crit_think_intro classroom slides class notes

crit_think_intro classroom slides class notes

1. C o n t e n t D e l i v e r y N e t w o r k s ( C D N )

2. C o n t e n t D e l i v e r y N e t w o r k s ( C D N ) What : Geographically distributed network of Web servers around the globe (by an individual provider, E.g. Akamai). Why: Improve the performance and scalability of content retrieval. How: Allow several content providers to replicate their content in a network of servers.

3. W e b B r o w s e r C a c h i n g Web browsers have their own caches. When a page is downloaded from a site the web page is put into the browser cache. This is especially useful in those cases when the back button is pressed. If a new copy is needed then a “ refresh ” can be done. No page stays permanently in the cache. There is limited room. A replacement algorithm is needed to determine which cached page should be purged.

4. C o n t e n t D i s t r i b u t i o n N e t w o r k s ( C D N ) Business Model: A content provider such as www.cnn.com or Yahoo pays a CDN company (such as Akamai ) to get its content to the requesting users with short delays. A CDN provides a mechanism for Replicating content on multiple servers in the Internet Providing clients with a means to determine the servers that can deliver the content fastest.

5. T e r m i n o l o g y Content : Any publicly accessible combination of text, images, applets, frames, MP3, video, flash, virtual reality objects, etc. Content Provider : Any individual, organization, or company that has content that it wishes to make available to users. Origin Server: Content provider ’ s server , where the content is first uploaded. Surrogate Server (sometimes called edge server): Content distributor ’ s server, where the replicated content is kept.

6. A B i g P i c t u r e

7. C D N s – C o n t e n t D e l i v e r y N e t w o r k s ( 1 ) CDNs scale Web servers by having clients get content from a nearby CDN node (cache)

8. C o n t e n t D e l i v e r y N e t w o r k s ( 2 ) Directing clients to nearby CDN nodes with DNS: Client query returns local CDN node as response Local CDN node caches content for nearby clients and reduces load on the origin server

9. C o n t e n t D e l i v e r y N e t w o r k s ( 3 ) Origin server rewrites pages to serve content via CDN P a g e t h a t d i s t r i b u t e s c o n t e n t v i a C D N T r a d i t i o n a l W e b p a g e o n s e r v e r

10. C D N – w h y ? One of the main goals of CDNs is to put content provider in control over how her content is cached Content provider signs a contract with CDN Contract specifies how content can be cached Contract also means CDN will follow what content provider wants CDNs typically charge per-byte of traffic served CDNs can be used for any kind of content Typically main use is for web content Streaming media has also been delivered over CDNs

11. H o w A k a m a i W o r k s Clients fetch html document from primary server E.g. fetch index.html from cnn.com URLs for replicated content are replaced in HTML E.g. <img src= “ http://cnn.com/af/x.gif ” > replaced with <img src=http://a73.g.akamai.net/7/23/cnn.com/af/x.gif> Or, cache.cnn.com, and CNN adds CNAME (alias) for cache.cnn.com  a73.g.akamai.net Client resolves aXYZ.g.akamaitech.net hostname Maps to a server in one of Akamai ’ s clusters <number>

12. C u r r e n t A k a m a i C u s t o m e r s

13. Content Distribution Networks (CDNs) Content providers are CDN customers Content replication CDN company installs thousands of servers throughout Internet In large datacenters Or, close to users CDN replicates customers ’ content When provider updates content, CDN updates servers <number> origin server in North America CDN distribution node CDN server in S. America CDN server in Europe CDN server in Asia

14. Problems Significant fraction (>50%?) of HTTP objects uncachable Sources of dynamism? Dynamic data: Stock prices, scores, web cams CGI scripts: results based on passed parameters Cookies: results may be based on passed data SSL: encrypted data is not cacheable Advertising / analytics: owner wants to measure # hits Random strings in content to ensure unique counting But…much dynamic content small, while static content large (images, video, .js, .css, etc.) <number>

15. Content Distribution Networks & Server Selection Replicate content on many servers Challenges How to replicate content Where to replicate content How to find replicated content How to choose among know replicas How to direct clients towards replica <number>

16. Server Selection Which server? Lowest load: to balance load on servers Best performance: to improve client performance Based on Geography? RTT? Throughput? Load? Any alive node: to provide fault tolerance How to direct clients to a particular server? As part of routing: anycast, cluster load balancing As part of application: HTTP redirect As part of naming: DNS <number>

17. DNS Redirection vs. URL Rewrite Discussion Comparison?

18. HTTP How Akamai Works End-user cnn.com (content provider) DNS root server 1 2 Akamai global DNS server Akamai regional DNS server Nearby Akamai cluster Akamai cluster 3 4 6 5 8 7 9 GET /foo.jpg Host: cache.cnn.com 12 11 GET foo.jpg

19. Players Content Provider H/W and S/W Vendor Content Distributor Hosting Provider Yahoo, MSNBC, CNN CBC Cisco, Oracle- Sun Akamai, Bell Sells servers Send content Install servers