applies when IMS-ALG1 allocates a TrGW during forwarding of the SDP offer for proactive transcoding with resource reservation, another IMS-ALG (IMS-ALG2) is the last signalling entity on the path before UA2, IMS-ALG2 does not need to allocate a TrGW to provide NAT, and UA2 signals in the SDP answer that transcoding is not required. There may be additional IMS-ALGs between IMS-ALG1 and IMS-ALG2.
A second SDP offer/answer transaction is required to remove the transcoder from the path. The scenario allows IMS-ALG2 to initiate the second SDP offer/answer transaction rather than requiring IMS-ALG1 to do this, thus saving some messaging.
As an alternative to the procedure shown, the IMS-ALG1 may forward connection information for a prior user plane segment without transcoding options while including an IP realm instance for the transcoding TrGW. This alternative avoids a second SDP offer/answer transaction if transcoding is not required, but does include a second SDP offer/answer transaction if transcoding is required.
IMS-ALG1 receives an SDP offer from realm R1. The SDP offer can include IP realm instances Rx associated with prior user plane segments and can include a visited realm instance for incoming realm R1.
IMS-ALG1 allocates TrGW1 to offer transcoding options to UA2.
IMS-ALG1 forwards connection information for TrGW1 in the SDP offer along with prior IP realm instance information Rx and visited realm instances for both its incoming and outgoing user plane segments. The visited realm instance for the outgoing side includes information about the codec changes associated with TrGW1.
IMS-ALG2 does not allocate a TrGW since its incoming and outgoing realms are compatible.
IMS-ALG2 forwards the SDP offer with no changes.
UA2 selects one of the original codecs, i.e., transcoding is not needed. In response to the SDP offer, UA2 sends an SDP answer to IMS-ALG2 with connection information for its address in realm R1.
IMS-ALG2 determines that transcoding is not necessary and sends a second SDP offer with the connection information from the visited realm instance for a prior user plane segment. This SDP offer includes those IP realm instances that remain options without transcoding.
UA2 updates its remote connection information and responds with a new SDP answer.
IMS-ALG2 determines that the prior transcoder is to be bypassed.
IMS-ALG2 forwards the SDP answer to IMS-ALG1 after including an IP realm instance for the UA2 address in realm R1. The connection information in the forwarded SDP answer cannot be used by the receiving IMS-ALG to establish this segment of the media path.
IMS-ALG1 de-allocates transcoder TrGW1 since there is no valid connection information available in the SDP answer.
IMS-ALG1 forwards the SDP answer after modifying the connection information to correspond to the IP address of UA2 in realm R1.
A user plane connection is now established in realm R1.