Let's begin by considering some important fundamentals and concepts related to CDN. So why CDN, why content delivery network? Why does this type of product exist? Alibaba Cloud is not the only major cloud to offer a CDN. In fact, almost all modern global scale thought providers, have some type of CDN offering and many companies specialize in providing CDN service. Why is this? Well, the reason is fairly simple. The world is very, very large and traffic between different countries often has to traverse great distances, over undersea cables. This not only causes long delays because of the physical distance, that the traffic must travel, but can also lead to network congestion on popular links. And as a result, you can achieve better speeds, better load speeds for your website content, if you host it vocally near your end users.

But what do you do if your end users aren't physically near you? Well, you need to keep local cached copies of your websites content, in multiple places, all around the world, in locations that are near to your end users, wherever they happen to be. And this is what CDN does. So, a content delivery network or CDN, uses a network of servers in geographically distinct dispersed locations, which we call edge locations to cache copies of content close to end users, lowering latency when they download or stream this content.

Content is replicated across edge locations, thereby providing redundancy. The major features of a CDN are low latency, which improves content load time. High bandwidth, which enables high bit rate HD video and other bandwidth heavy applications like say downloading software updates. Redundancy, which eliminates single points of failure. Scalability, which ensures great experience as your number of end users grows. And global distribution.

Most CDN networks are worldwide networks would provide a great user experience regardless of where your users happen to be relative to where your web application is hosted. And of course cost-effectiveness, Alibaba Clouds CDN uses a pay as you go model, which provides flexibility. The more CDN service you use in terms of the total amount of bandwidth used, the more you pay, the less you use, the less you pay, it scales with the utilization of your web application.

The basic concept behind a CDN is actually very simple. There's an origin server, which is where your website's content is hosted and then there are CDN servers, which you can think of as being like a type of cache that are located near where your users are. So if the origin server is where all of the original, authoritative copies of your websites, HTML, CSS, JavaScript, images, etc, are stored. All of that content is stored on the origin server. What happens is, when a client, when a user requests, a copy of a particular page on your site, or they request specific data from your web application backend, rather than always having to go back to the origin server, to fetch that content, the client's request can be sent to a CDN node. And what the CDN node will do, is deliver a cached copy of that content.

If the CDN node does not have the content, the user requested, it asks the origin for a copy of that content and then subsequently stores that copy locally, so that the next time a client makes a request to the CDN server, it can deliver the local cached copy, saving time and potentially bandwidth as well. In order to function correctly, Alibaba Cloud CDN needs to be able to resolve the domain name for a website into the IP address of a CDN node that's near the end user.

So let's take a look at how the two processes that make that happen work. The first is DNS resolution and the second is content retrieval and they're related. And let's assume that we have a website, a.com that's hosted in the United States and we have a user that's located in Beijing in China. So the user requests, a.com, which is to say, they type a.com into their web browser, and now they need to fetch the content for the website, a.com. The first thing their computer will do, is make a DNS request to convert a.com into a public IP address. That request will go to LDNS, the local DNS service, on that user's network. The LDNS service might not know the public IP address of a.com, so it will ask an authoritative DNS server, somewhere else on the internet, that's step two on the left side of the slide. That authoritative server, will return a CNAME record.

So it will first convert a.com into another domain name, a domain name that points to, Alibaba Clouds CDN service. It will return that, to the local DNS service. In step four, local DNS will ask, Alibaba Cloud DNS, how to convert the CNAME record into an IP address. This is where things get interesting. Alibaba Cloud DNS will have a look at where the local DNS service is located and will return an IP address for an Alibaba Cloud CDN node that is nearby. So the LDNS service will be in Beijing with the user. When Alibaba Cloud DNS sees this request to convert the tbcdn.com CNAME into an IP address, it will return the IP address of a CDN node located in Beijing, China, which is where the user and their local DNS service are located. So it returns that IP 2222 to the user.

The user's browser or application will then make an HTTP request to that public IP and that public IP points to a CDN node or a POP. So the content that the user fetches for a.com will from that CDN node. If the CDN node doesn't have a local copy of the content that the user requested, it will fetch that copy from the origin server, which again is hosted somewhere in the United States and will then store that local copy on the CDN server, the POP in Beijing, so that the next time a user in Beijing makes a request, it can be resolved directly by the Beijing CDN node without the need to go back to the origin.

So just to summarize, the way that CDN functions is, rather than fetching content from an origin site, which could be geographically very far away, instead, CDN routes users to a nearby CDN node that they can fetch content from more quickly. In order to achieve this, the CDN network has to be very large and there need to be CDN nodes all around the world. Alibaba's CDN is a global CDN. We have more than 120 terabytes per second, in total CDN bandwidth, covering more than 70 countries across six continents. In total, we have 2,800 plus nodes. 2300 of which are in China, with another 500 or so around the world. And each node has a storage capacity of between 40 terabytes and 1.5 petabytes with a minimum network bandwidth of 40 gigabytes per second, per CDN nodes. So we have a large, powerful, high bandwidth, high storage CDN network, with nodes that are near you, no matter where you happen to be. That's all for this section.

In the next section, we'll take a look at CDN architecture.