Downstream Forking Support

With the implementation of the following enhancements, SBC supports downstream forking call flows when dialog transparency is enabled.

• SBC supports multiple early dialogs at ingress leg when dialog transparency is enabled. It relays each forked 18x response received for the INVITE at egress leg towards the ingress leg by transparently passing the Call-ID, "From" tag, and "To" tag thus exposing multiple early dialogs at ingress leg.

  The offer/answer negotiation on a forked call depends on the Packet Service Profile configured on either legs. As a result, SDP sent in the forked 18x response(s) towards ingress leg may vary depending on whether pass-through mode or transcoding mode is selected by SBC. SBC continues to use the same media IP/Port that is sent in the first 18x SDP for all the forked 18x messages sent towards the ingress leg. Before sending a forked 18x response towards ingress leg, SBC sends UPDATE on the previous early dialog with SDP that contains "a=inactive" line. This is performed to prevent ingress peer from sending media on all the early dialogs exposed by SBC. With this call flow, media is expected only on the last active early dialog exposed by the SBC.

• SBC supports UPDATE with preconditions on each of the early dialogs exposed on the ingress leg and relays the same to the corresponding early dialog on the egress leg.
• SBC supports 199 response from the egress leg for an early dialog identified by the "To" tag. SBC removes the corresponding early dialog context after receiving 199 response and also relays the same to the ingress leg if configured. In this scenario, only the corresponding early dialog is removed and the call remains as it is.
• SBC supports BYE request within an early dialog identified by "To" tag. SBC removes the corresponding early dialog context after receiving a BYE request and also relays the same on the other leg.
• Currently, SBC supports CANCEL request for a regular call by tearing down the call and forwarding the CANCEL to the egress leg. With this feature, all the early forked dialogs are also cleared internally.

The following diagram is an example call flow scenario for downstream forking with dialog transparency.

Downstream forking with dialog transparency call flow