Content Delivery Network (CDN) Reference Architecture

Introduction

Every day, users of the Internet enjoy the benefits of performance and reliability provided by content delivery networks ↗ (CDNs). CDNs have become a must-have to combat latency and a requirement for any major company delivering content to users on the Internet. While providing performance and reliability for customers, CDNs also enable companies to further secure their applications and cut costs. This document discusses the traditional challenges customers face with web applications, how the Cloudflare CDN resolves these challenges, and CDN architecture and design.

Who is this document for and what will you learn?

This reference architecture is designed for IT or network professionals with some responsibility over or familiarity with their organization’s existing infrastructure. It is useful to have some experience with technologies and concepts important to content delivery, including caching, DNS and firewalls.

To build a stronger baseline understanding of Cloudflare, we recommend the following resources:

• What is Cloudflare? | Website ↗ (5 minute read) or video (2 minutes)

• What is a CDN? | Website ↗ (5 minute read)
• Analyst Report: Cloudflare named Leader in 2024 GigaOm Radar for Content Delivery Networks ↗ (20 minute read)

Those who read this reference architecture will learn:

• How Cloudflare CDN can significantly improve the delivery of content to your customers
• How anycast IP routing is important in ensuring reliable CDN performance
• The range of tiered caching options and how to choose the one for your needs

Traditional challenges deploying web applications

Over the last several years, especially with the advent of the COVID-19 pandemic and the focus on remote work, there has been a significant growth in Internet traffic, further growing the need to efficiently manage network traffic, cut latency, and increase performance.

Companies running their applications in the cloud or on-premise are faced with the challenges of:

1. Implementing solutions to increase performance
2. As demand grows, scaling out their architecture to meet availability and redundancy concerns
3. Securing their environments and applications from growing Internet threats
4. Reining in growing costs related to doing all of the above

With companies serving customers across the globe, the above challenges require a significant undertaking. Traditionally, a website/application is deployed centrally and replicated to another region for availability, or the website/application is deployed across a handful of servers, sometimes across multiple data centers for resiliency.

The servers hosting the websites are called origin servers. When clients access a website, they make a request for resources from the server. Navigating to one website can generate hundreds of requests from the browser for HTML, CSS, images, videos, etc. With versions of HTTP prior to HTTP/2, each of these HTTP requests would also require a new TCP connection.

Enhancements in HTTP/2 and HTTP/3 allow for multiplexing multiple requests to the same server over a single TCP connection, thus saving server resources. However, compute and network resources are still consumed as servers respond to these requests. As more clients access the website, the following can result:

• The origin server starts to become overloaded with requests, impacting availability; companies start looking at scaling out to handle the additional load
• As each request has to make its way to the origin server, performance and user experience is impacted due to latency
• The latency for end users becomes proportional to the distance between the client and origin server, thus resulting in varying experiences based on client location. This is especially true for specific countries that may experience latency due to traffic from or to that country, like China.
• As origin servers respond to the increasing requests, bandwidth, egress, and compute costs increase drastically
• Even as customers scale out to handle the increased demand in traffic, they are left exposed to both infrastructure-level and application-level distributed denial-of-service (DDoS) attacks

In Figure 1 below, there is no CDN present and there is an origin server sitting in the US. As clients access the website, the first step is DNS resolution, typically done by the user’s ISP. The next step is the HTTP request sent directly to the origin server. The user experience will vary depending on their location. For example, you can see the latency is much lower for users in the US, where the origin server is located. For users outside the US, the latency increases, thus resulting in a higher round-trip time (RTT).

As more clients make requests to the origin server, the load on the network and server increases, resulting in higher latency and higher costs for resource and bandwidth use.

From a security perspective, the origin server is also vulnerable to DDoS attacks at both the infrastructure and application layer. A DDoS attack could be initiated from a botnet sending millions of requests to the origin server, consuming resources and preventing it from serving legitimate clients.

Further, in terms of resiliency, if the origin server temporarily goes offline, all content is inaccessible to users.

Figure 1: HTTP Request with no CDN

How a CDN tackles web application challenges

A CDN helps address the challenges customers face around latency, performance, availability, redundancy, security, and costs. A CDN’s core goal is to decrease latency and increase performance for websites and applications by caching content as close as possible to end users or those accessing the content.

CDNs decrease latency and increase performance by having many data center locations across the globe that cache the content from the origin. The goal is to have content cached as close as possible to users, so content is cached at the edge of the CDN provider’s network.

