What is a CDN? The relevant technology of CDN! Lack of CDN?

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 enable users everywhere to make high-quality access, and as far as possible the resulting costs and web site management pressure. Content Publishing Network (Delivery Network,cdn) was born. CDN: Network speed up another recruit Zhang Xujun----with the development of the Internet, users in the use of the Web site to browse the speed and effect of more attention, but due to the proliferation of Internet users, network access path is too long, so that the quality of access to users has been 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 enable users everywhere to make high-quality access, and as far as possible 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, which is the 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. ----for a better understanding of CDN, let's look at the workflow of 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 the support of a variety of network technologies, 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 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----We all know that 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 networkAccess speed, so caching technology will be another major technology used by CDN.

　　 third, who needs CDN.

----Since the core role of CDN is to improve the speed of access to the network, then its users is a very large number of web sites, 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 things, in the existing model to bring about the improvement, there must be some limitations, the CDN is also the case. According to Guo Guangzhong, the technology manager of the Interconnection Network Limited, 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.

　　 Five, CDN products

----At present, the introduction of more mature CDN products manufacturers have Cisco, F5 and mutual unicom companies, the former two CDN is mainly hardware system, and the latter CDN is a software-oriented general-purpose platform, a good combination of lines and service advantages. The following is a brief analysis of the Cdn--smart CDN. ----Interconnect through the Integrated load Balancing (GSLB), caching technology (cache) and mirroring Technology (MIRROR), to provide customers with remote network acceleration services. The specific program is the main site outside the user focus on the city, the use of Internet-connected network acceleration Services, mutual unicom intelligent GSLB system through the interconnection of private networks, to guide the site users to visit the "best" cache/mirror server, thereby bypassing the Internet congestion link, reduce the main site server load, Realize the network acceleration in different places, and solve the problem that the simple cache/mirror can't solve. ----Smart CDN works through DNS resolution or HTTP redirection, and completes the transfer and synchronization of content through the cache server or remote mirror site. The accuracy rate of DNS method user location is more than 99% 85%,http. After the actual calculation, each cache server group's user accesses the data quantity and the cache server to the original website content data quantity ratio between 2:1 and 3:1, namely sharing 50%~70% to the original website repeatedly accesses the data quantity (mainly is the picture, the streaming media file and so on content). For mirroring, all but the data is synchronized, and the rest is done locally, without accessing the original server. ----Smart CDN System based on the user's actual access to the IP address to determine the user location, direct user access to the fastest response site. The whole system management is simple, the user can determine what content needs to do distributed distribution through GUI, the system will complete the whole process of content copy, update and database synchronization automatically. Moreover, the system has the ability of self-diagnosis and load balancing, and any link failure will not affect the accessibility of the whole system. ----It can be said that load balancing technology plays an important role in CDN, its ability has direct influence on CDN's performance. The load balancing used by Smart CDN has some advantages, mainly embodied in the following aspects.

----1. High-intelligent----The use of virtual IP (VIP) address to represent the target server and applications, Smart CDN will be the session to the highest availability of the server, the whole process of monitoring each session. Automatically re-registration after a service is restored and provides full address translation when forwarding client and server information packets. Smart CDN uses many different load balancing methods including cyclic method, least connection method, hashing method or least error method, and configures maximum connection quantity threshold and weighting value for individual servers, which can effectively avoid overloading the server.

----2. The High reliability----Smart CDN Architecture is above the high-speed backbone network dedicated to interconnect, the backbone network provides the network connectivity with minimal delay, thus guaranteeing the function of GSLB and high performance better than the GSLB based on public network. Furthermore, when the Internet exits of the main engine room are faulted, the Smart CDN can also improve the reliability of the network by automating and transparently directing the user from other branch site internet portals to the primary site server.

----3. High availability----The use of hot backup methods to detect and failover server links, switching ports, and switches over a very short period of time to protect applications from failure. Once any server or server cluster fails or blocks, the user is automatically booted to the next best backup server or site, further improving the availability of services and content.

----4. High redundancy----Smart CDN runs on the interconnected "three-ring nested" network backbone architecture, with high redundancy features, thus ensuring high reliability and high availability, which is very different from other GSLB solutions based on public networks. ----In addition, the advantage of the Smart CDN in the service is reflected in: through the interconnection of proprietary link system, can improve user access response speed, to ensure the Taiwan region multiple server content synchronization, eliminating the peak access (Peak Load) on the export link and server performance pressure , the bandwidth utilization of leased link is improved; Reduced the original WEB/FTP Web site load and so on. (IDC data Center, Guangdong)