Among other results, several protocols, such as Session Initiation Protocol (SIP) (Rosenberg 2002) and H.323 (ITU-T H.323 1998) have been proposed and promoted for the deployment of IP-based telephony service offerings. These protocols have been introduced progressively into operational plat- forms. Recently, new architectures such as IP Multimedia Subsystems (IMS) (Camarillo and Garcia-Martin 2005) and TISPAN Telecommunications and Internet converged Services and Protocols for Advanced Networks (TISPAN) (TR180001 2006) have been proposed to meet service providers' requirements (mainly to implement the fixed-mobile convergence). Most current big service providers deployments are inspired from IMS and/or TISPAN architectures. * Some parts of this chapter are based on "Migrating SIP-based Conversational Services to IPv6: Complications and interworking with IPv4," by M. BOUCADAIR and Y. NOISETTE which appeared in Proc. of Second International Conference on Digital Telecommunications (ICDT'07) © 2007 IEEE and "Towards Smooth Introduction of IPv6 in SIP-based Architec- tures," by M. BOUCADAIR and Y. NOISETTE, which appeared in Proc. Local Computer Networks (LCN'07) © 2007 IEEE. C 2009 by Taylor & Francis Group, LLC SIP Handbook: Services, Technologies, and Security In parallel and since the early nineties, service providers, manufacturers and standardization bodies have investigated the necessity of introducing a new IP to solve some critical problems encountered by the Internet architecture. After a large amount of standardization effort has been conducted, especially within the Internet Engineering Task Force (IETF), IPv6 has been adopted as the next generation IP connectivity protocol. Today, IPv6-related standards are judged mature enough to consider an operational deployment. Indeed, some service providers envisage adopting IPv6 as their new connectivity protocol not only for the abundance of IPv6 addresses, but also to take advantage of the routing hierarchy spirit of this new IP version (notably in order to red