CDN (Content Distribution Network)
CDN is the network for content distribution on the existing Internet, which features advanced flow distribution. It works between the to network layer and application layer (see the following figure). This concept is first put forward in Massachusetts Institute of Technology in 1998, which is a network solutions for user accessibility from near side.
In conventional interconnected network, the content is collectively located, although to which the management and maintenance is relatively convenient, but, when users of the operational companies from different regions access the web site, delay is unavoidable, resulted in degradation of access quality; meanwhile, when access quantity is increased abruptly due in any emergent case, the network supervisor will feel helpless. Therefore, in such content location situation, even the improvement of the server accessibility and increase of the user access band width, which only increase the access data flow of two sides, will not basically improve the network delay, but will cause waste of band width when access quantity drops down.
But CDN make it different, it adds a new network architecture between the existing Internet application layer and network layer, which distributes the content that is stored in the original website server to the Cache Server in different locations; a intelligent central management system is used to control the distribution of dynamic network flow, so that the user can obtain the requested information at quickest speed from the nearest location, during which, large amount of static data, like, picture and multimedia, will be read directly from the intelligent CDN server, this greatly reduced the size of data that are read from the source file server. In this way, the response speed of the user and accessibility of the service is increased, which solves the Internet congestion and increase the response speed of the website to the user. It completely solved the problems resulted from low band width, large user access quantity and uneven distribution of website nodes etc, enabling satisfactory access fro the users.
Besides access speed, network security is another critical problem for the website administration. As for conventional network, since the content is collectively placed in one source site, emergent situations, such as power failure, hacker’s attack and server down time etc., are very likely to cause inaccessibility of the website or service interruption, and even tremendous economic loss in worse cases. CDN system effectively increased the utilization efficiency of network resources by increasing the user’s access speed in one hand and increasing the website safety on the other hand; its distributed network structure solved the inextricable defect of the conventional website where content is collectively located. Even in case of emergency that the source site is inaccessible, not all the business will fall in paralysis because the access request of the user to the website will be automatically re-direct to the healthy website nodes by our system, this avoided great loss occurred to the customer due to the interruption of the website service. In addition, CDN can backup considerable data of the user in real time; undoubtedly, this increases the data security for the user.
In the general, content service sets basis on cache server, which is located at the edge of the network, at only "Single Hop" distance from the user. When the website adopts CDN, CDN service provider will be responsible for guaranteeing the information transmission; however the website that adopts CDN is not necessarily invest so much to purchase server or establish new sites, what he will do is to maintain the content without worrying about the flow issue. CDN provide warranty in terms of network speed, security, stability and extensibility.
Statistical data show that CDN technology can process 70% ~ 95 ％ of access of all the web pages, which not only alleviated the access pressure on the source server, but also increased the performance, extensibility and security of the website. By far, CDN technology is widely used in the scopes of portal sites, large and middle-sized companies, network teach, online game, securities, finance and insurance, ISP, ICP and online transaction etc. It is foreseeable that under the situation that technologies like stream media and tele-education which consume much band width resources are widely used, application of CDN will minimize the distance between the enquiry point and the delivery point by copying the content to the furthermost edge, which has significant meaning in increasing the performance of the website.
Dnion CDN solution can provide warranty in the aspects of speed, security, stability and extensibility of the network. Dnion CDN network structure consists of four parts: CDN management system (CDNM), CDN route system (CDNR), CDN edge server (CDNS) and CDN watch system (CDNW), where, the CDNM is mainly responsible for configuration of CDN equipment, data collection, security protection, content management and distribution tactics of network resources, CDNR is mainly responsible for distribute the nodes of quickest response speed to the user according to multiple optimization tactics, CDNS mainly provides content distribution service in response to the user access request, and CDNW monitors the network operation of CDN itself and the user’s website. The basic operating principle of Dion CDN is like this: various Cache servers are widely distributed to the zones or network where users are relatively concentrated; when users access the website, their access is directed to the nearest Cache server by means of load balancing technology, which directly responds to the request of the user. When Cache server does not contain the content needed by the user, it will automatically acquire the relevant pages from the source server and provide them to the user according to the configuration. Therefore, once Dnion CDN network works normally, content distribution and redirection of user access are all executed automatically.
The above figure is the user access flow chart after Dnion CDN is used; When the domain name which the user will visit is input, DNS request is sent to DNS server on the web, then it is directed to the route system in the CDN network. And CDN will send the IP of the node of the quickest response speed to that domain name to the user. The user’s web browser will send access request to the CDN node after it got this IP address, and because it is the first-time visiting, the CDN node will go back the source site to retrieve the requested data and send them to the user. At next time, when other user accesses the same content again, CDN will directly return the data to such user, thus the full process from request to service is completed.
1) User inputs the domain name for access in his web browser, which will request resolution of this domain name to the local DNS server;
2) The local DNS resolution server searches the authorized DNS record at the root DNS resolution server, which will return the record to local resolution server;
3) The local DNS resolution server requests resolution of host name at the DNS server authorized by the website, which will return the IP address as the resolution result to the local DNS resolution server;
4) The local DNS resolution server returns the IP to the user;
5) After the user has got the IP Address, he will access the source website server at such address, from which the relevant content and data are provided to the user.
1) After the request of domain name resolution is sent to the DNS server authorized by the website, the request is re-directed to the inelegant resolution system of Dnion CDN, which will send the IP of the nodes of quickest response speed to the user, who then sends access request to CDN.
2) At the time of first access, CDN node will send the content and data from the source website to the user. After that, when other user access the same content, CDN will directly return the content and data to such user, thus the full process from request to service is completed.
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