What is a content delivery network (CDN) and how does it reduce latency?

A CDN reduces latency by serving content from servers geographically closer to the user, reducing the number of network hops and physical distance data must travel. The speed of light imposes a hard limit — data from New York to Tokyo takes ~70ms just for the round-trip light travel time, regardless of server speed. A Tokyo CDN edge serves content in <5ms. Latency reduction mechanisms: (1) Geographic proximity: edge servers in 50-200+ cities worldwide — serve from the nearest one; (2) TCP connection optimization: pre-established persistent connections from edge to origin; edge handles TCP handshake with user (3-way handshake overhead is local); (3) Protocol optimization: CDNs use HTTP/2 or HTTP/3 (QUIC) by default — multiplexing, header compression, faster connection establishment; (4) Reduced origin load: cache hits at edge don't reach origin — origin only handles cache misses; (5) DNS-based routing: CDN DNS returns the IP of the nearest edge node; (6) Anycast routing: same IP address announced from multiple locations — BGP routes to the nearest one. Cache hierarchy: browser cache → CDN edge → CDN regional cache (shield) → origin server. The shield layer reduces origin requests further. Dynamic CDN: CDNs like Fastly and Cloudflare support edge computing (Cloudflare Workers, Fastly Compute@Edge) — run custom code at the edge, enabling dynamic personalization with minimal latency.