SIP Configurable Route Recursion
When the Oracle® Enterprise Session Border Controller routes SIP requests from a UAC to a UAS, it might determine that there are multiple routes to try based on a matching local policy. The Oracle® Enterprise Session Border Controller recurses through the list of routes in a specific order according to your configuration and the quality of the match. There are other scenarios when a UAS replies with a 3xx Redirect response to the Oracle® Enterprise Session Border Controller, the 3xx response can include multiple Contacts to which the request should be forwarded in a specific order. In both cases, the Oracle® Enterprise Session Border Controller needs to recurse through a list of targets.
When the Oracle® Enterprise Session Border Controller receives a non-successful (or non-6xx response) final response from the UAS, and there are multiple targets for the original request, the Oracle® Enterprise Session Border Controller will forward the request to the next target and wait for a response. While the process of forwarding the request to multiple targets as explained in the previous paragraph is called serial forking, and the process of forwarding the request to contacts received in redirect responses is called recursion, the term recursion is used for both processes in this notice.
Use the SIP Route Recursion feature when you want the Oracle® Enterprise Session Border Controller to forward a response to the UAC and stop recursing through the target list immediately after receiving the 3xx, 4xx, or 5xx response code that you configure. When this feature is disabled, the Oracle® Enterprise Session Border Controller only stops recursing when it receives a message with a 401 or 407 response code. Using this feature, you can configure a specific message or range of messages to stop recursing on when received. The Oracle® Enterprise Session Border Controller retains its default behavior to stop recursing on a 401 or 407 response code when SIP Route Recursion is configured on a SIP interface. The Oracle® Enterprise Session Border Controller will always stop recursing when it receives a global failure (6xx); this behavior is not configurable.
You can disable response recursion for either a SIP interface or for a SIP session agent, providing you with flexibility for various network architectures. For instance, a PSTN gateway might be the only hop to reach a given endpoint, whereas several session agents might need to be contacted if multiple devices map to a contacted address of record.
Example 1
A more detailed example is when a softswitch might return a list of contacts for multiple PSTN gateways in a Redirect message. If the PSTN target number contacted on redirection is busy, a 486 response will be sent to the Oracle® Enterprise Session Border Controller. Since the single target is located in the PSTN, a subsequent request through a different gateway will yield another 486 response. The Oracle® Enterprise Session Border Controller should be configured to return the 486 response to the UAC immediately. No other SIP requests should be sent to applicable targets/contacts that were enumerated in the redirect list. See the following example:
Example 2
The Oracle® Enterprise Session Border Controller might determine from a local policy lookup that several routes are applicable for forwarding a SIP message. The Oracle® Enterprise Session Border Controller will try each route in turn, but the SIP response recursion disable feature can be implemented to stop the route recursion when a configured responses message is received by the Oracle® Enterprise Session Border Controller. See the following example: