Optimizing the UNRESPONSIVETHRESHOLD value for your network

Once every monitoring interval, the resolver calculates the percentage of queries to a name server that failed in the previous 30-seconds or 5 minutes, and then compares this percentage to the threshold value that you set in the UNRESPONSIVETHRESHOLD statement to determine whether that DNS name server is unresponsive. If the resolver sends a query to a name server multiple times and the name server does not respond to multiple queries, each query is considered to be a unique failure to respond. When you specify the UNRESPONSIVETHRESHOLD value, consider the following factors that have an impact on the effectiveness of your setting:

• If you specify a small percentage for this value, an excessive number of operator notifications might occur. Short network disruptions that occur during the 30-second or 5-minute monitoring interval might result in some undeliverable resolver queries or name server responses, and a low threshold value might cause the resolver to alert the operator, and possibly stop using the name server, unnecessarily.
• If you specify a large percentage for this value, persistent issues with the network or the name server might be undetected even though a significant portion of resolver queries are not being processed by the name server.
• The setting on the RESOLVERTIMEOUT statement in the TCPIP.DATA file also affects the value that you should specify for the UNRESPONSIVETHRESHOLD setting. If you set a very short timeout value, even slight network disruptions might cause name server responses to be delayed longer than the amount of time specified by the RESOLVERTIMEOUT value. These delays are considered to be non-responses from the name server, which might cause unnecessary messages to be generated for this name server. A less aggressive (higher) percentage setting for the UNRESPONSIVETHRESHOLD value might be warranted in such a situation.
• The settings of the RESOLVERUDPRETRIES, SEARCH, and NAMESERVER statements in the TCPIP.DATA file can also contribute to high numbers of apparent failures on the part of the name server. See Examples of resolver monitoring of DNS name servers for information about how these settings can influence the statistics that are collected by the resolver.

One strategy that you can use to select the most optimal threshold value is to start with the default setting, which is 25%, and determine how many network operator messages are issued, if any, during normal operation of the network.

• If your network is operating in an acceptable manner (for example, no performance issues are detected and no host name or IP address resolutions delays are detected), examine the number of network operator alerts that are generated by the resolver:
  • If the number of network operator messages is zero or insignificant, leave the setting at the default value, or even decrease the threshold value slightly.
  • If the number of network operator messages is excessive, which suggests that a lot of false negative conditions were detected by the resolver, increase the threshold setting until the number of messages that is generated is appropriate for your network.
• If the name server is now responsive, but the failure rate is just slightly below the threshold value, the name server will probably become unresponsive again with a minor disruption in the network. If your network is currently operating in a satisfactory manner, consider increasing the threshold setting so that the resolver issues EZZ9308E messages only when your network conditions change significantly. Use the statistics that are displayed when message EZZ9309I is issued to modify the threshold setting to a more optimal value.
• If your network is experiencing performance issues that resolver delays might be contributing to (for example, unexplained application delays), consider decreasing the responsiveness threshold setting to determine whether issues with the name servers are being detected by the resolver but are not being reported as unresponsive. If this lower threshold value causes the resolver to generate network operator messages that identify name servers that are unresponsive and that are impacting network operations, consider using this lower value for normal operations to provide more timely identification of name server issues.

A second strategy that you can use to select the most optimal threshold value is to start with the lowest threshold setting, which is 1%. If your name servers are failing to respond to a small percentage of the overall resolver queries that are being sent, the resolver generates EZZ9308E messages. At 5-minute intervals, the resolver also generates EZZ9310I messages, which indicate the percentage of failures for the most recent 5-minute sliding window. Use the EZZ9310I messages to determine the highest failure rate during normal operation of the network, and then set the threshold value to that rate, or to a value slightly above that rate. For example, if the highest failure percentage displayed on the EZZ9310I messages is 4%, set the threshold value to 5% for your network. This value ensures that the resolver considers name servers to be unresponsive only when they experience a failure rate that is greater than the rate that typically occurs in your network.