Impacts

• Improved website load time: Instead of every client making a request to the origin server, which could be located a considerable distance away, the request is routed to a local server that responds with cached content, thus decreasing latency and increasing overall performance. Regardless of where the origin server and clients are located, performance will be more consistent for all users, as the CDN will serve locally cached content when possible.

• Increased content availability and redundancy: Because every client request no longer needs to be sent to the origin server, CDNs provide not only performance benefits, but also availability and redundancy. Requests are load balanced over local servers with cached content; these servers respond to local requests, significantly decreasing overall load on the origin server. The origin server only is contacted when needed (when content is not cached or for dynamic non-cacheable content).

• Improved website security: A CDN acts as a reverse proxy and sits in front of origin servers. Thus it can provide enhanced security such as DDoS mitigation, improvements to security certificates, and other optimizations.

• Reduced bandwidth costs: Because CDNs use cached content to respond to requests, the number of requests sent to the origin server is reduced, thus also reducing associated bandwidth costs.

Routing requests to CDN nodes

An important difference in some CDN implementations is how they route traffic to the respective local CDN nodes. Routing requests to CDN nodes can be done via two different methods:

DNS unicast routing

In this method, recursive DNS queries redirect requests to CDN nodes; the client’s DNS resolver forwards requests to the CDN’s authoritative nameserver. CDNs based on DNS unicast routing are not ideal in that clients may be geographically dispersed from the DNS resolver. Decisions on closest-proximity CDN nodes are based on the client’s DNS server instead of client’s IP address. Also, if any changes are needed for the DNS response, there is a dependency on DNS time to live (TTL) expiration.

Further, since DNS routing uses unicast addresses, traffic is routed directly to a specific node, creating possible concerns when there are traffic spikes, as in a DDoS attack.

Another challenge with DNS-based CDNs is that DNS is not very graceful upon failover. Typically a new session or application must be started for the DNS resolver with a different IP address to take over.

Anycast routing

The Cloudflare CDN, which is discussed in more detail in the next section, uses anycast routing. Anycast allows for nodes on a network to have the same IP address. The same IP address is announced from multiple nodes in different locations, and client redirection is handled via the Internet’s routing protocol, BGP.

Using an anycast-based CDN has several advantages:

• Incoming traffic is routed to the nearest data center with the capacity to process the requests efficiently.
• Availability and redundancy is inherently provided. Since multiple nodes have the same IP address, if one node were to fail, requests are simply routed to another node in close proximity.
• Because anycast distributes traffic across multiple data centers, it increases the overall surface area, thus preventing any one location from becoming overwhelmed with requests. For this reason, anycast networks are very resilient to DDoS attacks.

Introducing the Cloudflare CDN

Cloudflare provides a Software as a Service (SaaS) model for CDN. With Cloudflare’s SaaS model, customers benefit from the Cloudflare CDN without having to manage or maintain any infrastructure or software.

The benefits of the Cloudflare CDN can be attributed to the below two points, discussed in more detail in this section.

1. CDNs inherently increase performance by caching content on servers close to the user
2. The unique Cloudflare architecture and integrated ecosystem

Figure 2 shows a simplified view of the Cloudflare CDN. Clients are receiving their response back from a server on Cloudflare’s global anycast network closest to where the clients are located, thus drastically reducing the latency and RTT. The diagram depicts a consistent end-user experience regardless of the physical location of the clients and origin.

Figure 2: HTTP request to Cloudflare CDN with anycast

Cloudflare CDN architecture and design

Figure 3 is a view of the Cloudflare CDN on the global anycast network. In addition to using anycast for network performance and resiliency, the Cloudflare CDN leverages Tiered Cache to deliver optimized results while saving costs for customers. Customers can also enable Argo Smart Routing to find the fastest network path to route requests to the origin server. These capabilities are discussed in detail in the remainder of this document.

Figure 3: Cloudflare CDN with Tiered Cache on global anycast network

In the above diagram, there are a few important key points to understand about the Cloudflare CDN and the global anycast network it resides on:

