he second section describes why intro- ducing IPv6 into SIP-based architectures may be problematic. This section identifies a set of encountered technical issues. The third section discusses business and service concerns regarding IPv6 SIP-based architectures migra- tion. Then, this it describes three viable migration scenarios. Furthermore, this section lists a set of pertinent criteria to be taken into account when envisag- ing adopting IPv6 as an IP connectivity protocol. This section also analyzes the technical issues related to each migration scenario. Technical solutions to solve identified technical hurdles are finally introduced in the fourth section. Why Introducing IPv6 Could Be Problematic for SIP-Based Architectures SIP and IPv6 have been envisaged at the early stages of IMS specifications. SIP is an IETF signalling protocol used for initiating, modifying, and clos- ing multimedia sessions. SIP works jointly with Session Description Protocol (SDP) (Handley and Jacobson 1998). The latter is used to describe multime- dia parameters such as IP address and port numbers that will be activated for Real-Time Transport Protocol (RTP) (Schulzrinne 1996) streams. SIP manip- ulates IP addresses in order to route signalling messages. These addresses can be found, for example, in the Request URI, Contact, Via headers or in the SDP headers such as Originator, Contact and Media description headers. Since its first version, presented and documented in RFC 2547, the IPv6 address scheme is supported by SIP. SIP implementations should parse IPv6 addresses, which can even be stored as an Address of Record (AoR). Therefore, IPv6 protocol is not a problem per se; the challenge lies more in preventing session establishment failure when end users are not located in the same IP version routing plane. The following call flow illustrates the results of a SIP session establish- ment request between heterogeneous user agents. For the sake of simplicity, we consider a dual-stack (DS) proxy server and two UAs "A" and "B." A is an I