CDN Technical Principles

The rapid development of the Internet has brought great convenience to people's work and life. The demand for Internet service quality and access speed is getting higher and higher, the number of users is also increasing. due to factors such as the load and transmission distance of Web servers, slow response speed is still frequently complained and troubled. The solution is to use the Cache Technology in network transmission so that Web Service data flows can be accessed nearby. It is a very effective technology to optimize network data transmission, so as to achieve high-speed experience and quality assurance. The purpose of the network cache technology is to reduce network

Overview
The rapid development of the Internet has brought great convenience to people's work and life. The demand for Internet service quality and access speed is getting higher and higher, the number of users is also increasing. due to factors such as the load and transmission distance of Web servers, slow response speed is still frequently complained and troubled. The solution is to use the Cache Technology in network transmission so that Web Service data flows can be accessed nearby. It is a very effective technology to optimize network data transmission, so as to achieve high-speed experience and quality assurance.

The purpose of the network cache technology is to reduce the redundant data transmission in the network and minimize it, and convert the wide-area transmission to local or nearby access. Most of the content transmitted on the Internet is repeated Web/FTP data, Cache servers and network devices using the Caching technology, which can greatly optimize the performance of data links, eliminate node device congestion caused by peak data access. The Cache server has the Cache function, so most webpage objects (Web page objects), such as html, htm, PHP and other page files, gif, tif, png, bmp and other image files, for files in other formats, during the validity period (TTL), for repeated accesses, you do not have to re-transfer the file entity from the original website, but only need to pass the simple authentication (Freshness Validation) -Send dozens of bytes of headers to directly send local copies to visitors. Because the cache server is usually deployed near the user end, the response speed of the local area network can be obtained and the bandwidth consumption can be effectively reduced. According to statistics, over 80% of users on the Internet repeatedly access 20% of information resources, which provides a prerequisite for the application of cache technology. The architecture of the cache server is different from that of the Web server. The Cache Server delivers higher performance than the Web server. The cache server not only improves the response speed, but also saves bandwidth, it is very effective to effectively reduce the load on the source server.

The high-speed Cache Server is a professional function Server highly integrated with software and hardware. It is mainly used for high-speed Cache acceleration services and is generally deployed on the network edge. Different acceleration objects are divided into client acceleration and server acceleration. The client acceleration Cache is deployed at the network exit and the frequently accessed content is cached locally to increase the response speed and save bandwidth. Server acceleration, the Cache is deployed on the front-end of the server and serves as the front-end server of the Web server. This improves the performance of the Web server and speeds up access. If multiple Cache acceleration servers are distributed across different regions, You need to effectively manage the Cache network to guide users to access the nearest node and perform global load balancing. This is the basic idea of CDN content transmission network.

What is CDN?
CDN stands for Content Delivery Network (CDN. The purpose is to add a new network architecture to the existing Internet to publish website content to the "edge" closest to the user's network, so that users can obtain the desired content nearby, solves Internet network congestion and increases the response speed for users to access websites. Technically, this solution comprehensively solves the root cause of slow response speed when users access the website due to low network bandwidth, large user visits, and unevenly distributed outlets.

In a narrow sense, the content delivery network (CDN) is a new type of network construction mode, which is a network covering layer that is particularly optimized for publishing broadband rich media on the traditional IP network; in a broad sense, CDN represents a network service model based on quality and order. To put it simply, content delivery network (CDN) is an overall system that has been strategically deployed, including four requirements: distributed storage, Server Load balancer, network request redirection, and content management, content Management and global network Traffic Management are the core of CDN. Based on users' proximity and server load judgment, CDN ensures that the content provides services for users' requests in an extremely efficient manner. In general, content service is based on the cache server, also known as the proxy cache (Surrogate). It is located on the edge of the network and is only one Hop away from the user. At the same time, the proxy cache is a transparent image of the content provider source server (usually located in the data center of the CDN service provider. This architecture enables CDN service providers to provide end users with the best possible experience on behalf of their customers, that is, content providers. These users cannot tolerate any latency in request response time. According to statistics, CDN technology can be used to process 70%-of the entire website page ~ 95% of content access Traffic reduces the load on the server and improves the performance and scalability of the website.

