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 Internet Providing 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 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

CDN Distribution Content replication 5 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

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 HTTP cnn.com (content provider) DNS root server 9 How Akamai Works cnn.com (content provider) DNS root server GET index.html Akamai cluster Akamai global DNS server http://a.73.g.akamai.net/7/23/cnn.com/af/cnn.com/foo.jpg 1 2 HTTP Akamai regional DNS server Nearby Akamai cluster End-user

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: Redirection What 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 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

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

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

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

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 1.2.3.4 Nearby Akamai cluster End-user

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

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

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

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 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.

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).

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.