Today, the mobile Internet era, more and more people use mobile phones to watch videos, enrich their own entertainment.
However, everyone in the chase drama, there is no thought of a question - why sometimes obviously your phone's speed quickly, but when watching video, still Caton?
Before answering this question, let's do an arithmetic problem.
Prior to the fire of the "Yanxi Raiders" for example, it was once in a video APP achieved 10 million users simultaneously online viewing.
If you watch a 1080p resolution video (theoretically requires 4Mbps bandwidth), then the cumulative traffic bandwidth required is 10,000,000 × 4Mbps = 40,000,000Mbps = 40Tbps.
For Youku, love Fantastic Art video content providers such as the Internet, this is undoubtedly a very huge traffic pressure.
Our common computer network card is 1Gbps bandwidth. If Youku has a super server, then this super server needs 40000 network card, and must be one hundred percent run full speed, to be able to achieve smooth viewing this 1 million users.
In the case of some of the service providers is not enough strength or burst traffic increased sharply, it will cause congestion, resulting in Caton and delay.
There is such a saying: When a user opens a page, wait for more than four seconds, he would close this page. In other words, the user will be lost.
The loss of customers, it means the loss of money. No other Internet service providers want that to happen. Therefore, they must find ways to make their content presentation as soon as possible to shorten the waiting time for users, improve the user experience.
The CDN, is a very effective technique to shorten the delay.
CDN's birth
In the 1980s, Internet technology just into civilian areas.
People primarily access through dial-up network bandwidth is very low, and few users, so do not put pressure on the backbone network and server.
With the explosive growth of the Internet, more and more users, plus the emergence of broadband access networks, pressure source content server and backbone networks more and more, unable to respond to the needs of the user's access.
In 1995, MIT professor, one of the inventors of the Internet, Dr. Tim Berners-Lee found that network congestion is getting worse, it will become the biggest obstacle to development of the Internet.
Tim Berners-Lee
So he made an academic problem, hope someone can invent a new method to solve the problem fundamentally, to implement congestion-free Internet content distribution.
Next time Dr. Tim Berners-Lee, the office of professor Tom Leighton. He is a professor of applied mathematics at MIT.
Tom Leighton
He was Berners-Lee's challenge intrigued, so he asked graduate student Danny C. Lewin and several other top researchers together to crack this technical problem.
Danny C. Lewin
Eventually, they developed a dynamic routing algorithm technology uses mathematical algorithms to process the content, effectively solve this problem. This technique is CDN.
They also established for this purpose, and play to their commercial value. The company was later famous originator --Akamai CDN services company.
Akamai
CDN principle
CDN said that this technology is actually not very complicated, the original core idea is that the contents of the cache in the vicinity of the end user.
Content source is not far away? So, we are in a place close to the user, to build a cache server, the contents of the remote, copy, placed here, not to OK?
The difference between traditional network and CDN networks
Because this technology is to distribute the content, so its name is called CDN - Content Delivery Network, the content distribution network.
Specifically, CDN is to use more cache servers (CDN edge node), cloth on the regions with relatively dense user access or network. When a user accesses the site, the use of global load technology, the user's access point to the nearest cache server, the cache server in response to user requests. (A bit like the local electricity supplier's warehouse, right?)
You may think that this is not "mirror" thing? In fact, it is not the same. Mirror server is a complete copy of the source content server. The CDN, is part of the contents of the cache, a higher degree of intelligence.
Rather, CDN = more intelligent cache + mirror + traffic diversion.
But also it should be noted that, CDN cache is not the only video content, it can be distributed to a static resource site (for example, various types of images, html, css, js, etc.), static content for mobile applications of APP (such as installing pack apk files, pictures, videos, etc.) for distribution in the APP.
We give an example, look at the specific workflow of the CDN.
If a user wants to access Youku's video-on-demand content, then:
Specific steps:
①, when the user clicks on the content of APP, APP will seek an IP address based on the URL address to the local DNS (Domain Name System) resolution.
②、本地DNS系统会将域名的解析权交给CDN专用DNS服务器。
③、CDN专用DNS服务器，将CDN的全局负载均衡设备IP地址返回用户。
④、用户向CDN的负载均衡设备发起内容URL访问请求。
⑤、CDN负载均衡设备根据用户IP地址，以及用户请求的内容URL，选择一台用户所属区域的缓存服务器。
⑥、负载均衡设备告诉用户这台缓存服务器的IP地址，让用户向所选择的缓存服务器发起请求。
⑦、用户向缓存服务器发起请求，缓存服务器响应用户请求，将用户所需内容传送到用户终端。
⑧、如果这台缓存服务器上并没有用户想要的内容，那么这台缓存服务器就要网站的源服务器请求内容。
⑨、源服务器返回内容给缓存服务器，缓存服务器发给用户，并根据用户自定义的缓存策略，判断要不要把内容缓存到缓存服务器上。
CDN的好处
采用CDN技术，最大的好处，就是加速了网站的访问——用户与内容之间的物理距离缩短，用户的等待时间也得以缩短。
而且，分发至不同线路的缓存服务器，也让跨运营商之间的访问得以加速。
例如中国移动手机用户访问中国电信网络的内容源，可以通过在中国移动架设CDN服务器，进行加速。效果是非常明显的。
此外，CDN还有安全方面的好处。内容进行分发后，源服务器的IP被隐藏，受到攻击的概率会大幅下降。而且，当某个服务器故障时，系统会调用临近的健康服务器 进行服务，避免对用户造成影响。
正因为CDN的好处很多，所以，目前所有主流的互联网服务提供商，都采用了CDN技术。所有的云服务提供商，也都提供了CDN服务（价格也不算贵，按流量计费）。
CDN和通信行业
CDN是从传统IT行业发展起来的一项服务。但是，对于我们通信行业来说，CDN也有非常大的商业价值。
互联网服务提供商采用CDN，是以存储换时延。花钱购置CDN服务器或云计算服务，以此换取更好的用户体验。
Communications operators also sought CDN, but their purpose is to change the bandwidth of memory - through service "sink" to reduce traffic pressure of the upper backbone network, hardware expansion to avoid, reduce network construction costs.
This is well understood ah, if a large amount of data traffic running around in the backbone network, backbone network certainly too much to hard expansion. If the data in the underlying traffic was resolved, then the backbone network bandwidth pressure naturally reduced. Is not it?
CDN Communications Architecture
Many operators have sunk to the municipal CDN, in order to reduce stress, and can improve the user experience.
All along, with the continuous improvement of network capabilities, content resources and computing power are constantly "going up", went to the cloud computing center. It consists of a core of cloud computing centers to provide services to all end nodes.
As a result, people look back and found that, for a very large area of the region, a very large number of users, especially national or world-class service, whether you put this center in where, no matter how much ability you this powerful center , it can not overcome the physical barriers of distance, can cause intolerable delays and network congestion.
Ever since, people began to cloud computing center part of the "sinking", this has been calculated fog, haze calculations. Even people began to question whether centralized computing will eventually be replaced by a distributed computing?
In fact, who does not exist completely replace whom. Different scenarios bring different needs, different needs require different network architectures. Diversification of the scene is real, so flexible network architecture, but also the inevitable choice.
Upcoming 5G, presented three scenarios and slices, with a network to meet different needs, embodies the design concept, conforms to the development trend direction.
All in all, the network technology, whether it is black, white, short, catches mice is a good cat. Is not it?