While the RSS has relatively good global coverage, RSOs are interested in deploying additional instances, particularly to areas that are considered “underserved”. An underserved area has a reasonably-sized user base that perceives poor performance of the RSS due to its closest root server instances being topologically distant, resulting in high latency or low availability of the service. This is a subjective designation and it is not reasonable to expect that a tool could directly measure it. The metric described in this section serves only to inform a decision on placing new root server instances in an underserved area. A RSO would require multiple measurements run in diverse geographical locations.
The goal of this metric is to assess performance of the RSS at a measurement point compared to other measurement points. A local metric that is much worse than others would be an indicator of an underserved area. Availability and latency are the primary measurements and the impact of slow links should be discarded. Since metric is trying to assess the performance of the RSS versus individual RSIs (Root Server Identities), only the best NRSI measurements should be used. The measurements are direct queries to the individual root servers (versus via a recursive resolver).
A set of measurements should be taken at random intervals during a T=30 minute time period. All RSIs will be directly queried and their latencies recorded. Additionally, a normalization factor σ will be measured as the average latency of direct DNS queries to the first NTLD=20 somehow-chosen TLDs. If fewer than NTLD normalization measurements are available, the entire set of measurements are thrown away. Availability of an RSI is the success rate of queries to an RSI over the last 20 measurement periods that were not thrown away.
The performance metric is the inverse of the average of the best NRSI=3 RSI latencies, where the availability for each of those RSIs is > p=0.65, divided by the normalization factor.
Availability of an RSI: percentage of successful responses over the past 20 measurements
Normalization factor: σ = mean latency of NTLD queries
Normalized Latency = (best NRSI latencies where availability > p) / σ
Performance Metric = 1 / Normalized Latency
If fewer than NRSI latency measurements are available (due to timeouts), the performance metric should be multiplied by Navail / NRSI, where Navail is the number of RSIs that have availability > p.
When comparing performance metrics among locations, the last 20 successful performance metrics should be averaged before comparison. The bottom P percentile among a large set of local performance metrics or performance metrics below a certain threshold (Θ) may be used to inform a determination of an underserved area. Determining P or Θ will require further analysis and will not be defined in this document.