classical SIP Behavior The first alternative to notify the service platform about the type of the SIP user agent is to send a SIP REGISTER messa