About Overload Controls

Load Shedding occurs when a Diameter node (an MPE or MRA device) has insufficient resources to successfully process all of the Diameter requests that it receives. You can access Load Shedding Configuration controls from the MPE andMRA Advanced Configuration pages where you can configure rules for handling messages during overload conditions. Multiple congestion levels can be configured to accept, reject or drop selected messages at each level.

An MRA attempts to successfully process a message whenever possible using either Local or Remote Diversion:

Local Diversion: Selects an MPE device in the MPE pool to handle a new connection for a subscriber who is bound to a busy MPE device.
Remote Diversion: Selects an MRA device to handle a new connection for a subscriber who is bound to a busy MPE device. That MRA device creates a binding for the subscriber pointing to one of the MPE devices in the MPE pool.

MPE and MRA devices have configurable levels of congestion (busyness) for handling message overload. An MPE device has four congestion levels (Levels 1–4), and an MRA device has two congestion levels (Level 1 and 2). At each level you can define a default action for the level and create rules to handle specific message types. A level action is an action that is taken if none of the rules configured for the level match a message type. For example, for MPE and MRA Level 1, the default level action is Accept, which means to bypass load shedding rules instead of rejecting messages.

Note: When Local or Remote Diversion is not possible, the default result code is DIAMETER_TOO_BUSY. The NO_CAPACITY result code indicates an MRA server has a binding, but the MPE server it points to is currently overloaded, and the MRA server cannot perform local diversion to handle the request. The default result code is configurable.

An MRA device proactively rejects all messages destined for an overloaded MPE at all congestion levels. For example, if an MPE is configured to reject CCR-U messages at Level 2, the MRA device rejects the CCR-U message with DIAMETER_UNABLE_TO_COMPLY instead of forwarding it to the MPE device.

An MRA device subscribes to its pool of MPE devices for load notifications by issuing an LSR message after connection is established. It also subscribes to MPE devices in the backup MRA pool and to all other MRA devices in its association. MRA devices communicate their status using Load Notification (LNR) messages that include a Diversion-Status AVP to indicate whether that MRA device is available.

The Diversion-Status AVP indicates whether an MRA is available for diverting traffic to its MPEs (Remote Diversion). The diversion status is set to DIVERTABLE if none of the MPE devices in an MPE pool are overloaded. The status is set to NOT_DIVERTABLE if at least one MPE device in the MPE pool is overloaded.

The CMP system supports configuration of MPE load shedding rules on the MRA Advanced Configuration page. When you configure the admission rules for an MRA device to reject messages on behalf of an overloaded MPE, there may still be times when the MPE device responds to a message with DIAMETER_TOO_BUSY. In these cases, before forwarding the answer message, the MRA runs the original request through the MPE admission rules and updates the result code with the result code found in the MPE rules.