1. Content Distribution Network, Proxy CDN: Distributed Environment

2. Content Distribution Networks (CDN)
Business Model: A content provider such as 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.

3. Terminology
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.

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

5. CDN Distribution Content replication5
CDN Distribution
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
origin server
in North America
CDN distribution node
CDN server
in S. America
CDN server
in Asia
CDN server
in Europe

6. CDN: Functional Components
Distribution Service
Redirection Service
Accounting and Billing system

7. CDN:Distribution Service
The content provider determines which of its objects it wants the CDN to distribute.
The content provider tags and then pushes this content to a CDN node, which in turn replicates and pushes the content to all its CDN servers.

8. CDN: Redirection
When a browser in a user’s host is instructed to retrieve a specific object (specified using a URL), how does the browser determine whether it should retrieve the object from the origin server or from one of the CDN servers?
an example, suppose the hostname of the content provider is

9. How Akamai Works HTTP cnn.com (content provider) DNS root server9
How Akamai Works
cnn.com (content provider)
DNS root server
GET index.html
Akamai
cluster
Akamai global
DNS server 1 2
HTTP
Akamai regional
DNS server
Nearby Akamai
cluster
End-user

10. CDN: Redirection
Users get an html document from this could be index.html
The file index.html uses a modified URL for content that has been replicated.
Example: If the jpeg files are what has been replicated then
<img src=“
may be modified as follows:
<img src=
The browser needs to resolve a73.g.akamai.net hostname for replicated content.

11. Caching and Content Distribution(CDN)

12. CDN

13. CDN

14. CDN

15. CDN: Redirection What does this mean?
<img src=
host part: a73.g.akamai.net
Akamai control part: /7/23
Content URL: /af/foo.jpg

16. CDN: Redirection
DNS is configured so that all queries about g.akamai.net that arrive at a DNS server are sent to an authoritative DNS server for g.akamai.net.
This is referred to as a Akamai DNS server (authoritative DNS server)

17. How Akamai Works cnn.com (content provider) DNS root server DNS lookup
cache.cnn.com
Akamai
cluster
Akamai global
DNS server 3 1 2 4
ALIAS:
g.akamai.net
Akamai regional
DNS server
Nearby Akamai
cluster
End-user

18. CDN: Redirection
When the Akamai DNS server receives the query, it extracts the IP address of the requesting browser. .

19. How Akamai Works cnn.com (content provider) P DNS root server
DNS lookup
g.akamai.net
Akamai
cluster
Akamai global
DNS server 5 3 1 2 6 4
Akamai regional
DNS server
ALIAS
a73.g.akamai.net
Nearby Akamai
cluster
End-user

20. CDN: Redirection
Based on the IP address and information that it has about the Internet (called a map), the IP address of an Akamai regional server is returned to the requesting browser based on policy
e.g., select the server that is the fewest hops away.
The regional server may choose a surrogate server for content retrieval

21. How Akamai Works cnn.com (content provider) HTTP DNS root server
cluster
Akamai global
DNS server 5 3 1 2 6 4
Akamai regional
DNS server 7
DNS a73.g.akamai.net 8
Address
Nearby Akamai
cluster
End-user

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

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

24. CDN Redirection
The Akamai DNS server IP address is now in the cache of the local DNS server.
This implies that it is not always necessary to go to the root DNS server.
The TTL associated with the IP address of an Akamai server(surrogate) is relatively small.
This is done for performance reasons.
Akamai content distribution servers are caches

25. CDN Redirection What if content is not there?
If the request content is not found then the surrogate will ask other surrogates within a specified region for information.
If requested information is still not found or is stale, then a request is made to the original web site.

26. CDN Selection
The tricky issue is selecting which local content server to use for a particular request
Want to spread load evenly
Want minimal impact if server is added or removed.
In Akamai, each surrogate server sends measurement results to the Network Operations Communications Center (NOCC).
Measurement results include number of active TCP connections, HTTP request arrival rate, bandwidth availability, etc
This information is used by the Akamai DNS server.

27. Accounting Mechanism
Accounting mechanisms collect and track information related to request routing, distribution and delivery.
Information is gathered in real time and put into log files for each CDN component.
This gets sent to the Network Operations Communications Center (NOCC).

28. Full Site Delivery vs. Partial Site Delivery
Full Site Delivery : All the contents are delivered by the CDN (including HTML, images, and other objects).
Partial Site delivery: Only images, streaming media and other bandwidth intensive objects delivered by the CDN.