Introduction to Computer Network CDN Technology

Preface

This article combs CDN technology

What is CDN

CDN (Content Delivery Network, Content Delivery Network) is an intelligent virtual network built on the basis of the existing Internet. By deploying node servers throughout the network, the source site content can be distributed to all CDN nodes so that users can be nearby Get what you need.

Why do we need CDN

The CDN service shortens the access delay for users to view content, improves the response speed of users visiting the website and the availability of the website, and solves the problems of small network bandwidth, large user visits, and uneven distribution of outlets.

Advantages of CDN:

• Improve the response speed of users visiting the website

• In order to achieve cross-operator and cross-regional network coverage
  , various factors such as interconnection and non-interworking, regional ISP geographic limitations, and limited export bandwidth have caused the regional inaccessibility of the website. CDN acceleration can cover global lines. Through cooperation with operators, IDC resources can be deployed, and CDN edge distribution storage nodes can be reasonably deployed in backbone node vendors across the country to make full use of bandwidth resources and balance source site traffic.

• In order to ensure the security of your website,
  CDN's load balancing and distributed storage technology can strengthen the reliability of your website, and it is quite invisibly adding an umbrella to your website to deal with most Internet attacks. The anti-attack system can also avoid malicious attacks on websites.

• For remote backup.
  When a server fails unexpectedly, the system will call other nearby healthy server nodes to provide services, thereby providing close to 100% reliability, which allows your website to never be down.

• In order to save costs,
  use CDN to accelerate the deployment of websites across the country. You don’t have to consider purchasing servers and subsequent hosting operation and maintenance, mirroring synchronization between servers, and you don’t have to worry about managing and maintaining technicians, saving manpower, energy and financial resources. .

• In order to allow you to focus more on the business itself,
  CDN acceleration vendors generally provide one-stop services. The business is not limited to CDN, but also supporting cloud storage, big data services, video cloud services, etc., and generally provide 7x24 operation and maintenance monitoring support to ensure The network is open at any time, so you can use it with confidence. And put more energy into the development of their core business.

CDN basic workflow

1. When the user clicks on the content URL on the website page, after the local DNS system resolves, the DNS system will finally transfer the resolution power of the domain name to the CDN dedicated DNS server pointed to by CNAME.
2. The DNS server of the CDN returns the IP address of the CDN's global load balancing device to the user.
3. The user initiates a content URL access request to the CDN's global load balancing device.
4. The CDN global load balancing device selects a regional load balancing device in the area where the user belongs based on the user's IP address and the content URL requested by the user, and tells the user to initiate a request to this device.
5. The regional load balancing device will select a suitable cache server for the user to provide services. The selection basis includes: judging which server is the closest to the user according to the user’s IP address; judging which one is based on the content name carried in the URL requested by the user One server has the content required by the user; query the current load situation of each server to determine which server still has the service capacity. Based on the comprehensive analysis of the above conditions, the regional load balancing device will return the IP address of a cache server to the global load balancing device.
6. The global load balancing device returns the IP address of the server to the user.
7. The user initiates a request to the cache server, and the cache server responds to the user request and transmits the content required by the user to the user terminal. If there is no content that the user wants on this cache server, and the regional balance device still assigns it to the user, then this server will request the content from its upper level cache server until it is traced back to the origin server of the website Pull the content locally.

• CNAME
  canonical name record (Canonical Name) is used to describe the correspondence between aliases and domain names. This record allows you to map multiple names to the same computer.
  When the domain name server looks up a domain name and finds a CNAME record, it will replace the domain alias with the canonical name in the record, and then check the A record of the canonical name to find the IP address corresponding to the canonical name. In this way, Realize the IP address response to the request to find the domain name.
  

CDN system architecture

• Distribution service system
  The main function of the system is to push and store content from the content source center to the edge, and to undertake the entire network distribution of the actual content data stream and data request services for end users.

• Load balancing system
  It is the nerve center of a CDN system. Its main function is to schedule access to all users who initiate service requests and determine the final actual access address provided to users.

