Content delivery networks (CDNs) improve content delivery over the internet by replicating content on servers located at the network edge. This reduces latency, packet loss, jitter, and server load issues that occur when content must travel long distances and through multiple network providers to reach end users. CDNs deliver content through either passive caching methods or active replication of content on their distributed server networks with redirection techniques like HTTP redirects or DNS selection.

1. Content Delivery
Networks (CDN)
CS4262 Distributed Systems
Dilum Bandara
Dilum.Bandara@uom.lk
Some slides extracted from Dr. Srinath Perera & Dr. Rajkumar Buyya’s
Presentation Deck

2. Internet
Content
Providers
End
Users
The Web – Simple on Outside…

3. NAP
NAP
UUNet
Qwest
AOL
Network
Providers
Content
Providers
End
Users
Peering
Points
…But Problematic on Inside

4. Issues
 Latency
 Browser takes a long time to load pages
 Packet loss
 Browser hangs, user needs to hit refresh
 Jitter
 Streams are jerky
 Server load
 Browser connects but doesn’t fully load the page
 Bandwidth aggregation
 Broken/missing content
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5. Solution – Content Delivery Networks
(CDNs)
Servers
at Network Edge
Content
Providers
End
Users
NAP
NAP

6. Content Delivery Networks (CDNs)
 Passive
 Through browser cache & proxies
 If content is static, not much investment is needed from
content provider
 Significant fraction of HTTP objects not cacheable
 Need investment from content provider
 Provider lacks of control & customization
 Active
 Replicate content on a distribution network
 HTTP redirects
 DNS-based
 Provider has control & customization
 Low investment from content provider side
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7. Conventional vs. CDN
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Source: www.labnol.org/internet/setup-content-delivery-network-with-amazon-s3-cloudfront/

8. CDN Challenges
 How to replicate content?
 Where to replicate content?
 How to find replicated content?
 How to choose among known replicas?
 How to direct clients towards a replica?
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9. CDN
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10. CDN Options – HTTP Redirects
 Origin server rewrites pages to serve content via CDN
10Page that distributes content via CDN
Traditional Web page on server

11. CDN Options – DNS Based
11A. Su, D. R. Choffnes, A. Kuzmanovic, & F. E. Bustamante, “Drafting behind Akamai (travelocity-
based detouring), “ ACM SIGCOMM Comput. Commun. Rev. 36, 4, Aug. 2006, pp. 435-446.

12. CDN Options
 Application based (HTTP redirects)
 Pros
 Application-level, fine-grained control
 Cons
 Additional load & high RTTs, hard to cache
 Naming based (DNS selection)
 Pros
 Well-suitable for caching, reduce RTTs
 Cons
 Request by resolver not client, request for domain not URL,
hidden load factor of resolver’s population
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