Content for  TS 23.334  Word version:  18.1.0

Figure 6.2.20.1 shows a message sequence chart example for a terminating session setup with a dual stack UE using the SDP attributes "altc" inserted by the IMS-ALG in the SDP offer to indicate the IP addresses for potential alternative connection addresses.

Upon receipt of a session initiation request, the IMS-ALG shall extract the offerer's destination network address(es) and port number(s) from the SDP offer received from the calling party endpoint. Based on local policies, the IMS-ALG may decide to provide an alternative IP address and to insert then "altc" attributes in the SDP offer sent to the terminating side. The IMS-ALG shall then request the IMS-AGW to allocate transport resources for the terminating side (e.g. terminations T2 for IPv6 and T3 for IPv4) via two Reserve AGW Connection Point procedures, one per address type. Based on the responses received from the IMS-AGW, the IMS-ALG shall modify the offerer's destination addresses and/or ports initially contained in the SDP offer, insert the "altc" SDP attributes carrying the preferred and alternate IP addresses and propagate the session establishment toward the terminating party. On receipt of the SDP answer of the called party endpoint, the IMS-ALG shall pass the IP address, port and address type chosen by the called party endpoint for the session to the IMS-AGW in the Configure AGW Connection Point procedure and shall request the IMS-AGW to allocate transport resources (T1) for the calling party endpoint via the Reserve and Configure AGW Connection Point. Upon receiving the response from the IMS-AGW, the IMS-ALG shall modify the answerer's destination address(es) and/or port(s) contained in the application SDP answer and pass the information to the calling party endpoint. The IMS-ALG shall also request the IMS-AGW to release the transport resources (e.g. termination T3 with IPv4) reserved for the unused address type via the Release AGW Termination procedure.