y type and the content of the SDP offers (or SDP answers) carried in its SIP messages. This is a control operation that ensures the successfulness of the telephony communications. For a dual-stack SIP user agent, it is recommended to indicate its two addresses in the SDP offers (or SDP answers). In such a case, the SIP sessions towards both IPv4 and IPv6 SIP user agent have more chance to succeed without any alteration of SDP offers. In addition, this will reduce the number of assigned addresses in the case the proxy servers check the presence of two address types and inserts the missing one. This procedure can be offered if (Camarillo and Rosenberg 2005) and (Schulzrinne 2002) are supported. Furthermore, a double Record Route procedure may ease the routing of SIP messages in case SIP proxy servers are dual-stack. In this case, it could be convenient to add two Route headers each of them indicating an IP address (IPv4 and IPv6) of the crossed server/element. Of course, this can be achieved owing to FQDN usage (instead of "hard" IP addresses). 3.3.6 Focus on Quality of Service QoS is one of critical issues related to IP telephony services. Therefore, IPv6- enabled services should provide good QoS similar to the one experimented by IPv4-enabled ones. The introduction of IPv6 does not change the big picture of the QoS (since IPv6 does not generate bandwidth!) unless there is a need to interconnect IPv6 realms to IPv4 ones. As far as IPv4 is concerned, several initiatives have been launched within the IETF in this track, especially those related to resource reservation asso- ciated with call setup like the framework described in (Camarillo 2002). This proposal introduces a procedure for resource reservation negotiated during the call setup phase. Solutions adopted by service providers should evaluate if C 2009 by Taylor & Francis Group, LLC SIP Handbook: Services, Technologies, and Security this procedure applies for IPv6 and evaluate the impact of introducing IPv6 on such mechanisms. Within a heterogeneous context, 