Figure 5 shows the cumulative distribution of lookup latencies incurred by CoDoNS and the legacy DNS. Table 4 summarizes the results of Figure 5 by providing the median, mean, and the 90th percentile of the latency distribution. We aggregate the latency during the second half of the workload, allowing the first half to warm the caches of both CoDoNS and the legacy DNS. Note that the second half of the workload also contains DNS requests for domain names not present in the cache, and CoDoNS incurs the extra latency of redirecting these queries to the legacy DNS. In order to study the impact of legacy DNS redirections on latency, we separately evaluate the lookup performance of CoDoNS without redirections, by inserting the records at their home nodes before applying the work load. This study essentially evaluates the scenario after a complete take over of the legacy DNS by CoDoNS.
50% of the queries in CoDoNS are answered immediately by the local CoDoNS server without incurring network delay, since proactive replication pushes responses for the most popular domain names to all CoDoNS servers. Consequently, CoDoNS provides a significant decrease in median latency to about 2 milliseconds compared to about 39 milliseconds for the legacy DNS. The tail of the latency distribution indicates that cache misses leading to legacy DNS lookups have an impact on the worst-case lookup performance of CoDoNS. However, a complete take over from the legacy DNS would obviate the extra latency overhead. Overall, CoDoNS achieves low latencies in the mean, median, and the 90th percentile, for both deployment scenarios, with and without redirections to the legacy DNS.
Figure 6 shows the median latency of CoDoNS and the legacy DNS over time. The fluctuations in the graph stem from the changing relative popularities of names in the workload over time. CoDoNS reacts to these changes by continuously adjusting the extent of proactive caching. Initially, CoDoNS servers have an empty cache and redirect most of the queries to legacy DNS. Consequently, they incur higher latencies than the legacy DNS. But as resource records are fetched from legacy DNS and replication in the background pushes records to other CoDoNS servers, the latency decreases significantly. The initial surge in latency can be easily avoided by bootstrapping the system with records for well known domain names.