Pv4-enabled UA and B is an IPv6-enabled one. During the registration pro- cess, A (respectively B) has registered its IPv4 (respectively IPv6) address as AoR. Let's suppose now that the IPv6-enabled UA B sends an INVITE mes- sage destined to the IPv4-enabled UA A. Figure 3.1 illustrates the beginning of the SIP messages exchange that occurs between all involved parties. The SIP proxy server relays the received INVITE message to A. After receiv- ing this message, A parses its SDP part and formulates an answer. This could lead to a closure of the application or simply to sending back an error message because the media line of the SDP part doesn't include any IPv4 address. As a consequence, it is impossible to set up a SIP session initiated by an IPv6- enabled user agent toward an IPv4-enabled user agent. C 2009 by Taylor & Francis Group, LLC SIP and IPv6 Phone B Proxy server Phone A (1) Invite (2) Invite (3) Trying (4) Ringing (5) Ringing FIGURE 3.1 Example of heterogeneous call flow. A new proposal dedicated to dual-stack user agents (i.e., owning both IPv4 and IPv6 connectivity), (Chen and Wu 2005) describes a mechanism allow- ing multiple alternative network addresses to be enclosed in a single SDP offer. This proposal consists of introducing a new attribute called Alterna- tive Network Address Types (ANAT). This attribute allows inserting multiple media/connection lines in the same SDP offer (or SDP answer). IETF RFC 4092 (Schulzrinne 2002) defines how SIP can exploit the ANAT semantic by introducing a new option tag called "sdp-anat." This tag can be useful for SIP user agents to be aware of the capabilities of each other and then select from the supported media/network description lines the ones that are suitable for setting up the SIP communication (and also according to local preferences). A use case for illustrating the usage of this tag is a dual stack SIP user agent which can communicate either using its IPv6 or its IPv4 connectivity. This type of SIP user agent can also set a preference associated with 