Performance evaluation of redirection schemes in content distribution networks
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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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Performance Evaluation of Redirection Schemes inContent 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


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


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


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


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)


Effects of Bandwidth

  • After 1.5 Mbps, download time mostly based on RTT

  • Study only RTT and loss rate


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


Test Files

  • 100 pages from Hot100.com

  • Typical homepage 20 KB + 50 KB


Loss Free Network, Baseline

  • Big gains possible, up to 90%


Loss Free Network, Switch

  • Parallel connections, Medium page

  • Gains limited, maximum 30%


Loss in Network, Baseline

  • Big gains still possible, up to 85%

  • Not as good as no-loss case (slow-start)


Loss in Network, Switch

  • Parallel connections, Medium page

  • Maximum gains 20% (slow-start, loss of SYN)


Experiments

  • Repeat simulated experiment on Internet

  • Exclude DNS lookups

  • Results confirm results from simulations


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


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


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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