The IMS AAA Server maintains routing rules for each type of request. When the IMS AAA Server receives a request, it examines the request to determine what type of request it is:
Once the server determines the type of request, it consults the respective routing rules to determine how to route the request.
Figure 17 shows each of the routing rules maintained by the IMS AAA Server, and the Remote Network Element function to which the associated request type is routed.
The IMS AAA Server supports both explicit and implicit routing.
Certain network element functions require routing rules to be configured along with the function. These routing rules are called Implicit routing rules. For instance, when you assign the HSS function to a Diameter Remote Network Element, you need to specify which subscribers are served by the HSS. This is done by assigning the HSS function and configuring the implicit routing in the function configuration.
Implicit routing is usually based on subscriber identity or realm. In 3GPP WLAN networks, the subscriber identity is the IMSI of the mobile device (WLAN UE).
The functions that require you to configure implicit routing include the Downstream server function (supported in both RADIUS and Diameter) and the HSS, CDF and WAG (Diameter) functions.
Explicit routing rules provide greater flexibility for routing requests when implicit routing is not enough. To define an explicit routing rule, the network administrator defines a condition or set of conditions and a target. You then define the order in which you want the explicit routing rules to be evaluated. During the routing rule evaluation process, explicit routes are always evaluated before implicit routing rules. If a request matches all conditions defined by an explicit routing rule, the request is routed to the target specified in the routing rule.
Routing Rule Evaluation Process
Request routing rules are evaluated in the following order:
- Explicit routing rules
- Implicit routing rules
When a request is received, the IMS AAA Server identifies the request type and then evaluates the associated routing rules. First, the server evaluates any explicit routing rules that are defined for the request type. The order in which explicit routing rules are evaluated is user-defined. If no match is found, the server evaluates the implicit routing rules defined for the request type.
When a match is found, the IMS AAA Server processes the request by routing it to the specified destination. In some cases this may mean simply forwarding the request to the destination. In other cases, for example if the HSS is listed as the destination, the IMS AAA Server would consult the HSS for the subscriber credentials and then take the appropriate action.
If no routing rule match is found, the IMS AAA Server takes the appropriate action according to the respective RFC. For example, if the request is from a RADIUS based device, no response is provided, the request is simply discarded. If the request is Diameter based, a response with an appropriate result code is sent to the requestor.