IPv6 and IPv4 200 OK with IPv6 information information 200 OK message received by caller Caller SBC sends Caller SBC sends 200 OK message ACK with IPv4 received by caller information Caller SBC sends 200 OK message ACK with IPv6 received by caller ACK message information received by callee ACK message received by callee IPv4 media exchanges IPv6 media exchanges FIGURE 3.2 Overview of "blank SDP" procedure. This procedure ensures that IPv4 exchanges will occur between SBCs when an IPv4-only SIP user agent intervenes in this communication. Owing to the discrimination techniques described above, SBCs are also capable of triggering the type of the SIP user agent, and adapt the IP version used on the extremity legs. Consequently, SBCs can adapt the call context SO that most of the time it is full IPv4 or full IPv6 all along the media chain. As for the signaling plane, the content of the SDP offer doesn't need to be modified. Figure 3.3 illustrates the SDP procedure when activated within a SIP-based architecture. In the case the originating SBC is dual-stack and the Terminating SBC is IPv4-only, the main operations are: - Step 1: The calling SBC receives an INVITE message. In its mapping table, the SBC retrieves the IPv4 IP address associated with the calling SIP user agent toward core network. The SBC uses this latter IPv4 address to forward INVITE message with blank SDP offer. © 2009 by Taylor & Francis Group, LLC SIP Handbook: Services, Technologies, and Security Phone A P-CSCF P-CSCF Phone B INVITE INVITE INVITE INVITE INVITE INVITE 180 Ringing 180 Ringing 180 Ringing 180 Ringing 180 Ringing 180 Ringing PRACK PRACK PRACK PRACK PRACK PRACK 200 OK 200 OK 00 OK 200 OK 200 OK 200 OK 200 OK 200 OK 200 OK 200 OK 200 OK 200 OK RTP Media RTP Media RTP Media FIGURE 3.3 Example of call flows with blank SDP. - Step 2: the terminating SBC receives the INVITE with no SDP offer and forwards it to the Called user agent following classical operations, i.e., including a SDP offer. Step 3: upon reception of 200 OK message from the c