Broadsoft Extreme Overload Controls27 May 2010 #broadworks #sbc #sip
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.
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 To reduce the chances that a user’s registration is no longer valid (and cannot receive or possibly make calls), the extension should be set based on expected overload time, which is typically at least twice the non-callp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.
Extreme Overload Controls are an important element for all Broadworks deployments. Setting the proper thresholds between the
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.