Broadsoft Extreme Overload Controls

The BroadWorks platform uses an enhanced algorithm called Overload Control to offer protection when a cluster node is under severe conditions. The goal is that during an overload period of 150% the throughput will be no less than 90%.

Behavior of the Application Server is dependent upon a series of configuration parameters configured through bwcli. Extreme overload control provides message throttling at the decoding and encoding queues. The maximum packet age used during overload is different from that used during non-overload. Both the maximum packet age and maximum packet age during overload are configurable via the CLI.

Example:

AS_CLI/System/OverloadControls> get
enabled = true
mgcpOverloadAction = error
sipOverloadAction = error
percentMemoryInUseToEnterYellow = 85
percentMemoryInUseToEnterRed = 90
percentMemoryInUseToLeaveYellow = 85
percentMemoryInUseToLeaveRed = 85
allowEmergencyCallsInOverload = true
maxPacketAgeInMsecs = 3000
maxPacketAgeDuringOverloadInMsecs = 1500

One of the most critical parameters to configure is the sipOverloadAction. This determines how Broadworks will respond during a period of an Overload. The three possible options are:

• drop (silently discarded)
• redirect (302 Move Temp)
• error (503 Service Unavailable)

I do not recommend using drop as this is the least graceful approach and will cause the User Agent Client to rely on its SIP expiration timer before initiating additional SIP messages. While error should be a viable option it does not guarantee the UAC will reattempt another SIP invite upon receiving a SIP 503 Service Unavailable message. My recommendation and current best-practice is to use redirect. The Broadworks Application Server (AS1) will respond to the UAC with the address of the secondary Application Server (AS2) in the SIP contact header. The remaining dialog between the UAC and UAS will be carried out through AS2.

** NOTE: Registration Time Extension

During overload a SIP REGISTER messages may be discarded. To reduce the chances that a user’s registration is no longer valid (and cannot receive or possibly make calls), the extension should be set based on expected overload time, which is typically at least twice the non-callp minimum time in zone.

** Emergency Calls

If the system is configured to allow emergency calls during overload, then originations matching the call type Emergency in the system calling plan are allowed to progress.

** Summary

Extreme Overload Controls are an important element for all Broadworks deployments. Setting the proper thresholds between the Yellow and Red Zones will provide the appropriate alarming and graceful call redirection while at the same time notifying an administrator when the platform is reaching its capacity for growth.