Compared with the existing content publishing mode, CDN emphasizes the importance of the network in content publishing. By introducing active content management and global load balancing, CDN is fundamentally different from the traditional content publishing mode. In the traditional content publishing mode, content publishing is completed by the ICP application server, while the network is only a transparent data transmission channel, this transparency is manifested in the quality assurance of the network, which only stays at the layer of data packets, but cannot distinguish the quality of service based on different content objects. In addition, due to the "Best Effort" feature of the IP network, the quality assurance is achieved by providing sufficient end-to-end bandwidth between the user and the application server, far greater than the actual needs. In this content publishing mode, not only is a large amount of valuable backbone bandwidth occupied, but the load of the ICP application server also becomes very heavy and unpredictable. In the event of some hot events and surge traffic, local hot spots will be generated, so that the application server will be overloaded and quit the service. Another drawback of this center-based app server's content publishing model is the lack of personalized services and the distortion of the broadband service value chain, the content provider undertakes the content publishing services that they should not or are not doing well.

Throughout the value chain of broadband services, content providers and users are located at both ends of the entire value chain, and network service providers are used in the middle to connect them. With the maturity of the Internet industry and the transformation of business models, more and more roles in this value chain are becoming increasingly subdivided. Such as content/Application operators, managed service providers, backbone network service providers, and access service providers. In this value chain, each role must work in a division of labor and perform their respective duties to provide good services to customers, resulting in a win-win situation. From the perspective of the combination of content and network, content publishing has gone through two stages: content (Application) server and IDC. The IDC boom also gave birth to the role of hosting service providers. However, IDC cannot solve the issue of effective content publishing. The content in the center of the network does not address the occupation of backbone bandwidth and the traffic order on the IP network. Therefore, pushing content to the edge of the network to provide nearby edge services for users, thus ensuring the service quality and the access order on the entire network becomes an obvious choice. This is the CDN service mode. The establishment of CDN solves the dilemma of "centralization and decentralization" of content that troubles content operators. It is undoubtedly valuable and indispensable for building a good Internet value chain.

New CDN applications and customers
Currently, the CDN service is mainly used in securities, financial insurance, ISP, ICP, online trading, portal websites, media sites, large and medium-sized companies, network teaching, and other fields. In addition, it can be used in industry private networks, the internet, or even LAN optimization. Using CDN, these websites do not need to invest in expensive servers, set up sub-sites, especially the wide application of streaming media information, remote teaching courseware, and other media information that consumes a lot of bandwidth resources, and use CDN networks, copying content to the edge of the network minimizes the distance between the content request point and the delivery point, thus promoting the improvement of the Web site performance. CDN networks are mainly built by enterprises to serve enterprises. CDN networks of IDCs are mainly used for IDCs and value-added services. CDN networks built on network operations are mainly used for CDN networks, it mainly provides content push services. CDN network service providers and specially constructed CDN are used for services. Users cooperate with CDN institutions to transfer information and ensure normal information transmission, the website only needs content maintenance and does not need to consider traffic issues.

CDN can guarantee the speed, security, stability, and scalability of the network.

IDC establishes a CDN network. IDC operators generally need to have multiple IDCs located in different regions. The service targets customers hosted in IDCs. using existing network resources, there is less investment, easy to build. For example, if an IDC has 10 data centers in China and is added to the CDN network of the IDC, a Web server hosted on a node is equivalent to 10 backup servers, which are accessible to customers nearby. Broadband man, the speed of the intra-domain network is very fast, and the out-of-town bandwidth is generally a bottleneck. To reflect the high-speed experience of the man, the solution is to cache the content on the Internet to a local device at a high speed, deploy the Cache on various POP points in the Metropolitan Area Network to form an efficient and orderly network. Users can access most of the content in one hop. This is also an application to accelerate CDN for all websites.

How CDN works
After describing the implementation principle of CDN, Let's first look at the access process of the traditional cache service, so that we can understand the differences between the CDN cache access method and the cache access method: