King : A tool to estimate latency between Internet hosts

1. King : A tool to estimate latency between Internet hosts Krishna Gummadi Stefan Saroiu Steven D. Gribble

2. Goal C • Why is this useful? • Closest Server Selection • Distributed P2P Systems • Building topologically sensitive overlay networks (CAN, Chord, Gnutella, …) Measure the latency between any two IP addresses, from any point in the Internet Internet A B

3. Outline • Motivation • Techniques to predict network distances • Our DNS-based technique (King) • Evaluation of our tool • Conclusions

4. Related Techniques • Techniques to estimate the latency between any two arbitrary hosts • IDMAPS (U.Mich) • GNP (CMU) • Drawbacks of existing schemes • Require deployment of additional infrastructure in the Internet • They make assumptions about the topology of the Internet which don’t hold in practice • For example, triangle inequality

5. King: a DNS-based latency tool • Insights: • We can use recursive DNS queries to deduce latency between DNS servers • Most Internet hosts are located close to their DNS servers • Thus, we can approximate latency between the hosts as latency between their name servers • Later in this talk, we’ll show when this trick works, and when it fails

6. Resolve xyz.mit.edu using “Recursive” DNS Query My Machine DNS xyz.mit.edu Time taken to resolve recursive query xyz.mit.edu to Berkeley NS is T = t1 + t2 DNS abc.berkeley.edu

7. Resolve abc.berkeley.edu My Machine DNS xyz.mit.edu Time taken to resolve query abc.berkeley.edu to Berkeley NS is T’ = t1 DNS abc.berkeley.edu

8. How King Works ? My Machine DNS xyz.mit.edu DNS Latency between Name Servers, t2 = T- T’ Latency between Hosts ~ Latency between Name Servers abc.berkeley.edu

9. Potential Limitations of King • At least one of the authoritative name servers of either host has to support recursive queries • >75% of name servers support such queries • Multiple authoritative name servers that are geographically distributed • Name Servers tend to be better connected than End Hosts. • Our estimates tend to ignore the large last hop latencies for DSL/cable/modem users

10. Evaluating the Accuracy of King • Directly measure latencies between 50 public traceroute servers and 50 end hosts • Estimate latencies between the same servers/hosts using King and IDMaps. • Compare • Metric used : Estimated Latency Measured Latency

12. Additional evaluation • DNS servers tend to be close to IP addresses in practice • far less than 10 milliseconds, >80% of the time (although modems are trouble) • King tends to preserve order • Often more important than accuracy! • Can choose between accuracy and additional measurement costs • “tune” the tool for various applications • Can even measure latency between hosts that are offline or do not respond to ICMP ping messages !

13. Summary • We presented King, a DNS based tool to estimate latency • King works in the Internet as it exists today. • No need for any additional infrastructure. • We showed that King is fairly accurate for estimating latency between internet hosts

14. Future Work • Incorporating King into Internet services • e.g., closest server selection • Wide-area measurement studies using King • Generating a latency map of Gnutella overlay • Evaluating the efficiency of Akamai’s server selection • More extensive validation of Detour study

15. Questions ? More Info: www.cs.washington.edu/homes/gummadi/king