Content Distribution Network, Proxy CDN: Distributed Environment

Content Distribution Networks (CDN)

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

InternetProviding clients with a means to determine the

servers that can deliver the content fastest.

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.

Players

Content Provider

H/W and S/W Vendor

Content Distributor

Hosting Provider

Yahoo, MSNBC, CNNCBC

Cisco, Oracle-Sun

Akamai,

Bell

Sells se

rvers

Send content

Install

servers

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 Europe

CDN server

in Asia

5

CDN: Functional Components

Distribution Service Redirection Service

Accounting and Billing system

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.

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 www.cnn.com

How Akamai Works

End-user

cnn.com (content provider) DNS root server

1 2

Nearby Akamai

cluster

GET index.html

9

http://a.73.g.akamai.net/7/23/cnn.com/af/cnn.com/foo.jpg

HTTP

Akamai

clusterAkamai global

DNS server

Akamai regional

DNS server

CDN: Redirection• Users get an html document from

www.cnn.com; 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=“http://cnn.com/af/foo.jpg> may be modified as follows: <img src=http://a73.g.akamai.net/7/23/cnn.com/af/foo.jpg>

• The browser needs to resolve a73.g.akamai.net hostname for replicated content.

Caching and Content Distribution(CDN)

11

CDN

12

CDN

13

CDN

14

CDN: RedirectionWhat does this mean?

<img src=http://a73.g.akamai.net/7/23/cnn.com/af/foo.jpg>

host part: a73.g.akamai.net Akamai control part: /7/23 Content URL: /af/foo.jpg

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)

How Akamai Works

End-user

cnn.com (content provider) DNS root server

1 2

Nearby Akamai

cluster

DNS lookup

cache.cnn.comAkamai

cluster3

4 ALIAS:

g.akamai.net

Akamai global

DNS server

Akamai regional

DNS server

CDN: Redirection

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

.

P

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

cluster3

4 6

5

ALIAS

a73.g.akamai.net

DNS lookup

g.akamai.net

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

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

cluster3

4 6

5

8

7

DNS a73.g.akamai.net

Address

1.2.3.4

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

cluster3

4 6

5

8

7

9

GET /foo.jpgHost: cache.cnn.com

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

cluster3

4 6

5

8

7

9

GET /foo.jpgHost: cache.cnn.com

1211

GET foo.jpg

CDN RedirectionThe 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

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.

CDN SelectionThe tricky issue is selecting which local content

server to use for a particular requestWant to spread load evenlyWant 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.

Accounting MechanismAccounting 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).

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.