content distribution network, proxy cdn: distributed environment
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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
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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)
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CDN
12
CDN
13
CDN
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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.