• An important differentiator is that Cloudflare utilizes one global network and runs every service on every server in every Cloudflare data center, thus providing end users the closest proximity to Cloudflare’s services, with the highest scale, resiliency, and performance.
• Cloudflare is a reverse proxy, meaning it receives requests from clients and proxies the requests back to the customer’s origin servers. Thus, every request traverses through Cloudflare’s network before reaching the customer’s network. Since Cloudflare has hardened and protected its infrastructure at the edge (ingress), all customers are consequently also protected from infrastructure-level and volumetric DDoS attacks. Requests and traffic must go through the protected Cloudflare network before reaching the customer’s origin server.
• The Cloudflare CDN leverages the Cloudflare global anycast network. Thus the incoming request is routed to and answered by the node closest to the user.
• The inherent benefits of anycast are decreased latency, network resiliency, higher availability, and increased security due to larger surface area for absorbing both legitimate traffic loads and DDoS attacks. Cloudflare’s global anycast network spans hundreds of cities worldwide ↗, reaching 95% of the world’s Internet-connected population within 50 milliseconds while providing over 280 Tbps of network capacity and DDoS protection capability.
• Edge nodes within the Cloudflare network cache content from the origin server and are able to respond to requests via a cached copy. Cloudflare also provides DNS, DDoS protection, WAF, and other performance, reliability, and security services using the same edge architecture.
• Argo uses optimized routing and caching technology across the Cloudflare network to deliver responses to users more quickly, reliably, and securely. Argo includes Smart Routing and Tiered Cache. Cloudflare leverages Argo to provide an enhanced CDN solution.

Tiered Cache

Once a site is onboarded, standard caching is configured by default. With standard caching, each data center acts as a direct reverse proxy for the origin servers. A cache miss in any data center results in a request being sent to the origin server from the ingress data center.

Although standard caching works, it is not the most optimal design — cached content closer to the client may already exist in other Cloudflare data centers, and origin servers are sometimes unnecessarily overloaded as a result. Thus, it is best to enable Tiered Cache, which is included with every Cloudflare plan. With Tiered Cache, certain data centers are reverse proxies to the origin for other data centers, resulting in more cache hits and faster response times.

Tiered Cache leverages the scale of Cloudflare’s network to minimize requests to customer origins. When a request comes into a Cloudflare data center, if the requested content is not locally cached, other Cloudflare data centers are checked for the cached content.

Cloudflare data centers have shorter distances and faster paths between them than the connections between data centers and customer origin servers, optimizing the response to the client with a significant improvement in cache hit ratio. The Cloudflare CDN leverages Argo Smart Routing data to determine the best upper tier data centers to use for Tiered Cache. Argo Smart Routing can also be enabled as an add-on to provide the fastest paths between data centers and origin servers for cache misses and other types of dynamic traffic.

The Cloudflare CDN allows customers to configure tiered caching. Note that depending on the Cloudflare plan, different topologies are available for Tiered Cache. By default, tiered caching is disabled and can be enabled under the caching tab of the main menu. ​​

Tiered Cache topologies

The different cache topologies allow customers to control how Cloudflare interacts with origin servers to help ensure higher cache hit ratios, fewer origin connections, and reduced latency.

Smart Tiered Cache Topology (all plans)	Generic Global Tiered Topology (Enterprise only)	Custom Tiered Cache Topology (Enterprise only)
Recommended for most deployments. It is the default configuration once Tiered Cache is enabled.	Recommended for those who have high traffic that is spread across the globe and desire the highest cache usage and best performance possible.	Recommended for customers who have additional data on their user base and have specific geographic regions they would like to focus on.
Ideal for customers who want to leverage CDN for performance but minimize requests to origin servers and bandwidth utilization between Cloudflare and origin servers.	Generic Global Tiered Topology balances between cache efficiency and latency. Instructs Cloudflare to use all Tier 1 data centers as upper tiers.	Custom Tiered Cache Topology allows customers to set a custom topology that fits specific needs (ex: upper tiers in specific geographic locations serving more customers).
Cloudflare will dynamically find the single best upper tier for an origin using Argo performance and routing data.		Engage with a Customer Success Manager (CSM) to build a custom topology.

Traffic flow: Tiered Cache, Smart Tiered Cache topology

In Figure 4, Tiered Caching is enabled with Smart Tiered Cache Topology. The diagram depicts two separate traffic flows, summarized below. The first traffic flow (Client 1) is a request from a client that comes into Data Center 1. The second traffic flow (Client 2) is a subsequent request for the same resource into a different data center, Data Center 2.

Figure 4: HTTP requests and traffic flow through Cloudflare CDN

Request 1	Request 2
First request received in Data Center 1 results in cache miss, as request had not been made previously by any client.	Second request by a different client received in Data Center 3 results in cache miss, as request had not been made previously by any client served by Data Center 3.
No cached content found, so Data Center 1 checks with its upper tier data center to request a copy of the content.	No cached content found, so Data Center 3 checks with the upper tier data center to request a copy of the content.
Upper tier data center also does not have content cached locally, so it makes a request to the origin server for content. Upon receiving the content, the upper tier data center caches it locally and relays the content to the requesting lower tier data center. The lower tier data center caches the content and responds to the client.	Cached content found at the upper tier data center. Data Center 3 retrieves and caches this content locally and responds to the client.

