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CDNs & Replication

This resource examines how Content Delivery Networks (CDNs) and caching techniques can significantly improve HTTP performance. It explores the integration of forward and reverse caching, highlighting how CDNs like Akamai operate to cache content closer to users. Topics include cache management, load distribution, and strategies for dynamic web page processing. The document further discusses challenges in replication and caching, presenting various solutions for load balancing across multiple servers and how to handle dynamic responses effectively.

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CDNs & Replication

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  1. CDNs & Replication Prof. Vern Paxson EE122 Fall 2007 TAs: Lisa Fowler, Daniel Killebrew, Jorge Ortiz

  2. Improving HTTP Performance:Caching w/ Content Distribution Networks • Integrate forward and reverse caching functionality • One overlay network (usually) administered by one entity • e.g., Akamai • Provide document caching • Pull: Direct result of clients’ requests • Push: Expectation of high access rate • Also do some processing • Handle dynamic web pages • Transcoding

  3. Improving HTTP Performance:Caching with CDNs (cont.) Server CDN Backbone ISP ISP-1 ISP-2 Forward proxies Clients

  4. Improving HTTP Performance:CDN Example – Akamai Akamai: • Creates new domain names for each client content provider • e.g.,i.a.cnn.net = CNAMEcustom.i.cnn.net.edgesuite.net custom.i.cnn.net.edgesuite.net = CNAME a1921.g.akamai.net Customer/Content Provider: • Modifies content • Embedded URLs reference new domains • CNN page’s HREF’s refer to i.a.cnn.net • Pushes content out to Akamai as it changes • Or: Akamai pulls it on demand w/ usual caching mech. • Both CNN & Akamai have control over load distribution

  5. lookup a1921.g.akamai.net Improving HTTP Performance:CDN Example – Akamai akamai.net DNS servers cnn.com “Akamaizes” its content. Akamai servers store/cache secondary content for “Akamaized” services. a b DNS server for cnn.com c local DNS server GET http://cnn.com 1 - DNS Lookup 2 - Fetch page w/ “Akamaized” content 3 - DNS Lookup for Akamai URLs 4 - Fetch content “Akamaized” response object has inline URLs for secondary content at (after resolving CNAMEs) a1921.g.akamai.net and other Akamai-managed DNS names.

  6. Improving HTTP Performance:Caching vs. Replication • Why move content closer to users? • Reduce latency for the user • Reduce load on the network and the server • How? • Caching • Replicate content “on demand” after a request • Store the response message locally for future use • Challenges: • May need to verify if the response has changed • … and some responses are not cacheable • Replication • Planned replication of content in multiple locations • Update of resources handled outside of HTTP • Can replicate scripts that create dynamic responses

  7. Hosting: Multiple Machines Per Site • Replicate a popular Web site across multiple machines • Helps to handle the load • Places content closer to clients • Helps when content isn’t cacheable by proxies/CDNs • Problem: Want to direct client to a particular replica • Why? • Balance load across server replicas • Pair clients with nearby servers • Solution #1: Manual selection by clients • Each replica has its own site name • A Web page lists the replicas (e.g., by name, location) • … and asks clients to click on a hyperlink to pick

  8. Hosting: Multiple Machines Per Site • Solution #2: single IP address, multiple machines • Run multiple machines behind a single IP address • Ensure all packets from a single TCP connection go to the same replica Load Balancer 64.236.16.20

  9. Hosting: Multiple Machines Per Site • Solution #3: multiple addresses, multiple machines • Same name but different addresses for all of the replicas • Configure DNS server to return different addresses 12.1.1.1 64.236.16.20 Internet 173.72.54.131

  10. Hosting: Multiple Sites Per Machine • Multiple Web sites on a single machine • Hosting company runs the Web server on behalf of multiple sites (e.g., www.foo.com and www.bar.com) • Problem: GET /index.html • www.foo.com/index.html or www.bar.com/index.html? • Solutions: • Multiple server processes on the same machine • Have a separate IP address (or port) for each server • Include site name in HTTP request • Single Web server process with a single IP address • Client includes “Host” header (e.g.,Host: www.foo.com) • Required header with HTTP/1.1

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