Content Delivery Networks (CDNs)
Contents
A CDN is an overlay network of geographically distributed caching servers (reverse proxies) that reduces response time for clients and load on origin servers. Its primary benefit is the network itself, not just the cache. (→ Understanding Distributed Systems)
The Underlying Problem: BGP's Limitations
The internet's core routing protocol, BGP, was designed for correctness (reliable path discovery), not performance. BGP routes primarily on hop count, ignoring latency and congestion. This means the path BGP selects between two endpoints is often not the lowest-latency or highest-bandwidth path. (→ Understanding Distributed Systems)
Additionally, physical network latency is bounded below by the speed of light: a client on one side of the world talking to an origin on the other will see >100 ms of latency no matter how fast the machines are. This is irreducible without moving computation closer to the client.
The CDN Solution: Overlay Network
A CDN builds an overlay network on top of the internet — its own routing layer with different path-selection algorithms:
- Latency/congestion-aware routing: CDN backbone links are monitored continuously; traffic is routed around congestion and high-latency paths.
- TCP optimizations: persistent connection pools between CDN servers eliminate per-connection setup overhead; TCP window sizes are tuned for maximum bandwidth.
- Internet Exchange Point (IXP) placement: CDN servers are co-located at IXPs, where ISPs interconnect. Traffic from origin to client flows almost entirely over CDN-controlled links; only the last mile uses the public internet.
Client Routing: Global DNS Load Balancing
When a client resolves a CDN URL, the DNS response returns the address of the closest CDN cluster, taking into account:
- Client IP geolocation
- Current cluster health
- Network congestion toward each cluster
This is called global DNS load balancing — an extension to standard DNS that makes routing decisions based on real-time network state.
Caching Architecture
CDN caching is typically hierarchical:
Client → Edge cluster (geographically near client)
↓ (on miss)
Intermediary caching cluster (regional)
↓ (on miss)
Origin server
Edge clusters are deployed in many geographic locations. High coverage = more clients served locally, but more clusters = lower cache hit ratio (each cluster caches a smaller fraction of total content).
Intermediary clusters exist at fewer locations and cache more of the total content. They absorb misses from multiple edge clusters, reducing origin load significantly.
Within a cluster: content is partitioned (sharded) across multiple servers — no single server can hold all content.
Cache expiry follows the same HTTP semantics as client-side caching: Cache-Control, ETag, If-None-Match (→ Caching).
Dynamic Content and DDoS Shielding
CDNs are not limited to static content. Even for dynamic content that cannot be cached, the CDN overlay network reduces latency by routing requests efficiently to the origin.
In this mode, the CDN acts as a DDoS shield: volumetric attack traffic hits the CDN's distributed edge capacity (spread across hundreds of PoPs) rather than the origin. The CDN absorbs or filters the traffic before it reaches the backend.
Trade-offs
| Concern | Implication |
|---|---|
| Cache hit ratio vs. coverage | More edge clusters = better geographic coverage but lower hit ratio; intermediary clusters help |
| Content freshness | CDN respects TTLs, but cache purges (on deploy) must be explicitly triggered |
| Cost | CDN traffic is metered; high-volume static assets are cheap per GB; origin-touching dynamic traffic is more expensive |
| Consistency | CDN edge caches may serve stale content up to TTL; combine with URL versioning for correctness |
Well-Known CDNs
- AWS CloudFront — tightly integrated with AWS origin services
- Akamai — oldest and most widely deployed; historically dominant for media delivery
- Cloudflare — strong DDoS mitigation and DNS integration; also functions as a reverse proxy
(→ Technology Glossary for brief entries on each)
Related Pages
- Caching — HTTP caching mechanics that CDN edge servers implement
- Dns — global DNS load balancing routes clients to the nearest CDN cluster
- Http — CDN benefits from HTTP/2 multiplexing and persistent connections
- Availability — CDN improves availability via geographic redundancy and DDoS absorption
Sources
- (→ Understanding Distributed Systems ch. 15) — overlay network, BGP limitations, global DNS load balancing, IXP placement, caching layers, dynamic content/DDoS shield, cache hit ratio vs. coverage trade-off.