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Proactive DNS Caching: Addressing a Performance Bottleneck

Proactive DNS Caching: Addressing a Performance Bottleneck. Edith Cohen AT&T Labs-Research. Haim Kaplan Tel-Aviv University. Talk Overview. Overview and Motivation DNS architecture DNS lookup latency Proactive DNS caching Renewal Policies Simultaneous Validation Conclusion.

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Proactive DNS Caching: Addressing a Performance Bottleneck

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  1. Proactive DNS Caching:Addressing a Performance Bottleneck Edith Cohen AT&T Labs-Research Haim Kaplan Tel-Aviv University SAINT ‘01

  2. Talk Overview • Overview and Motivation • DNS architecture • DNS lookup latency • Proactive DNS caching • Renewal Policies • Simultaneous Validation • Conclusion

  3. Domain Name System hostname IP-address • Essential for Internet name-based communication • Many-to-many mapping (virtual hosting, mirrors, aliases) • Distributed database maintained by a hierarchy of name-servers www.research.att.com 135.207.23.30

  4. resolving www.research.att.com ? ? Local Name-Server ? DNS Hierarchy

  5. Resolution may involve multiple remote name-servers DNS Lookup Client root • Root DNS server • returns NS for att.com • dnsprime.att.com • returns NS for research.att.com • ns0.research.att.com • returns IP-address for www.research.att.com dnsprime.att.com ns0.research.att.com

  6. Resolving Hostnames • Browser: if no answer in browser cache, query is sent to the local DNS server. • Name-server: use own cache. For missing info, iteratively query remote name-servers, while following referrals/ delegations.

  7. DNS Caching Mechanism • Data is stored in Resource Records (RR) • Each record has a TTL value (Time To Live) • TTL values are assigned by respective domain administrators. • Record may be cached and used only for TTL duration.

  8. Latency of DNS Lookups All requests > 60 sec after previous, ATT log

  9. Latency of DNS Lookups AltaVista referrals requests, ATT proxy log

  10. Issues with DNS Latency • RTTs to (several) remote name servers • Not addressed by fatter pipes, faster high-capacity content servers. • Highly sensitive to packet loss • Inconsistent - fraction of lookups suffer long/pathological delays • As Internet service improves, will increasingly become more noticeable.

  11. Passive DNS caching Used by BIND name-server software • Query remote NS only to answer a current client request • Cache (use) results till TTL expires

  12. Proactive DNS caching Guidelines: • Renewal Policies: auto-refresh entries just before TTL expires • Simultaneous Validation: Concurrently validate & use “expired” address Respect TTL values (be transparent to client) Minimize overhead to DNS servers Our Proposals:

  13. Methodology and Logs • Proxy logs • Simulate associated DNS cache • Separately-issued DNS queries obtain: TTL values, rate-of-change of IP-address.

  14. Renewal Policies - Issue a renewal query upon expiration. - Policy determines when to renew. - Tradeoff of overhead/reduced-latency. • R-LRUrenew r times past the most-recent cache hit • R-LFUgrant r additional renewals per hit ( TTL interval) • R-FIFOgrant r renewals at entry time to the cache • R-OPToptimal omniscient offline renewal policy

  15. Performance of Renewal Policies ATT proxy log

  16. Performance of Renewal Policies UC (NLANR) log

  17. Renewal Policies: Conclusions • R-LRU and R-LFU performed equally well across logs • R-FIFO did not perform as well • Reduction in misses corresponds to reduction in long DNS query times • More effective for more clients

  18. Renewal Policies: Implementation issues • Preferred Implementation: • within the name-server • Overhead control: • pre-expiration renewals (~1 RTT) • off-peak renewals

  19. Challenge: How do we benefit from valid expired addresses while still respecting TTL values. TTL vs. Rate-of-change • TTL values are set conservatively: Rate-of-change of addresses is significantly lower than TTL value. • So, when “expired” records are discarded, we often lose valuable and valid information

  20. Simultaneous Validation • Keep expired address records. • When a client request arrives, concurrently: • Initiate a connection to host, using expired IP-address, and start fetching content • Issue a validating DNS query • If validation is successful, serve the content to the client

  21. Client DNS GAIN Web Server Web Server SV Latency Gain Client DNS • DNS lookup • session with Web server: • Establishing TCP connection(s), sending HTTP request(s), ...

  22. Simultaneous Validationsuccess rate

  23. Simultaneous Validation:deployment issues • browser or proxy • requires maintenance of a separate name-to-address cache • single-entity implementation • name-server (using its internal cache) • requires protocol support for 2-phase resolutions • requires separate proxy or browser support • SV is more effective for a larger user base.

  24. Summary • DNS lookup delays can be addressed by increasing the local availability of RRs • Renewal Policies • incur overhead of additional queries • limited deployment is effective • inter-request-time < c * TTL • Simultaneous Validation • minimal overhead • more involved implementation • inter-request-time < IP-address-lifetime

  25. Future Work • Large, local, hostname database + SV • Co-operative DNS caching • SV and Renewal at the RR level • Combine SV and Renewal

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