Adding Multi-Homing and Dual-Stack Support to the Session Initiation Protocol

Although the SIP protocol claims a complete dual-stack support, some aspects, such as interoperability between different address realms and support for multi-homed hosts, are not taken into consideration. This leads to an extensive usage of proxies as gateways, e.g., between different address realms. ALEX ("address list extension") is a simple extension to the SIP header that addresses these limitations, providing additional scalability for SIP proxies and allowing the establishment of direct channels between peers, while still guaranteeing backward compatibility with traditional SIP implementations.