CDN (Content Delivery Network) Knowledge

CDNThe full name is content delivery network, that is, content delivery network. The basic idea is to avoid bottlenecks and links on the Internet that may affect data transmission speed and stability, so that content transmission can be faster and more stable. A layer of smart virtual network formed by placing node servers in various parts of the network on the basis of the existing Internet, the CDN system can redirect users' requests to the nearest service node in real time based on the network traffic and connection, load status, distance to the user and response time of each node.. The aim is to allow users to obtain the desired content nearby, solve the problem of Internet congestion, and improve the response speed when users access the website.

I. graphic functions of CDN:

Before CDN acceleration:

In traditional networks, website servers are generally connected to the Internet through a single line, which cannot meet the needs of users in different regions and lines for high-speed access.

Websites cannot be opened, and web pages are displayed slowly, which is not conducive to website operation and development!

 

After CDN is used:

CDN nodes are deployed on different lines in China and different data centers to achieve nearby reading of website data and reduce the waiting time for Data Reading.

Because CDN nodes share a large amount of data requests, the access pressure on the Web source server is greatly reduced.

Because the CDN node and the website source server may not have the same line, wdcdn implements BGP multi-line back-to-Source request data.

Visitor access data is read from the CDN node, and the IP address of the website source server can be hidden. Ensure security and prevent malicious attacks!

Ii. CDN service mode

In fact, content delivery network (CDN) is a new network construction method. It is a network covering layer that is particularly optimized to publish broadband rich media on a 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 a strategically deployed overall system, which includes 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, the content service is based on a cache server, also known as a 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.

III,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:

As you can see, the process for users to access websites that are not cached using CDN is as follows:

1) The user provides the domain name to be accessed to the browser;

2) the browser calls the domain name resolution function library to resolve the domain name to obtain the corresponding IP address of the domain name;

3) when the browser uses the obtained IP address, the service host of the domain name sends a data access request;

4) the browser displays the webpage content based on the data returned by the domain name host.

Through the above four steps, the browser completes the whole process from receiving the domain name accessed by the user to obtaining data from the domain name service host. The CDN network adds a cache layer between the user and the server. How to direct users' requests to the cache to obtain data from the source server is mainly achieved by taking over DNS, let's take a look at the process of accessing the website cached by CDN:

We can see that the website access process after CDN cache is changed:

1) The user provides the domain name to be accessed to the browser;

2) the browser calls the domain name resolution library to resolve the domain name. Because CDN has adjusted the domain name resolution process, the resolution function library generally obtains the cname record corresponding to the domain name, to obtain the actual IP address, the browser needs to resolve the obtained cname domain name again to obtain the actual IP address. In this process, the Global Server Load balancer DNS resolution is used, for example, you can resolve the corresponding IP address based on the geographic location information so that users can access the IP Address nearby.

3) the IP address of the CDN Cache Server is obtained through this resolution. After obtaining the actual IP address, the browser sends an access request to the cache server;

4) The Cache Server obtains the actual IP address of the domain name based on the domain name to be accessed provided by the browser through private DNS resolution within the cache, and then submits access requests to the actual IP Address by the cache server;

5) after the Cache Server obtains the content from the actual IP address, it saves the content locally for future use, and returns the obtained data to the client to complete the data service process;

6) after the client obtains the data returned by the cache server, it displays the data and completes the browsing data request process.

Through the above analysis, we can see that in order to achieve transparency to normal users (that is, after the cache is added, the user client does not need to make any settings, and can directly access the original Domain Name of the accelerated website ), in addition, when providing acceleration services for a specified website and reducing the impact on ICP, you only need to modify the domain name resolution Section during the entire access process to achieve transparent acceleration service, the following describes how to implement the CDN network.

1) As an ICP, you only need to give the domain name Interpretation Right to the CDN operator, and do not need to make any changes to other aspects. During the operation, the ICP modifies the resolution record of your domain name, generally, cname is used to point to the address of the CDN network cache server.

