the full name of the DN is the content Delivery network, which is the contents distribution networks. The goal is to add a new layer of network architecture to the existing Internet, publish the content of the site to the "Edge" of the network closest to the user, so that users can get the content they need, solve the congestion on the Internet, and increase the responsiveness of users to the site. The problem of the slow response speed of the user visiting website caused by the small network bandwidth, the large number of users, and the uneven distribution of dot is comprehensively solved technically.
in fact, the content sub-distribution network (CDN) is a new type of network construction, it is to be able to publish broadband rich media in the traditional IP network to optimize the network coverage layer, and from a broad perspective, the CDN represents a quality and order based network service model. Simply put, the Content Publishing network (CDN) is a strategic deployment of the overall system, including distributed storage, load balancing, network request redirection and Content Management 4 elements, while content management and global network traffic Management (traffic Management) is the core of the CDN. By judging the user's proximity and server load, the CDN ensures that the content serves the user's requests in an extremely efficient manner. In general, the content service is based on a cache server, also known as the proxy cache (surrogate), which is located at 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's source server, which is typically located in the CDN service provider's Datacenter. Such architectures enable CDN service providers to provide the best possible experience to end users on behalf of their customers, content providers, who cannot tolerate any delay in request response time. According to statistics, the use of CDN technology, can handle the entire Site page 70%~array5% content access, reduce the pressure on the server, improve the performance and scalability of the site.
compared with the current content publishing model, CDN emphasizes the importance of the network in content publishing. By introducing the active Content management layer and global load balancing, the CDN is fundamentally different from the traditional content publishing model. In the traditional content publishing mode, the content is published by the application server of the ICP, and the network is only represented as a transparent data transmission channel, which is manifested in the quality assurance of the network only stay at the level of the packet, but not according to the different content objects differentiated service quality. In addition, because of the "best effort" nature of the IP network, its quality assurance relies on the end-to-end provision of sufficient bandwidth flux between the user and the application server, which is much larger than the actual requirement. In such a content publishing mode, not only a large number of valuable backbone bandwidth is occupied, but also the application server of the ICP load is very heavy, and not predictable. When there are some hot events and surge flow, there is a local hotspot effect, which causes the application server to overload and exit the service. Another drawback of this hub-centric application server's content publishing model is the lack of personalized services and the distortions in the value chain of broadband services, which content providers assume they should not do or do poorly in content publishing services.
Throughout the value chain of broadband services, content providers and users are located at both ends of the entire value chain, relying on network service providers to connect them together. With the maturity of the Internet industry and the transformation of business model, the role of this value chain is more and more subdivided. such as content/application operators, managed service providers, backbone network service providers, access service providers, and so on. Each role in this value chain has to be a division of labor to provide customers with good service, resulting in a multi-win situation. From the combination of content and network mode, the release of content has gone through the content of ICP (application) server and IDC two phases. The boom in IDC also spawned the role of managed service providers. However, IDC does not address the issue of effective publishing of content. Content located in the center of the network does not solve the backbone bandwidth consumption and establish the traffic order on the IP network. As a result, the content is pushed to the edge of the network, providing a near edge service to the user, thus guaranteeing the quality of the service and the order of access throughout the network becomes an obvious choice. And this is the Content publishing network (CDN) service model. The establishment of CDN solves the dilemma of "centralization and decentralization" of content operators, which is undoubtedly valuable for building a good Internet value chain, and is also an indispensable and optimal website accelerator service.
Application of CDN
The current CDN services are mainly used in securities, financial insurance, ISPs, ICP, online transactions, portals, large and medium-sized companies, network teaching and other fields. In addition, in the industry special network, the Internet can be used, and even network optimization of LAN. Using CDN, these sites do not need to invest in expensive various kinds of servers, set up sub-sites, especially the wide application of streaming media information, long-distance teaching courseware, such as the use of more bandwidth-intensive media information, the application of CDN Network, the content to the edge of the network, so that the content demand point and delivery point to minimize the distance between Therefore, it is of great significance to improve the performance of Web site. CDN Network Construction mainly has the Enterprise Construction CDN Network, serves for the enterprise, the IDC CDN Network, mainly serves the IDC and the value-added service, the network operation main constructs the CDN network, mainly provides the content pushes the service, the CDN Network service provider, specially constructs the CDN to do the service, The user cooperates with the CDN organization, the CDN is responsible for the information transmission work, guarantees the information normal transmission, maintains the transmission network, but the website only needs the content maintenance, no longer need to consider the traffic question.
CDN can guarantee the fast, safe, stable and extensible network.
