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
4

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
6

7. Conventional vs. CDN
7
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?
8

9. CDN
9

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
12