• CDN operation management system
  It is similar to the general telecom operation management system, divided into two subsystems: operation management and network management.

  • The operation management subsystem is the business management functional entity of the CDN system. It is responsible for processing some collection, sorting, and delivery tasks necessary for interaction with external systems at the business level, including customer management, product management, billing management, statistical analysis and other functions.
  • The network management subsystem realizes the network equipment management, topology management, link monitoring and fault management of the CDN system, and provides an interface for the administrator to centralize the management and operation of the resources of the entire network, usually based on the Web.

CDN system classification

From the perspective of the types of content carried by CDN, there are mainly static web content, dynamic web content, streaming media, downloadable files and application protocols. Therefore, we divide CDN services into:

• Web acceleration
• Streaming acceleration
• File transfer acceleration
• Application protocol acceleration

CDN system model

Among them, the horizontal X-axis direction is divided according to different types of business capabilities, and the
vertical Y-axis direction is divided according to the three-level network structure of center, area and edge (according to the network scale, the area level is optional); the
Z axis direction is based on the data plane and management The plane and the control plane are divided.

• X-axis direction
  It is a vertical system that runs from the system head to the terminal. Each business capability can have its own center, area, and edge layer equipment, as well as its own independent management system, load balancing system, and distribution service system. Therefore, in the X-axis direction, each CDN service capability can be a set of relatively independent subsystems.
• In the Y-axis direction,
  its hierarchical division is based on the requirements of CDN gradual caching, hierarchical scheduling, and hierarchical services. The management functions, scheduling functions, caching functions, and service functions of the CDN are all deployed hierarchically, and the same functional entities at different levels cooperate with each other. From the point of view of control function, the next level of control entity is often the executor of the upper level of control entity; from the perspective of cache function, the upper level cache is the content provider of the next level of cache entity; from the perspective of service function, the upper level The first-level service entity is the capability supplement and backup of the next-level service entity.
• Z-axis direction The
  management layer mainly completes the CDN network management and business logic processing. Network management provides configuration management, fault management, network performance management, and network security management at the IP network level. It is responsible for abstracting the network and equipment resources of the entire CDN system, extracting its business capabilities, and providing them to the upper business platform. Business logic processing means that the CDN system assists its service business platform to complete the realization of some application layer functions, completes the interface adaptation with other systems, and is responsible for system authentication, billing, statistical analysis, system management, user management, and SP management And other functions. The control plane is responsible for managing the content distribution of the entire CDN network, and controlling content distribution and access routing strategies. It is the control point of the content data flow of the entire content distribution system. The data plane is responsible for the actual content distribution work. According to the specific requirements of the intersection of the Z-axis direction and the X-axis and the Y-axis, the data plane device has different application service provision capabilities.

Global load balancing GSLB

• Load balancing
  is to allocate a large number of requests to different service nodes through a certain strategy. It can improve the server's impact speed and utilization efficiency, avoid a single point of failure of software or hardware modules, solve network congestion problems, realize geographic independence, and provide users with more consistent access quality.

• Global load balancing GSLB is
  due to various unstable factors in reality, such as a power outage of a data center where a server cluster is located, floods or earthquakes causing data center paralysis and so on. In a data center, no matter what technology is adopted, there may always be some irresistible factors that cause its paralysis. Therefore, servers are usually deployed in multiple data centers to minimize the probability and extent of disasters affecting service quality.
  In addition, the CDN system always wants to use the device closest to the user to provide services for it, which also requires the deployment of multiple nodes in different regions.
  The GSLB system addresses this problem. It is responsible for cooperating among multiple CDN nodes, keeping the load of each node and device at a level that is conducive to providing quality services. The load balancing result of GSLB may directly assign users to RS, or it may deliver users to the next level of load balancing system.

• Comparison of GSLB implementation methods
  Currently commonly used DNS-based resolution methods
  

Conclusion

This article combs CDN related knowledge to have a more overall understanding of the network.

Reference:
Basic introduction to CDN concept Brief introduction to
CDN principle