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Performance Evaluation of Redirection Schemes in Content Distribution Networks

Performance Evaluation of Redirection Schemes in Content Distribution Networks. Jussi Kangasharju, Keith W. Ross Institut Eurecom. Jim W. Roberts France Telecom R&D. Outline. Content Distribution Networks Redirection Schemes Simulations Experiments Discussion Future Work Conclusion.

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Performance Evaluation of Redirection Schemes in Content Distribution Networks

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  1. Performance Evaluation of Redirection Schemes inContent Distribution Networks Jussi Kangasharju, Keith W. Ross Institut Eurecom Jim W. Roberts France Telecom R&D

  2. Outline • Content Distribution Networks • Redirection Schemes • Simulations • Experiments • Discussion • Future Work • Conclusion

  3. Content Distribution Networks • Content distribution on the Web is changing • Content placed on content servers • Clients redirected to nearby content servers • Better performance for clients • Load balancing on content servers • Better fault tolerance

  4. Redirection Schemes • Full redirection • All client requests directed to content servers • Advantage: Simple • All content servers must be able to serve any content -> full mirrors or proxies • Selective redirection • Only some requests redirected • Need to select what content to serve from content server

  5. DNS Redirection • Currently CDNs use DNS redirection • Two strategies • CDN controls the DNS resource records of the origin server • References to objects are changed to point to a content server • First strategy = Full redirection • Second strategy = Selective redirection

  6. Simulation Model • Use the NS network simulator • Simple model: Client, Server, and Link • Vary RTT, bandwidth, and loss rate on link • Client requests HTML file, then requests images • Look at user-perceived performance (total time) • Does not account for server load (yet)

  7. Effects of Bandwidth • After 1.5 Mbps, download time mostly based on RTT • Study only RTT and loss rate

  8. Different Clients • Two different clients: Pipelining and parallel • Assumption: Parallel client cannot retrieve all objects with one set of connections • Baseline: Retrieve all from origin server • RTTs: 10, 20, 60, 100, 120, and 160 ms • Does not include DNS lookup delay • Assume parallel connections to be independent

  9. Test Files • 100 pages from Hot100.com • Typical homepage 20 KB + 50 KB

  10. Loss Free Network, Baseline • Big gains possible, up to 90%

  11. Loss Free Network, Switch • Parallel connections, Medium page • Gains limited, maximum 30%

  12. Loss in Network, Baseline • Big gains still possible, up to 85% • Not as good as no-loss case (slow-start)

  13. Loss in Network, Switch • Parallel connections, Medium page • Maximum gains 20% (slow-start, loss of SYN)

  14. Experiments • Repeat simulated experiment on Internet • Exclude DNS lookups • Results confirm results from simulations

  15. Discussion • Switching servers limits performance: • Either new server is not fast enough or the client should have used it for all the objects • Easy with full redirection, can be done with selective redirection, but have to be careful • Persistent connections • No server load -> switching degraded • No redirection cost -> switching improved

  16. Future Work • Improve simulation topology and model • Take into account server load in simulations • Include cost of redirection • Perform more extensive experiments • Evaluate the server selection schemes of existing CDNs

  17. Conclusion • Evaluate effects of redirection on user-perceived performance • Both simulations and experiments • Switching servers during download of a page hurts performance • Need to replicate Web pages with images to guarantee best performance

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