1. Introduction to CDN
The full name of CDN is Content Delivery Network, that is, Content Delivery Network . The basic idea is to avoid bottlenecks and links that may affect the speed and stability of data transmission on the Internet as much as possible, so that content transmission is faster and more stable. By placing node servers everywhere in the network to form a layer of intelligent virtual network on the basis of the existing Internet, the CDN system can real-time based on network traffic and the connection and load status of each node, as well as the distance to the user and response time Such comprehensive information redirects the user's request to the service node closest to the user. Its purpose is to enable users to obtain the desired content nearby, solve the situation of Internet network congestion, and improve the response speed of users visiting websites.
Simply put, a content distribution network is a strategically deployed overall system, including four requirements . Content management and global network traffic management are CDN's. where the core lies. Judging by user proximity and server load, CDNs ensure that content is served user requests in an extremely efficient manner.
In general, content serving is based on cache servers, also known as proxy caches, located at the edge of the network, just "one hop" away from the user. At the same time, the proxy cache is a transparent mirror of the content provider's origin server. Such an architecture enables CDN service providers to provide the best possible experience on behalf of their clients, the content providers, to end users who cannot tolerate any delay in request response time.
2. The principle of CDN acceleration/CDN acceleration
CDN acceleration caches the content of the website at the edge of the network (the place closest to the user's access to the network), and then when the user accesses the website content, the user's request is routed or guided through the scheduling system to the closest to the user's access to the network or the effect of the access. On the best cache server, the cache server provides content services for users; compared with direct access to the origin site, this method shortens the network distance between the user and the content, thereby achieving the effect of acceleration.
3. The benefits of using CDN
1. Don't worry about your website visitors, you can quickly open the website at any time, any place, and any network operator.
2. Purchasing costs such as bandwidth of various server virtual hosts, including post-operation and maintenance costs, will be greatly reduced.
3. The benefits directly brought to the website are: traffic, consultation volume, customer volume, and order volume will be greatly improved.
4. Disadvantages of CDN
Any new thing, while bringing improvements to the existing model, must also have certain limitations. The same is true for CDNs. The lack of real-time performance is the shortcoming of CDNs. However, with the growing demand for CDNs, this deficiency has been improved to keep web content pages from remote servers in sync with pages in replica servers or caches. The solution is to directly transfer the new network content from the server to the cache when the network content changes, or copy the network content of the data source server to the cache server in real time as much as possible when the access to the network content increases.
V. Principles of CDN Technology
The basic principle of CDN is reverse proxy. Reverse proxy means that the proxy server accepts connection requests on the internet, then forwards the request to the server on the internal network, and returns the results obtained from the server. To a client requesting a connection on the internet, the proxy server acts as a node server to the outside world. By deploying more reverse proxy servers, the effect of multi-node CDN can be achieved.
In describing the implementation principle of CDN, let us first look at the access process of the traditional uncached service in order to understand the difference between the CDN cache access method and the uncached access method:
The user submits the domain name → the browser parses the domain name → obtains the IP address of the destination host → sends a request according to the IP address access → obtains the request data and replies
It can be seen from the above that the process for a user to access a website that does not use CDN cache is as follows:
1) The user provides the browser with the domain name to be accessed;
2), the browser calls the domain name resolution function library to parse the domain name to obtain the IP address corresponding to the domain name;
3) The browser uses the obtained IP address to send a data access request to the service host of the domain name;
4) The browser displays the content of the web page according to the data returned by the domain name host.
Through the above four steps, the browser completes the entire process from receiving the domain name to be accessed from the user to obtaining data from the domain name service host. The CDN network adds a Cache layer between users and servers. How to direct user requests to the Cache to obtain data from the source server is mainly achieved by taking over DNS.
In order to achieve the realization of CDN, it is not only transparent to ordinary users (that is, after adding the cache, the user client does not need to perform any settings, and can directly use the original domain name of the accelerated website to access, but also to provide acceleration services for designated websites while reducing the need for ICP. As long as the domain name resolution part of the entire access process is modified to achieve transparent acceleration services, the following is the specific operation process of CDN network implementation.
1) As an ICP, you only need to hand over the domain name interpretation right to the CDN operator, and other aspects do not need to be modified; during operation, the ICP modifies the resolution record of its own domain name, and generally uses the cname method to point to the address of the CDN network cache server.
2) As a CDN operator, it is first necessary to provide public resolution for the domain name of the ICP. In order to implement the sortlist, the result of the domain name interpretation of the ICP is generally pointed to a CNAME record;
3) When the sortlist needs to be performed, the CDN operator can use DNS to perform special processing on the domain name resolution process pointed to by the CNAME, so that the DNS server can return different IP addresses of the same domain name according to the client's IP address when receiving the client's request. ;
4) Due to the IP address obtained from cname and with hostname information, after the request reaches the Cache, the Cache must know the IP address of the source server, so an internal DNS server is maintained within the CDN operator to explain the user access. The real IP address of the domain name;
5) When maintaining the internal DNS server, it is also necessary to maintain an authorization server to control which domain names can be cached and which ones are not cached, so as to avoid the occurrence of an open proxy.
6. Main features of CDN
1. Local Cache acceleration Improves the access speed of enterprise sites (especially sites with a large number of pictures and static pages), and greatly improves the stability of sites with the above properties
2. Mirroring service Eliminates the impact of the bottleneck of interconnection between different operators, realizes network acceleration across operators, and ensures that users in different networks can get good access quality.
3. Remote acceleration The remote access user intelligently selects the Cache server automatically according to the DNS load balancing technology, selects the fastest Cache server, and speeds up the speed of remote access
4. Bandwidth optimization Automatically generate a remote mirror (mirror) cache server of the server, read data from the cache server when remote users access, reduce the bandwidth of remote access, share network traffic, and reduce the load of the original site WEB server.
5. Cluster anti-attack The widely distributed CDN nodes and the intelligent redundancy mechanism between nodes can effectively prevent hacker intrusion and reduce the impact of various DDoS attacks on the website, while ensuring better service quality.
7. Key functions of CDN
(1) Content publishing : It publishes or delivers content to the remote service point closest to the user by means of technologies such as indexing, caching, stream splitting, and multicasting (Multicast);
(2) Content routing : It is an overall network load balancing technology. Through the redirection (DNS) mechanism in the content router, users' requests are balanced on multiple remote POPs, so that user requests get the fastest response from the content source;
(3) Content exchange : It intelligently balances load traffic on the POP cache server using technologies such as application layer switching, traffic classification, and redirection (ICP, WCCP) according to the availability of content, server availability, and user background. ;
(4) Performance management : It obtains status information of network components through internal and external monitoring systems, measures the end-to-end performance (such as packet loss, delay, average bandwidth, startup time, frame rate, etc.) in optimum operating condition.