2) as a CDN carrier, you must first provide public resolution for the ICP domain name. To achieve sortlist, the ICP domain name interpretation result is generally directed to a cname record;

3) When sorlist is required, the CDN operator can use DNS to perform special processing on the domain name resolution Process pointed to by cname, so that the DNS server can receive client requests according to the IP address of the client, returns different IP addresses of the same domain name;

4) because the IP address obtained from cname carries the Hostname Information, after the request arrives at the cache, the cache must know the IP address of the source server. Therefore, the CDN operator maintains an internal DNS server, it is used to explain the real IP address of the domain name accessed by the user;

5) when maintaining the internal DNS server, you also need to maintain an authorization server to control which domain names can be cached and which do not, so as to avoid opening the proxy.

Iv. Key Technologies
(1) content publishing: with the help of indexing, caching, stream splitting, and Multicast Technologies, the content is published or delivered to the remote service point (POP) closest to the user);
(2) content routing: it is a holistic network load balancing technology. Through the redirection (DNS) mechanism in the Content Router, user requests are balanced on multiple remote POP, to get the response from the latest content source for user requests;
(3) content exchange: Based on the Content availability, server availability, and user background, it uses application layer exchange, stream splitting, and redirection (ICP and WCCP) on the POP cache server) and other technologies to intelligently balance load traffic;
(4) Performance management: it obtains the status information of network components through internal and external monitoring systems, measure the end-to-end performance of content publishing (such as packet loss, latency, average bandwidth, start time, and frame rate) to ensure that the Network is in the optimal running state.

V. Application Examples
Currently, large websites with high access volume in China, such as Sina and Netease, all use the CDN network acceleration technology. Although the Website access is huge, it will feel fast wherever it is accessed. Generally, if the website server is in China Netcom, the access by telecom users is very slow. If the server is in China Telecom, the access by Netcom users is very slow.
It adopts a distributed network cache structure (that is, the internationally popular Web Cache Technology). By adding a new network architecture to the existing Internet, publish the website content to the Cache Server closest to the user. Using the DNS Server Load balancer technology, you can determine that the user source accesses the Cache Server nearby to obtain the required content and solve Internet network congestion, to speed up the response of users to visit websites, as if multiple accelerators are provided in various regions, to achieve fast and redundant acceleration for multiple websites.
The CDN service was initially used to ensure fast and reliable distribution of static content, which can be cached and is most suitable for storing and distributing content on a network with a high speed, the network covers CDN network servers in dozens of countries.
Because dynamic content must be transmitted over the Internet, to provide a fast network experience to meet the requirements of today's users, it can only be achieved through a unique perspective on Internet traffic. Advanced technologies must be used to learn about the best routes over the Internet and make full use of these optimal routes to accelerate data communication over the Internet Protocol to improve the user's network experience.

Appendix:

1. What are the advantages of CDN over dual-line data centers?
Common dual-line data centers can only solve the problem of slow access between China Netcom and China Telecom. The problem of intercommunication between other ISPs (such as CERNET, mobile network, and China tietong) is still not solved. CDN allows visitors to retrieve data from the nearest node, while CDN nodes are distributed across ISPs, which ensures the access speed from the website to any ISP. In addition, because of the principle that CDN distributes traffic to each node, it is naturally able to resist network attacks.

2. Why is my website updated and I can see the webpage or the old Webpage Through CDN?
Because CDN uses the cache mechanism of each node, after the static webpages and images of the website are modified, if the CDN cache is not updated accordingly, the old webpages are still displayed. To understand
To solve this problem, the CDN Management Panel provides the url push Service to notify each CDN node to refresh its own cache.
In the URL push address bar, enter a specific URL or image address. The cached content on each node is deleted and takes effect immediately.
If you need to push too many URLs and images, you can select directory push and enter

Http://www.kkk.com/news refreshes all the web pages and images under the news directory under the website.

From: http://kb.cnblogs.com/page/121664/

Http://wiki.baiwanzhan.com/term/seo/cdn/

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