What is a CDN? | NOC.org

Understanding Content Delivery Networks

A Content Delivery Network (CDN) is a geographically distributed network of servers that delivers web content to users based on their location. Instead of every visitor fetching files directly from your origin server, a CDN caches copies of your content on edge servers around the world. When a user requests a page, the CDN serves it from the nearest edge server, dramatically reducing latency and load times.

CDNs were originally built to solve a simple physics problem: data travels at the speed of light through fiber optic cables, but over long distances, the round-trip time adds up. A user in Tokyo requesting a page from a server in New York experiences roughly 200 milliseconds of latency just from the physical distance. A CDN eliminates this by placing cached content within a few milliseconds of virtually every user on the planet.

How Edge Caching Works

When a CDN edge server receives a request for content it does not yet have cached, it fetches the content from your origin server, stores a copy, and delivers it to the user. Subsequent requests for the same content are served directly from the edge cache without contacting the origin. This process is called a "cache miss" on the first request and a "cache hit" on subsequent requests.

CDNs use HTTP cache headers to determine how long content should be cached. The Cache-Control header is the primary mechanism, with directives like max-age specifying the time-to-live (TTL) in seconds. For example, Cache-Control: public, max-age=86400 tells the CDN to cache the content for 24 hours.

Static assets like images, CSS, JavaScript, and font files are ideal candidates for edge caching because they rarely change. Dynamic content like personalized pages or API responses can also be cached with careful configuration, though shorter TTLs and cache key variations are typically used.

Points of Presence (PoPs)

A Point of Presence (PoP) is a physical location where a CDN has deployed edge servers. Major CDN providers operate hundreds of PoPs across six continents. Each PoP typically contains multiple servers with high-speed connections to the local internet exchange points (IXPs) and transit providers.

The number and distribution of PoPs directly affects a CDN's performance. More PoPs mean shorter distances to users, which translates to lower latency. The placement of PoPs is strategic: CDN providers prioritize locations near major population centers and internet hubs where traffic volumes are highest.

Anycast Routing

Most modern CDNs use Anycast routing, a network addressing method where the same IP address is announced from multiple locations simultaneously. When a user's DNS query resolves to the CDN's IP address, their request is automatically routed to the nearest PoP by the internet's Border Gateway Protocol (BGP). This happens transparently without any action from the user or the website owner.

Anycast provides two key benefits beyond performance: automatic failover and DDoS absorption. If a PoP goes offline, BGP automatically reroutes traffic to the next nearest location. During a DDoS attack, the attack traffic is distributed across all PoPs rather than concentrated on a single server, making it far easier to absorb.

Performance Benefits

Reduced latency: Content is served from the nearest edge server, cutting round-trip times from hundreds of milliseconds to single digits.
Lower origin load: Cached content is served without contacting your origin server, reducing CPU, memory, and bandwidth consumption on your infrastructure.
Improved Core Web Vitals: Faster content delivery directly improves metrics like Largest Contentful Paint (LCP) and First Contentful Paint (FCP), which affect both user experience and search engine rankings.
Bandwidth savings: CDNs handle the majority of your traffic, significantly reducing the bandwidth your hosting provider needs to serve.
High availability: With content cached across multiple locations, your site remains available even if your origin server experiences downtime.
Connection optimization: CDNs maintain persistent connections between PoPs and use optimized routing protocols to deliver content faster than the public internet.

Security Benefits

Modern CDNs provide substantial security benefits beyond performance optimization:

DDoS protection: The distributed nature of a CDN's network absorbs volumetric attacks across dozens of PoPs, preventing any single point from being overwhelmed.
Origin IP hiding: When traffic is proxied through a CDN, your origin server's IP address is not exposed to the public internet, making it harder for attackers to target directly.
TLS termination: CDNs handle SSL/TLS encryption at the edge, offloading the computational cost from your origin server while ensuring encrypted connections for all users.
Bot management: Many CDNs include bot detection capabilities that identify and block malicious automated traffic before it reaches your application.
WAF integration: CDN providers frequently bundle Web Application Firewall functionality, inspecting traffic at the edge and blocking application-layer attacks like SQL injection and XSS.

Types of CDN Architecture

Push vs. Pull

In a pull CDN, edge servers fetch content from the origin on demand when a user requests it. This is the most common model because it requires no manual content management. In a push CDN, you upload content directly to the CDN's storage, which then distributes it to edge servers. Push CDNs are used for large media files or content that does not change frequently.

Reverse Proxy CDN

A reverse proxy CDN sits in front of your origin server and intercepts all incoming traffic. It caches what it can and forwards the rest to the origin. This model is the most popular for website acceleration because it requires only a DNS change to deploy and provides both caching and security benefits.

When You Need a CDN

Any website with a geographically distributed audience benefits from a CDN. E-commerce sites, media publications, SaaS applications, and API-driven services all see measurable improvements in performance and reliability. Even small websites benefit from the security features, particularly DDoS protection and origin shielding.

The cost of not using a CDN is measured in slower load times, higher bounce rates, lower search rankings, and increased vulnerability to attacks. For most websites, a CDN is no longer optional; it is a fundamental component of modern web infrastructure.