What is the principle of CDN and CDN Acceleration

with the development of the Internet, users pay more attention to the browsing speed and effect of the Web site, but because of the proliferation of netizens, the network access path is too long, so that the quality of access of users is seriously affected. Especially when the link between the user and the website is congested with large traffic data, the poor quality of access is an urgent problem for the region with rapid increase of internet users. How to make high-quality access to users everywhere, and minimize the resulting costs and web site management pressure. Content Publishing Network (Delivery Network,cdn) was born.
A, CDN is what.
The full name of CDN is content Delivery network, that is, contents distribution network. The goal is to add a new layer of network architecture to the existing Internet, publish the content of the Web site to the nearest user's network "edge", so that users can get the required content nearby, improve the user's response speed to visit the site. CDN is different from mirroring because it is smarter than mirroring, or it can be likened to a more intelligent image + cache + Flow diversion. cdn= Therefore, CDN can obviously improve the efficiency of information flow in Internet network. From the technical comprehensive solution because of the network bandwidth is small, the user accesses the quantity to be big, the dot distribution unequal problem, enhances the user to visit the website the response speed.
To better understand the CDN, let's look at the workflow of the Cdn. When a user accesses a Web site that has already joined CDN Services, it first determines the best CDN node of the nearest user through DNS redirection technology, and points the user's request to the node. When the user's request arrives at the specified node, the CDN server (the cache on the node) is responsible for providing the user with the content requested by the users. The specific process is: users in their own browser to enter the site to visit the domain name, browser to the local DNS request resolution of the domain name, local DNS will send requests to the site's primary DNS, the primary DNS based on a series of policies to determine the most appropriate CDN node at that time, and the resolution of the results (IP address) to the user, The user requests the content of the corresponding Web site to the given CDN node.
Second, the relevant technology of CDN
CDN implementation needs to rely on a variety of network technology support, including load balancing technology, dynamic content distribution and replication technology, caching technology is a relatively major few, let us briefly look at these technologies.
Load Balancing Technology
Load balancing technology is not only applied in CDN, but has been widely used in many fields of network, such as load balance of server and load balance of network traffic. As the name implies, the Network load balance is the network flow as evenly as possible to a few can complete the same task server or network nodes, thereby avoiding some network nodes overload. This can not only improve the network traffic, but also improve the overall performance of the network. In CDN, load balance is divided into server load balancing and server overall load balancing (also some called server Global load Balancing). Server load balancing refers to the ability to perform task assignment between servers with different performance, not only to ensure that the poor performance of the server is not a bottleneck of the system, but also to ensure that the high performance of the server resources are fully utilized. Overall server load Balancing allows Web network hosts, portals, and businesses to distribute content and services geographically. Improve fault tolerance and availability by using multi-site content and services to prevent failures caused by outages, power outages, or natural disasters on a local or regional network. In the CDN scheme, the overall server load balance will play an important role, and its performance will directly affect the performance of the whole CDN. dynamic content distribution and replication technology
As we all know, the speed of Web site access response depends on a number of factors, such as whether the network bandwidth bottlenecks, transmission routes have congestion and latency, Web server processing capacity and access distance. In most cases, the site response speed and the distance between the visitor and the Web server are closely related. If the distance between the visitor and the website is too far, the communication between them needs to be routed and processed as much as possible, and the network delay is unavoidable. An effective way is to use content distribution and replication technology to replicate the majority of static Web pages, images and streaming media data distribution to the local acceleration nodes. Therefore, dynamic content distribution and replication technology is also a major requirement of CDN technology. Caching technology
Caching technology is no longer a new technology. Web caching services improve user response times in several ways, such as proxy caching services, transparent proxy caching services, transparent proxy caching services using redirected services, and so on. The Web caching service allows users to minimize the traffic to a wide area network when they visit a Web page. For intranet users, this means caching content locally without having to retrieve the Web page through a dedicated WAN. For Internet users, this means storing content in their ISP's cache without having to retrieve Web pages over the Internet. This will undoubtedly improve the user's access speed. The core function of CDN is to improve the access speed of the network, so the caching technology will be another main technology adopted by CDN. third, who needs CDN. since CDN's core role is to improve the speed of access to the network, then its users are very large access to the site, such as ICP, ISP, large enterprises, e-commerce sites and government websites. Using CDN Technology, these sites do not need to invest in expensive types of servers to set up a branch site. Through the adoption of CDN,CDN will be responsible for information transmission work, to ensure the normal transmission of information, and technicians only need to maintain the content of the site, do not need to consider traffic problems. In this way, the site can ensure that users get more new business, can quickly access the content on the network, to obtain better quality of service. For example, for a relatively large number of visits, and the content of the update cycle of a longer period of the site, such as government sites, users often do a lot of query work. This kind of website is more suitable to use CDN. Also, do you notice that in the so-called broadband community, the bottleneck is the external export of the community. In this way, if the use of CDN undoubtedly to the community users to use video on demand, network education and other broadband applications provide a guarantee. Four, the lack of CDN any new thing, in the existing model to bring improvements, but also must have some limitations, the CDN is also the case. The real time is not very good is a CDN's fatal flaw. As the demand for CDN grows, this flaw will be improved to keep the Web content pages from the remote server synchronized with the Web pages in the replica server or cache. The solution is to transfer new network content directly from the server side to the cache when the content of the network changes, or to replicate the data source server's network content to the cache server as quickly as possible when access to the network content increases. transferred from "http://www.cnblogs.com/seanxyh/archive/2013/04/16/3023499.html"