Skip to content

CONCEPT Cited by 1 source

Anycast origin detection

Definition

Anycast origin detection is the technique of determining whether a given origin IP address responds from a single physical location or from multiple geographically distributed locations (i.e., is anycast). This distinction is critical for CDN topology decisions: unicast origins have a definitive "closest" data center; anycast origins do not.

The Speed-of-Light Method (Cloudflare)

Cloudflare's Smart Tiered Cache detects anycast using a constraint from physics:

  1. Measure probe latency from multiple checkpoint data centers around the world to the origin IP.
  2. For any pair of checkpoint data centers, compute the minimum physical travel time between them (speed of light in fiber ≈ 2/3 c, accounting for the fiber path between the two cities).
  3. If the sum of the two probe latencies (checkpoint A → origin) + (checkpoint B → origin) is less than the speed-of-light floor between checkpoint A and checkpoint B, then the origin cannot be at a single location.

Intuition: if the origin were at one fixed point, then the triangle inequality holds — the sum of two legs must be at least as long as the direct path. Violation implies the "origin" is answering from different physical locations for each probe.

Why This Matters

When anycast is detected, Smart Tiered Cache knows its latency probes are unreliable for that origin: - It won't pin the origin to a single upper tier (which could cause hairpin routing). - It falls back to multiple upper tiers, or awaits a region hint from the customer.

Limitations

  • Requires sufficient geographic spread of checkpoint data centers.
  • Very short inter-checkpoint distances may not provide enough resolution to detect anycast.
  • Assumes fiber-path latency (not straight-line distance) — practical but approximate.

Seen in

Last updated · 575 distilled / 1,757 read