DNS Performance and the Effectiveness of Caching

This is a study of DNS in the wild, using traces of DNS requests and responses leaving two adminstrative domains along with TCP connections and disconnections. They found typical lookup performance was better than the '88 paper, but, still, many lookups would take an exceptionally long time from a user's perspective, apparently due to network or configuration problems. About 20% of all lookups got no answer (and no error repsonse) providing both high lookup latency and a probably uncachable negative result. Around 4% of the unanswered lookups were authority forwarding loops — an obvious misconfiguration. Some popular negative responding lookups were found to be common misconfigurations (e.g. asking the root servers to resolve 'loopback' or 'loghost').

The authors of the paper, however, were not mostly concerned with misconfiguration but with the effect of caching and TTL settings. The authors generally found that caching worked well for nameserver records, so most queries required few referrals (each of which is an extra round-trip to a server), substantially decreasing the latency of lookups. The authors found that the names requested followed a Zipf-like distribution and therefore most of the benefit of caching was obtained by caching relatively few names. They also found, based on the TCP trace, that because accesses to the same domain were highly clustered, short TTLs (popular for load balancing), did not cause substantial increases in the number of DNS queries or user-perceived latency.