IMG 1010 - Multiple SIP 183s prior to 200 OK
Overview:
The IMG supports receiving multiple SIP 183 responses prior to a 200 OK for an INVITE request, when SIP responses can potentially be received from multiple remote user agents. A proxy or an Application server that receives an INVITE from the IMG can then fork the request to multiple destinations.
The IMG will handle multiple inbound 183 if and only if the To header tags are different in each response. Multiple 18x responses are to be processed during the initial INVITE transaction only. A maximum number of five 18x responses will be accepted per call, with different To header tags.
Benefits:
Multiple 183 responses are used in Follow-me services, and request forking scenarios. Applications that can benefit from the IMG handling multiple 18x responses are:
Application Servers
Proxy Servers
Configuration
This feature is implemented by default and no configuration is required.
Feature Interaction: Comedia Mode
When the Comedia Mode is set to Passive, the outbound RTP packets are to be sent to the second RTP destination, even if the first destination continued to send RTP packets. If Comedia Mode is set to active, the IMG behavior is to be the same as if it were disabled.
See IMG 1010 - Symmetric NAT Traversal for more information on Comedia Mode.
Call Tracing
Call tracing will reflect SDP decoding and the overlap logic for multiple 18x responses.
Call Flows
Standard Behavior
The typical signaling call flow is shown in the diagram below. The initiating gateway can receive multiple 18x responses, each containing an SDP answer. This SIP behavior is defined in RFC 3261.
183 Followed by 180
If a SIP 183 with SDP (or a SIP 180 with SDP) is followed by a 180 (no SDP), the IMG will ignore the 180 and Local ring back will not be played.
Failure Scenario
If one of the subsequent 18x responses is syntactically incorrect or the SDP offer/answer overlap fails, the IMG will ignore the 18x response and will not send a SIP CANCEL request to the external gateway.
