Multiple SIP 183s prior to 200 OK

There are two types of proxies, stateful and stateless. The stateless proxy acts simply as a forwarding element that forwards  each request to a single network element determined by routing decision and then discards the information. A stateful proxy remembers information about each incoming request and any requests it sends as a result of processing the incoming request.  It uses this information to affect the processing of future messages associated with that request which speeds up call processing. A stateful proxy may choose to "fork" a request, and route it to multiple destinations. Any request that is forwarded to more than one location must be handled statefully.

Because multiple 183 Session Progress message can be transmitted or forked from a stateful proxy, the IMG 2020 supports receiving the multiple 183 Session Progress messages which all come in prior to receiving a 200 OK message. In this case, each of the 183 Session Progress messages if and only if the To: header tags are different in each response. Multiple 18x responses can 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. The Benefits to the 2020 IMG handling multiple 183 messages before receiving a 200 OK is this behavior is used in Follow Me services and request forking scenarios. Both Proxy Servers and Application Servers can benefit from this functionality.

Note: Feature Interaction: Comedia Mode - When Comedia Mode field in the IP Bearer Profile is set to Passive, the outbound RTP packets are to be sent to the second RTP destination even though the first destination continues to send RTP packets. If Comedia Mode is set to active, the 2020 IMG behavior is the same as if it were disabled. See Symmetric NAT Traversal for more information on Comedia Mode.


Call Flows

Standard Feature 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

Case of a SIP 180 (no SDP) followed by a 183 with SDP (or a 180 with SDP). The 2020 IMG is to play local ring back until an SDP is received and open a media path with remote RTP connection.




Case of a SIP 180 with SDP followed by a 183 with SDP (or a 180 with SDP). The IMG is to behave as if two 183 responses with SDP had been received.



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.




This feature is implemented by default and no configuration is required.


Additional Information