The Contribution of Dns Lookup Costs to Web Ob Ject Retrieval

There are a number of factors that contribute to the performance between clients and servers in the World Wide Web. In this work we have concentrated on the contribution of DNS lookup to the overall Web object retrieval time. We found that the DNS mechanism performed better for popular Web servers than for random Web servers. Performance was better both in terms of local cache hit rates, which would be expected, but also for authoritative and non-local, non-authoritative response times. We also found that the DNS lookup time contributed more than one second to approximately 20% of retrievals for the Web objects on the home page of a larger list of popular servers. While these numbers are signi cant, they do not re ect the expected access patterns for URLs. Results from real user request log show that only about 20% of DNS requests are not cached locally. If 20-30% of these non-cached lookups take more than one second then only 4-6% of all DNS lookups should take more than one second. These numbers appear to be less of a problem for overall Web retrieval, particularly if one considers that many DNS lookups resulting from small DNS TTL values do not yield di erent IP addresses for successive requests.