user agent can be taken into account to adapt the IP version used within a given call, i.e., influence the SDP offer returned by the terminating SBC. This is summarized in Figure 3.4. C 2009 by Taylor & Francis Group, LLC Legend IPv4-only Type of the caller Dual-stack Caller side Policy: IP version Callee side Originating SBC sends coherence Originating SBC sends invite with empty SDP invite with empty SDP part to destination SBC part to destination SBC Control operation Policy: IPv6 promotion Normal flow Dual-stack UA Destination SBC Ipv4-only sends invite to IPv4-only Destination SBC Dual-stack UA Policy: No IPv6 promotion sends invite to called UA Policy: IPv6 promotion When reading this called UA diagram, choices must remain coherent: once a Dual-stack UA path has been chosen Dual-stack UA Policy: IPv6 promotion (blue for IPv6 promotion, Policy: IP/6 promotion red for IP version Invite embeds coherence) must be Invite embeds IPv4 offer kept all along down to IPv4 offer Invite embeds Invite embeds the media exchanges. Invite embeds Invite embeds ANAT offer IPv6 offer IPv6 offer ANAT offer 200 OK sent by called UA by called UA 200 OK sent Destination SBC 200 OK sent Destination SBC Destination SBC 200 OK sent Destination SBC 200 OK sent by called UA sends 200 OK by called UA sends 200 OK sends 200 OK by called UA sends 200 OK by called UA Pv4-only core nodes Destination SBC IPv4 -only core nodes IPv4-only core nodes Destination SBC iPv4-only core nodes can parse IPv6 sends 200 OK can parse IPv6 can parse IPv6 sends 200 OK can parse IPv6 information as per with IPv4 information as per information as per with IPv4 information as per RFC3261 information RFC3261 RFC3261 information RFC3261 IPv4-only core nodes IPv4- only core nodes IPv4-only core nodes IPv4-only core nodes can parse multiple can parse multiple can parse multiple can parse multiple contact information in contact: information contact: information in contact information in in SDP Destination SBC Destination SBC Destination SBC Destination SBC Destinat