the positioning of intermediary nodes (those elements acting as relays between IPv4 and IPv6 domains) should take into consideration the end-to-end QoS as perceived by end users. The routing to a given intermediary node should be optimal SO as the delivered QoS is similar to communication within a homogenous realm. This routing issue is tricky since routing is usually (except when static routes are activated) done based on shortest path, and thus the end-to-end route is the result of two sub optimal functions in two realms. These two sub optimal operations can result in a non end-to-end optimal solution. Solutions deployed by service providers should be able to redirect IP packets destined to heterogeneous realms to cross these intermediary nodes. In addition, as far as QoS is concerned and when DiffServ (Differentiated Services) are implemented, these solutions should pro- vide techniques SO as to preserve the signalling of the requested QoS and that consistent and coherent treatment is experienced by IP packets destined to remote destined located in heterogeneous domains. 3.3.7 Focus on Security Solution adopted by service providers to serve both IPv4 and IPv6-enabled customers must support both hop-by-hop and end-to-end security modes as described in (Rosenberg 2002). In addition, these solutions may support an end-to-middle security scheme as described in (Ono and Tachimoto (RFC4189) 2005) and (Ono and Tachimoto 2005). Both Signalling and media streams must be secured. For instance, Secure Real-Time Transport Protocol (SRTP) (Baugher 2004) and Internet Protocol Security (IPSec) (Kent and Atkinson (RFC 2402) 1998) (Kent and Atkinson (RFC 2406) 1998) may be activated to secure media streams. In addition, these solutions should ensure that when interworking between heterogeneous realms, security mechanisms are not broken. Previous work has been conducted like the one introduced in (Xing and Atwood 2005), which transcribes a mechanism for guaranteeing end-to-end security between SIP entities in the IPv4 domain an