Understanding CDN: How Content Delivery Networks Accelerate Web Access

Introduction

CDN (Content Delivery Network) is a technology that distributes data from the origin server to users via nearby edge servers, allowing users to retrieve content quickly while reducing the load on the origin.

Motivation

Direct delivery from the origin can cause congestion at several points: the "first mile" (the outbound link of the origin server), the "last mile" (the user's inbound link), ISP interconnections, and long‑haul backbone transmission. These bottlenecks may lead to slow access or overload.

"First mile": the first exit of web traffic from the origin server; limited bandwidth can cause congestion when request volume exceeds capacity. "Last mile": the final segment to the user; improved ISP bandwidth has largely mitigated this issue. ISP interconnection: traffic crossing between different ISPs (e.g., China Telecom to China Unicom) can become a minor congestion point. Long‑haul backbone: long‑distance transmission delay and backbone congestion further affect flow.

If all data were delivered directly from the origin, these congestion points would frequently cause access delays.

By caching data at locations close to users, a CDN reduces the origin's outbound bandwidth pressure and alleviates network congestion.

Basic Process

Traditional request flow without CDN:

User enters the domain name in the browser.

Browser queries a DNS server for the domain's IP address.

DNS returns the IP address.

Browser sends a request to the origin server using that IP.

Origin server returns the requested content to the browser.

Request flow with CDN:

User enters the domain name in the browser.

Browser queries DNS; the domain's CNAME points to a CDN‑specific DNS server.

CDN DNS returns the IP address of a load‑balancing device.

User requests the content URL from the CDN load balancer.

The load balancer selects an appropriate cache server based on user IP, requested URL, and server load.

User sends the request to the chosen cache server.

If the cache contains the content, it is returned directly; otherwise the cache fetches the content from the next‑level cache or the origin server and then serves it.

Summary

After introducing a CDN, users experience no visible change in the website but benefit from faster access because the CDN serves content from edge servers located in the user's ISP network. This shortens the transmission path, speeds up page loads, and significantly reduces the load on the origin server.