IDC set up a CDN network, IDC operators generally need to have a number of IDC centers distributed around the customer is hosted in the IDC Center, the use of existing network resources, less investment, easy to build. For example, an IDC has 10 rooms in the country, joined the IDC CDN Network, hosted on a node of the Web server, equivalent to 10 mirror servers, the nearest customer access. Broadband metropolitan Area Network, the speed of the Internet, the city bandwidth will generally be the bottleneck, in order to reflect the high-speed experience of the metropolitan Area Network, the solution is to cache the Internet content to the local, the cache will be deployed in the metropolitan area of the pop points, so as to form an efficient and orderly network, Users can access most of the content in just one jump, which is an app that accelerates all CDN applications.
the technical principle of CDN
in describing the implementation principle of CDN, let us first look at the traditional non-cached service access process, in order to understand the way CDN cache access and non-cached access to the difference:
by visible, the process by which a user accesses a site that is not using a CDN cache is:
the user provides the browser with the domain name to access;
The browser calls the domain name resolution function library to parse the domain name to obtain the corresponding IP address of the domain name;
The browser uses the resulting IP address, the domain name of the service host to send data access requests;
The browser displays the contents of the Web page based on the data returned by the domain host.
with the above four steps, the browser completes the process of receiving the domain name from the user to get the data from the Domain Name Service host. The CDN network is to add the cache layer between the user and the server, how to direct the user's request to the cache to get the data of the source server, mainly by taking over the DNS implementation, let's look at the process of accessing the website after using CDN cache:
through this, we can see that the access process of the site after the use of the CDN cache becomes:
the user provides the browser with the domain name to access;
The browser calls the domain name Resolution Library to resolve the domain name, because the CDN to the domain name resolution process has been adjusted, so the analytic function library generally obtains the domain name corresponding CNAME record, in order to obtain the actual IP address, the browser needs to parse the obtained CNAME domain name again to obtain the actual IP address In this process, the use of global load balancing DNS resolution, such as based on geo-location information to resolve the corresponding IP address, so that users can access the nearest.
This resolution obtains the IP address of the CDN cache server, the browser sends the access request to the cache server after obtaining the actual IP address;
Cache server According to the browser provided by the domain name to access, through the cache internal private DNS resolution to obtain the actual IP address of this domain name, and then by the cache server to this actual IP address to submit access requests;
cache server from the actual IP address to get content, on the one hand in the local storage for later use, two of the data to return to the client to complete the data service process;
The client obtains the data that is returned by the cache server and completes the entire browse data request process. Through the above analysis, we can obtain, in order to achieve both transparent to the ordinary user (that is, the user client does not need to make any settings after adding the cache, directly use the original domain name of the accelerator site can be accessed), but also to provide accelerated services for the designated site while reducing the impact on the ICP, As long as you modify the domain name resolution part of the entire access process to achieve transparent acceleration services, the following are the specific operating procedures of the CDN network implementation.
as an ICP, only need to give the domain name interpretation rights to the CDN operator, other aspects do not need to make any changes; when the operation, ICP modify their own domain name of the resolution record, generally use the CNAME method to point to the address of the CDN network cache server.
as a CDN operator, the first need to provide an open interpretation of the domain name of the ICP, in order to achieve sortlist, generally, the ICP's domain name interpreting results point to a CNAME record;
When sorlist is required, the CDN operator can take advantage of DNS to special process the domain name resolution process that the CNAME points to, so that the DNS server can return different IP addresses of the same domain name according to the client's IP address upon receiving the client request;
due to the IP address obtained from the CNAME and with the hostname information, the cache must know the IP address of the source server after the request reaches the cache, so an internal DNS server is maintained within the CDN operator to interpret the real IP address of the domain name accessed by the user;
when maintaining an internal DNS server, you also need to maintain a licensing server, control which domain names can be cached, and which do not cache, in order to avoid the occurrence of open proxies.
Network Architecture of CDN
The CDN Network architecture mainly consists of two parts, the center and the edge, the center means CDN Network Management Center and DNS Redirection resolution Center, responsible for global load balancing, equipment system installed in the Management Center room, edge mainly refers to the remote node, CDN distribution Carrier, Mainly consists of cache and load balancer.
when a user accesses a website that joins a CDN service, the domain name resolution request is eventually handed over to the global load Balancer DNS for processing. Global load Balancing DNS uses a predefined set of policies to provide the user with a node address that is closest to the user, enabling users to get a quick service. At the same time, it maintains communication with all CDNC nodes distributed around the world, collects the communication state of each node, ensures that the user's request is not assigned to the unavailable CDN node, and actually does global load balancing through DNS.
for ordinary Internet users, each CDN node is the equivalent of a web that is placed around it. With global load balancing DNS control, the user's request is transparently directed to the node closest to him, and the CDN server in the node responds to the user's request as if it were the site's original server. Because it is closer to the user, the response time must be faster.
Each CDN node consists of two parts: a load balancer device and a cache server
Load balancer is responsible for the load balancing of each cache in each node, which ensures the efficiency of the nodes, and the load balancing device is responsible for collecting the information of the nodes and the surrounding environment, maintaining the communication with the global load DNS, and realizing the load balance of the whole system.
the cache server (cache) is responsible for storing a large amount of information about a customer's website, responding to a local user's access request as if it were a Web server close to the user.
the management system of CDN is the guarantee that the whole system can operate normally. It can not only monitor the subsystems and equipment in real time, but also produce corresponding alarms for various faults, and can monitor the total traffic and the traffic of each node in real-time, and keep in the database of the system, so that the network manager can make further analysis conveniently. Through the perfect network management system, the user can modify the system configuration.
Theoretically, the simplest CDN network has a DNS that is responsible for global load balancing and one cache for each node to run. DNS supports resolving different IPs according to the user source IP address to achieve the nearest access. To ensure high availability, it is necessary to monitor the traffic, health status, etc. of each node. A node of a single cache bearer is not enough, only need more than one cache, more than one cache at the same time, only need a load balancer, so that the cache group work together.
What is a CDN?