In Figure 4, the top end user traffic flow displays the traffic flow when a client request is received by a data center closest to the client, Data Center 1. Since there is nothing locally cached on the ingress data center and tiered caching is enabled, a request is sent to the upper tier data center to request a copy of the content to cache. Because the upper tier data center also does not have the content cached, it sends the request to the origin server, caches the received content upon response, and responds to the lower tier data center with the cached content. The lower tier data center caches the content and responds to the client.

Notice that when a new request for the same content is made to another data center (bottom end user traffic flow), Data Center 3, the content is not locally cached; however, the content is retrieved from the upper tier data center, where it was cached from the first request for the same content.

With the upper tier data center returning the cached content for the second request, the trip to the origin server is prevented, resulting in higher cache hit ratios, faster response times, saved bandwidth cost between the Cloudflare network and the origin server, and reduced load on the origin server responding to requests.

Regional Tiered Cache

The main difference between Smart Tiered Cache and Global tiered cache is the number of upper tiers that can talk to the origin servers. With Smart Tiered Cache the closest upper tier to the origin is selected using Argo performance and routing data. This means that all requests that experience a cache MISS at a lower tier will funnel through this single upper tier and have a higher percentage chance of a cache HIT to avoid sending traffic to an origin server. However, the downside to this architecture is that the lower tier could be located across the globe from the upper tier. Even if the upper tier can fulfill the request from its cache, the distance between the upper tier and lower tier could still add latency to the response depending on the distance traveled. To summarize, Smart Tiered Cache ensures that all requests for cache flow through a single upper tier cache location which increases cache HIT percentages, and reduces requests to the origin server, however it can result in higher latencies fulfilling those requests since the upper tier could be located far away from the lower tier that originated the request.

With Generic Global Tiered Cache, Cloudflare uses its largest data centers around the globe as upper tier cache which means, in general, that the upper tier cache is much closer to the lower tier cache. This can greatly reduce latency when lower tiers need to pass requests to upper tiers. However, this ultimately will increase the amount of requests serviced by the origin as each upper tier cache will need to populate from the origin. To summarize, Generic Global Tiered cache can improve response times when cache is populated, but will also increase load on the origin servers.

Regional Tiered Cache combines the best of both of these strategies together by adding an additional layer of cache to the architecture. Using the Regional Tiered Cache option with Smart Tiered Caching means that while a single upper tier cache location exists closest to the origin, a Regional Tier layer has been added between the upper and lower tier that is geographically closer to the lower tier. Now, requests from lower tiers will now check a Regional Tier for cache before being sent to an upper tier. A single Regional Tier can accept requests from several lower tier caches and because of that, can greatly improve performance and latency for globally available applications.

Regional Tiered Caching is recommended for use with Smart Tiered Caching and Custom Tiered Caching. However, Regional Tiered Cache is not beneficial for customers with many upper tiers in many regions like Generic Global Tiered Cache.

Traffic flow: Tiered Cache, Smart Tiered Cache with Regional Tiered Cache

In Figure 5, Tiered Caching is enabled with Smart Tiered Cache Topology. The diagram depicts the topology of Smart Tiered Cache with Regional Tiered Cache enabled. Lower tier caches, when they experience a cache MISS will first send those requests to a more local, regional hub data center to see if the cache can handle the request. If not, the request will continue on to the upper tier and then origin server, if necessary.

Figure 5: Cloudflare CDN with Tiered Cache and Regional Tiered Cache

Argo Smart Routing

Argo Smart Routing is a service that finds optimized routes across the Cloudflare network to deliver responses to users more quickly. As discussed earlier, Cloudflare CDN leverages Argo Smart Routing to determine the best upper tier data centers for Tiered Cache.

In addition, Argo Smart Routing can be enabled to ensure the fastest paths over the Cloudflare network are taken between upper tier data centers and origin servers at all times. Without Argo Smart Routing, communication between upper tier data centers to origin servers are still intelligently routed around problems on the Internet to ensure origin reachability.

Argo Smart Routing accelerates traffic by taking into account real-time data and network intelligence from routing nearly 50 million HTTP requests per second; it ensures the fastest and most reliable network paths are traversed over the Cloudflare network to the origin server. On average, Argo Smart Routing accounts for 30% faster performance on web assets.

Traffic Flow: Tiered Cache, Smart Tiered Cache Topology with Argo Smart Routing

Figure 6 details the traffic flow when Tiered Cache and Argo Smart Routing are not enabled. The request comes into the closest data center, and, because content is not locally cached and Tiered Cache is not enabled, the request is sent directly to the origin server for the content. Also, since Argo Smart Routing is not enabled, a reliable, but perhaps not the fastest, path is taken when communicating with the origin server.

