WebSphere Application Server Network Deployment, Version 6.1
             Operating Systems: AIX, HP-UX, i5/OS, Linux, Solaris, Windows

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SIP overload protection

Session Initiation Protocol (SIP) overload protects the system from two overload conditions: memory overload and CPU overload. Container managed overload protection (CMOP) and proxy managed overload protection (PMOP) allow for real-time protection based on the overload settings information.

SIP container managed overload protection

In a stand-alone server deployment, CMOP provides protection against memory and CPU overload conditions. An administrator can set up several static thresholds using SIP container settings. When these thresholds are exceeded, the container begins to discard new requests by responding to requests that initiate new dialogs with a 503 response until the container is no longer overloaded. CMOP includes settings that affect both memory and CPU usage.

In a network deployment environment, CMOP allows the container to use the SIP container settings to notify the proxy server when the container is in an overloaded state. Then the proxy server begins to discard new requests instead of routing them to the containers. CMOP prevents all memory overload conditions in a network deployment environment regardless of the configuration.

Overload protection is calculated based on the virtual memory settings and the maximum throughput that a container can handle. You can specify a value for the following SIP container settings in the administrative console for CMOP. See the information center topic about SIP container settings.
  • Maximum application sessions
  • Maximum messages per averaging period
  • Maximum response time
  • Maximum dispatch queue size
You can specify a value for the following SIP container custom properties for CMOP. See the information center topic about SIP container custom properties.
  • thread.message.queue.max.size
  • weight.overload.watermark

SIP proxy managed overload protection

PMOP is optimal way to avoid container overload. In a typical deployment, CMOP, alone, does not provide optimal results. The following conditions might occur when CMOP is deployed at the proxy server without deploying PMOP.
  • The on or off mechanism might be too granular.
  • Admission rates might fluctuate.
  • It might be difficult to establish an absolute cap on load.
  • Unstable loads might be sent to the containers.

When PMOP is deployed, the proxy server utilizes admission rate controllers for each container. When a container is overloaded, instead of accepting or discarding new requests for a period of time, new requests are sent to the container without completely stopping the flow of new traffic. PMOP allows the proxy to offer a consistent load to the containers without exceeding the maximum values for the container settings.

The SIP proxy server calculates a maximum value for message throughput to each container based on a percentage of the configured Maximum messages per averaging period (MMAP) setting specified for each container. The maximum value for message throughput is called the Maximum throughput factor (MTF).

The MTF custom property is disabled by default, and it can only be enabled using the maxThroughputFactor custom property in the SIP proxy settings. The value specified for the MTF custom property must be less than 100 percent to prevent the CPU of the container. For example, you might set this value to 90 percent. When the MTF value is set to less than 100 percent, the proxy server prevents the total throughput to the container from exceeding the maximum value specified for the MMAP setting.

The MTF value is specified when stable and accurate overload protection is required. Specifying the MTF value provides optimal results for loads that range up to twice the capacity of the system. Consider your system capacity when configuring for overload protection.

The algorithm utilized by the SIP proxy relies on the following features to offer steady and predictable loads to the container.
  • Per-server rate control managed at the proxy
  • Auto-adjusting, per second admission rate controller
    • Ratio of in-dialog to non-dialog averages used to control rate
    • Auto-rate reduction when in an overloaded state
  • Ability to absorb fast load transitions
  • Burst tolerance (BT) to allow for short occasional load burst without triggering overload
  • Stabilization control to prevent excessive overload when there is a transition in the cluster
You can specify a value for the following SIP proxy server custom properties to configure PMOP. See the information center topic about SIP proxy server custom properties.
  • burstResetFactor
  • deflatorRatio
  • dropOverloadPackets
  • inDialogAveragingPeriod
  • maxThroughputFactor
  • outDialogAveragingPeriod
  • perSecondBurstFactor
  • proxyTransitionPeriod
  • sipProxyStartupDelay



Related tasks
Setting up the proxy server
Related reference
SIP container settings
[AIX HP-UX Linux Solaris Windows] [i5/OS] SIP proxy server custom properties
[AIX HP-UX Linux Solaris Windows] [i5/OS] SIP container custom properties
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Last updated: Feb 25, 2009 9:32:38 AM CST
http://publib.boulder.ibm.com/infocenter/wasinfo/v6r1/index.jsp?topic=/com.ibm.websphere.nd.multiplatform.doc/info/ae/ae/cjpx_sippxovldpro.html