they may receive messages containing no SDP offer from other SBC peers. Anyway, they must keep answering these messages as usual (i.e., adopting the guidelines described in (Rosenberg 2002)). The procedure operations are captured in Figure 3.2. As far as dual-stack SBCs are concerned, the implications are the following: - When serving as originating SBC, a dual-stack SBC will never enclose a SDP offer. - When receiving a message with IPv4 contact information only in the SDP part, the dual-stack SBC must consider the originating peer is IPv4-only. Therefore, it must process the message (and subsequent mes- sages) as an IPv4-only SBC. - When receiving a message with no SDP offer, the dual-stack SBC may, depending on the service provider preference, either (1) consider the originating peer is IPv6-enabled then answer with IPv6 information enclosed (2) or consider the originating peer is dual-stack and include both IPv6 and IPv4 information. - Once an answer has been received from the destination SBC, the dual- stack SBC will treat the enclosed contact information as described in (Rosenberg 2002), and generate an answer accordingly (IPv4, respec- tively IPv6, if the enclosed data is IPv4, respectively IPv6). If both IP versions SDP offers are available, the originating SBC will decide which version to use and formulate a SDP answer. This SDP answer is to be enclosed in an ACK message sent to the terminating SBC. C 2009 by Taylor & Francis Group, LLC SIP and IPv6 IPv4-only Type of the caller Dual-stack Legend Caller side SDP part of INVITE contains SDP part of Callee side IPv4 information INVITE is empty Control operation INVITE sent to callee IPv4-only Type of the callee Dual-stack Callee SBC sends 200 OK with IPv4 information Callee SBC sends IPv4-only core nodes can 200 OK with IPv4 parse IPv6 information as information per RFC3261 IPv4-only core nodes can 200 OK message parse multiple contact received by caller information in SDP 200 OK message received by caller Callee SBC sends 200 OK with both Callee SBC sends 