Figure 6: Cloudflare CDN without Tiered Cache or Argo Smart Routing

Figure 7 articulates the traffic flow with both Tiered Cache and Argo Smart Routing enabled. When a request is received by Data Center 1 and there is a cache miss, the cache of the upper tier data center, Data Center 4, is checked. If the cached content is not found at the upper tier data center, with Argo Smart Routing enabled, the request is sent on the fastest path from the upper tier data center to the origin.

The fastest path is determined by the Argo network intelligence capabilities, which take into account real-time network data such as congestion, latency, and RTT.

With the Cloudflare CDN, Argo Smart Routing is used when:

1. There is a cache miss and the request needs to be sent to the origin server to retrieve the content.
2. There is a request for non-cacheable content, such as dynamic content (ex: APIs), and the request must go to the origin server.

Figure 7: Cloudflare CDN with Tiered Cache and Argo Smart Routing

Cache Reserve

Expanding on the idea of Tiered Cache, Cache Reserve further utilizes the scale and speed of the Cloudflare network while additionally leveraging R2, Cloudflare’s persistent object storage, to cache content even longer. Cache Reserve helps customers reduce bills by eliminating egress fees from origins while also providing multiple layers of resiliency and protection to make sure that content is reliably available which improves website performance by having content load faster. Basically, Cache Reserve is an additional higher tier of cache with longer retention duration.

While Cache Reserve can function without Tiered Cache enabled, it is recommended that Tiered Cache be enabled with Cache Reserve. Tiered Cache will funnel, and potentially eliminate, requests to Cache Reserve which eliminates redundant read operations and redundant storage of cached content reducing egress and storage fees. Enabling Cache Reserve via the Cloudflare dashboard will check and provide a warning if you try to use Cache Reserve without Tiered Cache enabled.

Cache Reserve has a retention period of 30 days which means it will hold cached content for 30 days regardless of cached headers or TTL policy. The TTL policy still affects the content’s freshness which means when content cache TTL expires inside of Cache Reserve, the content will need to be revalidated by checking the origin for any updates. The TTL policy can be set by any number of methods, such as Cache-Control, CDN-Cache-Control response headers, Edge Cache TTL, cache TTL by status code, or Cache Rules. Every time cache is read from Cache Reserve, the retention timer is reset to 30 days. After 30 days, if the cached content has not been read from Cache Reserve, the cache will be deleted.

There are three main criteria to match for content to be considered cacheable via Cache Reserve:

1. The content must be cacheable. See the Cache documentation for more details on cacheable content.
2. TTL is set to at least 10 hours. This can be set by any method from the previous paragraph.
3. The Content-Length header must be used in the response header. Please note, this means that the [Transfer-Method “chunked” will prevent Cache Reserve from being populated.

When combined with Tiered Caching and Argo Smart Routing, Cache Reserve can be a powerful tool for increasing cache hits and in turn reducing load on origin servers while also improving performance by bringing the content closer to the end user.

Note

Using Image Resizing with Cache Reserve will not result in resized images being stored in Cache Reserve since Image Resizing takes place after reading from Cache Reserve. Resized images will be cached in other available tiers when they are served after resizing.

Traffic flow: Cache Reserve topology

Figure 8 illustrates how Cache Reserve can help reduce load on an origin server while also helping repopulate cache stores in both upper and lower tier data centers.

Figure 8: Cloudflare CDN with Tiered Cache and Cache Reserve

China Network & China Express for clients in China

Latency is not only proportional to the physical distance between the client and the origin or cache, but it may also increase significantly when traffic comes from or goes to certain countries, such as China. To mitigate the latency, Cloudflare offers two solutions:

1. China Network ↗ provides in-China caching for end users located in China, regardless of the origin location. This solution is provided by collaborating with JD Cloud and uses their data centers to ensure the fastest and most reliable cache performance for Chinese users compared to data centers outside of China.
2. China Express ↗ offers reliable and secure connectivity to streamline content from origins to JD Cloud data centers in China. This is particularly beneficial for dynamic content like web applications and API calls.

Summary

To summarize, the Cloudflare CDN is SaaS that helps address the challenges customers face around latency, performance, availability, redundancy, security, and costs. The Cloudflare CDN leverages Cloudflare’s global anycast network and Tiered Cache to deliver optimized results while saving costs for customers. Customers can also enable Argo Smart Routing to ensure the fastest network path is used to route requests to the origin server and also choose to enable Cache Reserve to increase cache hits to further save costs and increase performance of their website or application.