Overview of CDN

Content Delivery Network (CDN)

A computer network that has multiple copies of data stored at various points of the network.
The end user connects to a CDN and will access the data from the nearest server (middle) instead of connecting to a central server.
A few of the applications include media distribution, multiplayer gaming, and distance learning.
The end user can be a wired or wireless unit, that tries to access the content.

The middle servers (or several servers forming a cluster) store the images of the content from the central (main) server. They are located at the edge of the ISP network and may be geographically separated from each other.

Elements of CDN

Request: A request for a specific content (for example a webpage) is made from the end user, that is redirected to the nearest image server. This is done by the use of a protocol known as a Web Cache Communication Protocol (WCCP).
Distribution: Once the request is received, a distribution element in the CDN forwards the request based on the point of origin, content availability, location and server's global load.
Delivery: Delivery of the requested content is made by this element using routing and switching protocols.

Algorithms/Protocols used in Request Routing

A variety of algorithms are used for this purpose. These include Global Server Load Balancing, DNS-based request routing, Dynamic metafile generation and so on.
Global Server Load Balancing (GLSB) enables the content to be obtained from a server pool in a sequential manner using a round-robin method and redirects the request in the case of inactive server sessions.
DNS-based request routing: Here when a request is made (URL), the local DNS server provides the IP address of the nearest matching CDN node. If the Local DNS is unable to resolve the URL, it forwards the request to the Root DNS server, that then provides the nearest possible CDN server IP.
Dynamic metafile generation includes creation of a metafile, that has an ordered hierarchy of CDN domains connected to a Main server and helps in the load balancing on each of the CDN nodes connected to it.
Internet Content Adaptation Protocol (ICAP), Open Pluggable Edge Services (OPES) and Edge Side Includes (ESI) are the protocols used for accessing data through a request in CDNs.
ICAP is a high level protocol that helps in generating HTTP requests and delivers contents from the CDN servers.
OPES uses a processor to share content to the end users. This processor duplicates the content at each CDN node and traces the route followed by each request made by the user and notifies the user once the content is found.

ESI avoids back end processing delays hence providing dynamic contents with ease. It breaks web content into fragments and delivers dynamic contents to end users.

Benefits of CDN

Accelerates web-based applications
Low connectivity latency
Optimization of capacity per user
Faster and reliable access to contents
Low network loads