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

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